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10th Class - Physical Science-TS

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10th Class - Physical Science-TS
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I. Reflections on concepts

Question 1.
Are the magnetic field lines closed? Explain.

Answer:
1. The field lines appear to be closed loops but you can’t conclude that lines are closed or open loops by looking at the picture of field lines because we do not know about alignment of lines that are passing through the bar magnet.

2. The direction of a magnetic line at any point gives the direction of the magnetic force on a north pole placed at that point.

3. Since the direction of magnetic field line is the direction of force on a north pole, the magnetic lines always begin from the ‘N’ pole of the magnet and end on the ‘S’ pole of the magnet.

4. Inside the magnet however the direction of magnetic lines is from S-pole of the magnet to the N-pole of the magnet.

5. Thus the magnetic field lines are closed.

Question 2.
See figure, magnetIc lines are shown. What is the direction of the current flowing through the wire?

Answer:
1) The magnetic field lines are in the anti-clockwise direction.
2) The direction of current is vertically upwards. This can be demonstrated with right-hand thumb rule.

Question 3.
A bar magnet with North pole facing towards a coil moves as shown In figure given below. What happens to the magnetic flux passing through the coil?

Answer:
The magnetic flux passing through the coil induces current in the coil. This current is called induced EMF. This induced N EMF is equal to the rate of change of magnetic flux passing through it.

Question 4.
A coil is kept perpendicular to the page. At P, current flows into the page and at Q it comes out of the shown in figure. What is the page as direction of magnetic field due to the coil?

Answer:
To know the direction of magnetic field, we use right-hand rule i. e., ‘when you curl your right-hand fingers ¡n the direction of current, thumb gives the direction of magnetic field’. According to this the direction of magnetic field is as shown in the following figure.

Application of Concepts

Question 1.
The direction of current flowing in a coil Is shown in figure. What type of magnetic pole Is formed at the face that has flow of current as shown in fig.

Answer:
The top surface of the coil shown in the fig. behaves as North pole and direction of magnetic field points towards us.

Question 2.
Why does the picture appear distorted when a bar magnet is brought close to the screen of a television? Explain.

Answer:
Picture on a television screen Is due to motion of the electrons reaching the screen. These electrons are affected by magnetic field of bar magnet. This is due to the fact that the magnetic field exerts a force on the moving charges. This force is called magnetic force. Due to this magnetic force, the picture is distorted when you remove the bar magnet away from the screen, the motion of electron is not affected by the magnetic force and the picture will be normal.

Question 3.
Symbol ‘X’ indicates the direction of a magnetic field into the page. A straight long wire carrying current along its length is kept perpendicular to the magnetic field. What A is the magnitude of force experienced by the wire? In what direction does it act?

Answer:
From the figure, a straight wire carrying current which Is kept perpendicular to a uniform magnetic field B. This ‘B’ is directed Into the page. Let the field be confined to the length L. We know that the electric current means charges in motion. Hence they move with a certain velocity called drift velocity V.

The magnetic force on a single charge is given by F0= qv B.
Let total charge inside the magnetic field be Q. So magnetic force on the current carrying wire s given by F=QvB ……………………………. (1)
The time taken by the charge (Q) to cross the field be
t = L//v⇒v=Lt ............ (2)
∴ (1) ⇒ F = QLB/t⇒Q/t (LB) ........... (3)
We know that Qt Is equal to the electhc current in the wire.
I = Q/t
∴ (3) ⇒ FILB.

The direction of the force: The direction of force can be find by using right-hand rule. Fore finger points towards the velocity of current, middle finger points to the direction of magnetic field (B), then the thumb gives direction of force when the three fingers are stretched in such away that they are perpendicular to each other.

Question 4.
An 8N force acts on a rectangular conductor 20 cm long placed perpendicular to a magnetic field. Determine the magnetic field induction if the current in the conductor is 40 A.

Answer:
Formula, F = ILB (or) B = F/IL
Where F = magnetic force = 8N
I = Electrical current in the conductor = 40 A
L, length of conductor = 20 cm. = 20/100 = 0.2 mt
∴ B =?
B= 8/40×0.2=80/40×2 = 1
Magnetic field induction = 1 tesla.

Question 5.
As shown in figure both coil and bar magnet move in the same direction. Your friend is arguing that there is no change in flux. Do you agree with his statement’ If not what doubts do you have? Frame questions about the doubts you have regarding change in flux.

Answer:
Agree: Yes. I will agree.

  1. The induced EMF will not produce when the coil and magnet are moving in the same direction with same velocity.
  2. Hence my friend’s argument is correct.

Disagree:

  1. What happens if both magnet and coil move in same direction?
  2. What happens if both magnet and coil move in opposite direction?
  3. What is the direction of the current in the coil?
  4. If both move in same direction, is there any linkage of flux with the coil?
  5. When N pole is moved towards the coil what is the direction of current?
  6. If magnet is reversed, what is the direction of current in the coil?
Question 6.
Give a few applications of Faraday’s law of induction in daily life.

Answer:
The daily life applications of Faraday’s law of induction are :

  1. Generation of electricity.
  2. Transmission of electricity.
  3. Metal detectors in security checking.
  4. The tape recorder.
  5. Use of ATM cards
  6. Induction stoves
  7. Transformers
  8. Induction coils (spark plugs in automobiles)
  9. Break system in railway wheels
  10. AC and DC generators
  11. Windmills etc.
Question 7.
Explain the working of an electric motor with a neat diagram.

Answer:
1. Consider a rectangular coil kept in uniform magnetic field as shown in figure.
2. Switch on the circuit so that the current flows through the coil. The direction of current is shown in the figure.
3. The sides AB and CD of the coil are always at rigi angles to the magnetic field.
4. According to right-hand rule, at AB the magnetic force acts inward perpendicular to the field of magnet ai
on CD, it acts outward.
5. The top view of coil is shown in the figure.
6. The force on the sides BC and DA varies because they make different angles at different positions of the coil in the field. At BC, magnetic force pulls the coil up and at DA magnetic force pulls it down.
7. The net force acting on AB and on CD is zero because they carry equal currents in the opposite direction.
Similarly, the sum of the forces on sides BC and DA is also zero. So, net force is zero on the coil.
8. But the rectangular coil comes into rotation in clockwise direction because equal and opposite pair of forces acting on the twos sides of the coil.

9. If the direction of current in the coil is unchanged, it rotates, upto half rotation In one direction and the next half In the direction opposite to previous like to and fr0 motion.
10. It the direction of current in the coil is changed the coil will rotate continuously in one and the same direction.
11. To achieve this, brushes B, and B2are used.
12. These brushes are connected to the battery. The ends of the coil are connected to slip rings C1and C2which rotate along with the coil.
13. Initially C1is In contact with B1and C2Is in contact with B2
14. After half rotation, the brushes come into contact with the other slip rings in such a way that the direction of current through the coil Is reversed. This happens every half-rotation.
15. Thus the direction of rotation of the coil remains the same. This is the principle used in electric motor.
16. In electric motors electrical energy is converted Into mechanical energy.

Question 8.
Explain the working of AC electric generator with a neat diagram.

Answer:
(i) Electric Generator or Dynamo: It is a device which converts mechanical energy into electrical energy.
(ii) Principle: It works on the principle of the electromagnetic induction.
(iii) Construction:
It consists of
(i) Armature coil,
(ii) Brushes,
(iii) Slip rings,
(iv) Strong magnet and
(v) Rotating mechanism (or) motor.
The two A and B of the coil ABCD are connected to the slip rings.

Working:

  • When the coil is at rest ¡n vertical position, with side (A) of coil at top position and side (B) at bottom position, no current will be induced in it. Thus current in the coil is zero at this position.
  • When the coil is rotated in clockwise direction, current will be induced in it and it flows from A to B, in this position the current increases from zero to a maximum.
  • If we continue the rotation of coil current decreases during the second quarter of the rotation and once again becomes zero when coil comes to vertical position with side B at top side and side A at bottom position.
  • During the second half of the rotation, current generated follows the same patterns as that in the first half, except that the direction of current is reversed.
  • Thus, after every rotation of the current ¡n the respective arm changes, there by generating an alternating current. This device is called A.C. generator.
Question 9.
Explain the working of DC generator with a neat diagram.

Answer:

  1. Consider a rectangular coil. Let it be held between the poles of curve-shaped permanent magnets as shown in figure.
  2. As the coil rotates the magnetic flux passing through the coil changes.
  3. According to the law of electromagnetic induction, an induced current is generated in the coil.
  4. If two half-slip rings are connected to the ends of the coil as shown in figure, this generator works as DC generator to produce DC current.

Working:

  1. When the coil is in vertical position the induced current generated during the first half rotation, rises from zero to maximum and then falls to zero again.

2. As the coil moves further from this position, the ends of the coil go to other slip rings.

3. Hence, during the second half rotation, the current is reversed in the coil itself, the current generated in the second half rotation of the coil is identical with that during the first half of the direct current (DC), for one revolution.

4. Hence, this current ¡s called direct current (DC).

Question 10.
How do you appreciate Faraday’s law, which s the consequence of conservation of energy?

Answer:

  1. Faraday’s law of electromagnetic induction, which is the outcome of law of conservation of energy, is very much useful.
  2. The law led to the conclusion that mechanical energy can be converted into electrical energy.
  3. The fruits of electrical energy, namely, the electric lights, the stoves, electric heaters, electrical irons, the fridge, the electric motor the television, the electronic locomotives and all electric gadgets are serving mankind and making people more comfortable.
  4. All these gadgets work on electricity and electrical energy that can be produced by generators on large scale where energy Is created by law of conservation of energy on a very large scale.
  5. So, we must appreciate the utilïtanan value of Faraday’s law which is the consequence of conservation of energy.
Question 11.
The value of magnetic field Induction which is uniform is 2T. What is the flux passing through a surface of area 1.5m1 perpendicular to the field?

Answer:
Magnetic Induction of uniform field B = 2T
Flux Φ =?
Surface area A = 1.5 m2
We know B = φ/A ⇒ BA cosθ
Φ= BA
Where B ¡s perpendicular to area is
ΔΦ = 2x 1.5=3webers

Question 12.
Which of the various methods of current generation protects nature well? Give examples to support your answer. )

Answer:
The methods of current generation that protect the nature well:
1. Hydra power plant A power plant that produces electricity by using flowing of water to rotate a turbine Is called hydropower plant.

Advantages:

  • It does not produce any environmental pollution.
  • It will never get exhausted.
  • The construction of dams on rivers helps In controlling floods and in irrigation.

2. Wind generators: A wind generator s used to generate electricity by using wind energy.

Advantages:

  • It does not cause any pollution.
  • It will never get exhausted.
  • Wind energy is available free of cost.

3. Solar cell: Solar cell is a device which converts solar energy directly into electricity.

Advantages:

  1. They require no maintenance.
  2. They can be set up in remote inaccessible and very sparsely inhabited areas where the laying of usual power transmission is different and expensive.
  3. As seawater flows in and out of the tidal barrage during high and low tides It turns the turbines to generate electricity.
  4. The energy of moving sea waves can be used to generate electricity.
  5. The energy available due to the difference In the temperature of water at the surface of the ocean and at deeper levels is called ocean thermal energy. The ocean thermal energy can be converted into a usable form of energy like electricity.
  6. Geothermal energy is the heat energy from hot rocks present inside the earth. It is also used to produce electricity. It is a clean and environmental friendly source of energy.
  7. The energy produced during nuclear fission reactions is used for generating electricity at nuclear power plants.

Suggested Experiments

Question 1.
Explain with the help of two activities that current-carrying wire produces magnetic field.

Answer:
Activity -1.

  1. Take a wooden plank and make a hole as shown in adjacent figure. Table
  2. Place the plank on the table.
  3. Place a retort stand on the plank as shown in figure.
  4. Pass 24 gauge copper wire through a hole of the plank and rubber knob of the retort stand In such a way that the wire be arranged In a vertical position and not touch the stand.
  5. Connect the two ends of the wire to a battery via switch.
  6. Place 6 to lo compass needles In a circular path around the hole so that its centre coincides with the hole. Use 3-volt battery In the circuit.
  7. Switch on current.
  8. We see the compass needles change the direction In such a way as tangents to the circle.
  9. This activity helps us to prove current carrying wire produces magnetic field.

Activity -2
1. Take a thin wooden plank covered with white paper.
2. Make equidistant holes on its surface as shown in adjacent figure.
3. Pass copper wire through the holes. This forms a coil. Join the ends of the coil to a battery through a switch.
4. Switch on the circuit. Current passes through the coil.
5. Now sprinkle iron filings on the surface of the plank around the coil.
6. Give small jerk to It. An orderly pattern of iron filing is seen on the paper.
7. This activity proves that a current-carrying wire develops a magnetic field around it.

Question 2.
How do you verify experimentally that the current-carrying conductor experiences a force when it is kept in a magnetic field?
(Or)
List out the apparatus and experimental procedure for the experiment to observe a current-carrying wire experiences a magnetic force when it is kept in uniform magnetic field.

Answer:
Required apparatus:

  • Horse-shoe magnet
  • conducting wire
  • battery, switch
  1. Take a wooden plank.
  2. Fix two long wooden sticks on it.
  3. These wooden sticks are split at their top ends.
  4. A copper wire is passed through splêts of wooden sticks.
  5. Connect the wire to 3 volts battery.
  6. Close the switch to make the circuit. Current passes through the wire.
  7. Bring a horseshoe magnet near the wire.
  8. Then a force is experienced on the wire, following right thumb rule.
  9. Reverse the polarities of the magnet, then the direction of the force is also reversed.
  10. The night hand rule helps the direction of magnetic force exerted by the magnetic field on the current carring wire.
Question 3.
Explain Faraday’s law of induction with the help of activity.

Answer:

  1. Connect the terminals of a coil to a sensitive ammeter or galvanometer as shown in the adjacent figure.
  2. Now push a bar magnet towards the coil, with its north pole facing the coil.
  3. While the magnet is moving towards the coil the needle in galvanometer deflects, showing that a current has been set up in the coil.
  4. The needle of galvanometer does not deflect If the magnet is at rest.
  5. If the magnet Is moved away from the coil, the needle In the galvanometer again deflects, but ¡n the opposite direction.
  6. It means that a current is set up in the coil in the opposite direction.
  7. If we use the end of south pole of a magnet instead of north pole in this activity, the experiment works just as described but the reactions are exactly in opposite directions.
  8. Further experimentation enables us to understand that the relative motion of the magnet and coil sets up a current in the coil. It makes no difference whether the magnet is moved towards the coil or the coil towards, the magnet.
  9. This proves the Faraday’s law: “Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated In the coil.
Question 4.
What experiment do you suggest to understand Faraday’s law? What Instruments are required? What suggestions do you give to get good results of the experiment? Give precautions also.

Answer:
Aim: To understand Faraday’s law of induction. Materials required: A coil of copper wire, a bar magnet Galvanometer etc.

Procedure:

  1. Connect the terminals of a coil to a sensitive galvanometer as shown In the figure.
  2. Normally we would not expect any deflections of needle in the galvanometer because there is to be no electromotive force In this circuit.
  3. Now if we push a bar magnet towards the coil, with its north pole facing the coil, we observe the needle in the galvanometer deflects, showing that a current Is set up in the coil.
  4. The galvanometer does not deflect if the magnet Is at rest.
  5. If the magnet is moved away from the coil, the needle in the galvanometer again deflects, but in the opposite direction, which means that a current is set up in the coil, in the opposite direction.
  6. If we use end of south pole of a magnet instead of north pole In the above activity, the deflections are exactly reversed.
  7. This experiment proves “whenever there is a continuous change of magnetic flux linked with a closed coil, a current Is generated in the coil.

Precautions:

  • The coil should be kept on an insulating surface.
  • Bar magnet should be of good pole strengths.
  • The centre of the Galvanometer scale must be zero.
  • The deflections in the galvanometer must be observed while Introducing the bar magnet Into the coil and also while withdrawing It.
Question 5.
How can you verify that a current-carrying wire produces a magnetic field with the help of an experiment?

Answer:
Aim: To demonstrate that a magnetic field is produced around a current-carrying wire.
Apparatus: Copper wire, slits, 3volt battery, key, and connecting wires, thermocol sheets.

Procedure:

  • Take a hermocoI sheet and fix two wooden slits of 1 cm at the top of their ends.
  • Arrange a copper wire which passes through slits and make a circuit with the elements as shown in the figure.
  • Now, keep a magnetic compass below the wire.
  • Bring a barmagnet dose to the compass.

Suggested Projects

Question 1.
Collect Information about generation of current by using Faraday’s law.

Answer:
Faraday’s law is useful In generation of current.

  1. According to this law, the change in magnetic flux Induces EMF in the coil.
  2. He also proposed electromagnetic induction.
  3. Electromagnetic induction is a base for generator, which produces electric Current.
  4. Transformer also works on the principle of electromagnetic induction, which is helpful In transmission of electricity.
  5. Hence Faraday’s law is used in the generation and transmission of current.
Question 2.
Collect information about material required and procedure of making a simple electric motor from Internet and make a simple motor on your Own.

Answer:
Material required:

  1. A plastic pipe of 3 or 4 cm. diameter and 5 cm long.
  2. A copper wire 30 cm long.
  3. A horseshoe magnet.
  4. Two corks that exactly fit Into the bore of the plastic pipe.
  5. A wooden plank used as base for the whole arrangement.
  6. Cello tape and small nails.

Procedure:

  1. Wind the copper wire around the plastic pipe.
  2. Connect the ends of the wire to the two brushes.
  3. Fit in the corks into the hollow of the pipe such that they close either end of the pipe.
  4. By means of vertical pieces of wood and the nails arrange the plastic pipe such that it can rotate freely on the pieces of wood. This acts as the armature.
  5. Arrange the horseshoe magnet on the wooden base such that the armature lies between both of its poles.
  6. Now connect the ends of the copper wire to a sensitive galvanometer.
  7. Rotate the armature freely.
  8. You can observe reflection in the galvanometer.
  9. Thus it is an Improvised electric motor.
Question 3.
Collect information of experiments done by Faraday.

Answer:
Experiment -1

  1. Connect the terminals of a coil to a sensitive galvanometer as shown n the figure.
  2. Normally, we would not expect any deflection of needle In the galvanometer because there Is no EMF In the N – circuIt.
  3. Now, If we push a bar magnet towards the coil, with N Its north pole facing the coil, the needle In the galvanometer deflects, showing that a current has been set up in the coil, the galvanometer does not deflect if the magnet is at rest.
  4. If the magnet is moved away from the coil, the needle in the galvanometer again deflects, but in the opposite direction, which means that a current is set up in the coil in the opposite direction,
  5. If we use the end of south pole of a magnet Instead of north pole, the results i.e., the deflections in galvanometer are exactly opposite to the previous one.
  6. This activity proves that the change in magnetic flux linked with a closed coil, produces current.
  7. From this Faradays law of induction can be stated as ‘whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil” This induced EMF is equal to the rate of change of magnetic flux passing through it.

Experiment -2
1. Prepare a coil of copper wire C1and connect the two ends of the coil to a galvanometer.
2. Prepare another coil of copper wire C2similar to C1and connect the two ends of the coil to a battery via switch.
3. Now arrange the two coils C1and C2nearby as shown in the figure.
4. Now switch on the coil C2We observe a deflection In the galvanometer connected to the coil C1.
5. The steady current in C2produces steady magnetic field. If coil C2is moved towards the coil C1, galvanometer shows a deflection.
6. This indicates that electric current Is induced in coil C1.

7. When C2is moved away, the galvanometer shows a deflection again, but this time in the opposite direction.
8. The deflection lasts as long as coil C2in motion.
9. When C2is fixed and C1is moved, the same effects are observed.
10. This shows the induced EMF due to relative motion between two coils.

Page 209

Question 1.
How do they (most of the appliances) work?

Answer:
The electronic motor, generators, electric cranes, rice cookers, electrical irons, washing machines, grinders, fans, and many more appliances work on electricity.

Question 2.
How do electromagnets work?

Answer:
By passing D.C. current through a conductor wound around a core, a magnetic field Is developed around the current-carrying conductor and work as a magnet.

Question 3.
Is there any relation between electricity and magnetism?

Answer:
Yes. A current-carrying conductor develops a magnetic field around it.

Question 4.
Can we produce magnetism from electricity?

Answer:
Yes. By passing electric current (D.C) through an insulated wire wound around an iron rod we can produce magnetism by electricity.

Page 210

Question 5.
Does the needle get deflected by the bar magnet?

Answer:
Yes, the needle gets deflected.

Question 6.
why does the needle get deflected by the magnet?

Answer:
When the N-pole of compass needle and N-pole of magnet come close to each other the needle gets deflected due to repulsion. (or) When the S-pole of compass needle and S-pole of magnet come close to each other (S-pole and S-pole are like poles they repel each other) and the needle gets deflected.

Question 7.
What do you notice?

Answer:
The needle is deflected, as there is magnetic field around the wire.

Question 8.
Is there any change in the position of the compass needle?

Answer:
Yes.

Question 9.
Which force Is responsible for the deflection of the compass needle?

Answer:
The magnetic force developed around the current-carrying wire is responsible for the deflection of compass needle.

Question 10.
Does the current-carrying wire apply a force on the needle?

Answer:
Yes. The current-carrying wire applies a repulsive force on the needle.

Question 11.
What do we call this force?

Answer:
This force is called magnetic force of attraction and repulsion.

Page 211

Question 12.
How was this fleid produced?

Answer:
This field was produced due to the magnetic field developed ¡n the wire by the electric current.

Question 13.
Can we observe the field of a bar magnet?

Answer:
Yes.

Question 14.
Why does this happen?

Answer:
The needle of the magnetic compass is affected by the bar magnet without any physical contact.

Question 15.
Is there any change in the direction of the needle of the magnetic compass? Why?

Answer:
Yes. The change in direction of needle of magnetic compass is due to the magnetic force developed in the conductor.

Question 16.
What is the nature of force that acts on the needle?

Answer:
The force which acting on the needle from a distance s the magnetic field of the bar magnet.

Question 17.
What do you observe?

Answer:
Almost the needle of compass shows single direction pointing north at far places from bar magnet.

Page 212

Question 18.
What does It mean?

Answer:
It means that strength of field vanes with distance from the bar magnet.

Question 19.
How can we find the strength of the field and direction of the field?

Answer:
The direction of field can be determined by using the compass. To determine the strenath we have to perform activity 3.

Question 20.
What are these curves?

Answer:
These curves are technically called as magnetic field lines,

Page 213

Question 21.
Are these field lines closed loops or open loops?

Answer:
The field lines appear to be closed loops. But we can’t conclude that lines are closed or open loops by looking at the figure of field lines because we do not know about alignment of lines that are passing through the bar magnet.

Question 22.
Can we give certain values to magnitude of the field at every point in the magnetic field?

Answer:
Yes. The number of lines passing through the plane of area ‘A’ perpendicular to the field is called magnetic flux and is deoted by φ(Phi).

Page 214

Question 23.
What is the flux through unit area perpendicular to the field?

Answer:
It Is equal to φ/A. (A = Area)

Question 24.
Can we generalize the formula of flux for any orientation of the plane taken in the field?

Answer:
The magnetic flux density denoted by ‘B’ is given by, B = magnetic flux / effective area i.e.; B= φ /A cos θ)
Where ‘θ’ Is the angle between magnetic field (B) and normal to the plane with area ‘A’.

Question 25.
What is the flux through the plane taken parallel to the field?

Answer:
Since θ is zero between parallel lines. Cos0° = 1 and So, φ= BA.

Question 26.
What Is the use of Introducing the Ideas of magnetic flux and magnetic flux density?

Answer:
The ideas of magnetic flux and magnetic flux density help us to estimate the strength of the magnetic field.

Question 27.
Are there any sources of magnetic field other than magnets?

Answer:
Yes. The current-carrying conductor develops a magnetic field around It.

Question 28.
Do you know how the old electric calling bells work?

Answer:
Yes. They work on the principle of magnetic effect of electric currents.

Page 215

Question 29.
How do the directions of the compass needles change?

Answer:
They are directed along tangents to the circle.

Question 30.
What is the shape of the magnetic field line around wire?

Answer:
The shape of magnetic line around a current-carrying wire is almost a circle,

Question 31.
What is the direction of magnetic field Induction at any point on the field line?

Answer:
If the current flows vertically in upward direction from the page (plane of paper),the field lines are in anti-clockwise direction.

Page 216

Question 32.
Can you tell the direction of the magnetic field of the coil?

Answer:
Yes. The direction In which compass needle comes to rest Indicates the directton of the field due to the coil. So, the direction of field is perpendicular to the plane of the coil.

Question 33.
Why does the compass needle point In the direction of field?

Answer:
We know that south pole is attracted by the north pole. The needle Is oriented in such a way that Its south pole points towards the north pole of the coil.

Page 217

Question 34.
How do they adjust ¡n such an orderly pattern?

Answer:
A solenoid Is a long wire wound in a closed-packed helix. The magnetic field lines set up by solenoid resemble those of a bar magnet indicating that a solenoid behaves like a bar magnet.

Question 35.
What happens when a current-carrying wire is kept in a magnetic field?

Answer:
When a current-carrying wire is kept in a magnetic field it undergoes orientation.

Question 36.
Do you feel any sensation on your skin?

Answer:
Yes. The hair on my skin rises up when I stand near TV screen.

Question 37.
What could be the reason for that?

Answer:
It is due to the magnetic field produced by electric charges in motion.

Question 38.
Take a bar magnet and bring it near the TV screen. What do you observe?

Answer:
I observed that the picture on the TV screen is not clear. It is distorted.

Question 39.
Why does the picture get distorted?

Answer:
It is because the motion of electrons that form the picture is affected by the magnetic field of bar magnet.

Page 218

Question 40.
Is the motion of electrons reaching the screen affected by magnetic field of the bar magnet?

Answer:
Yes.

Question 41.
Can we calculate the force experienced by a charge moving In a magnetic field?

Answer:
Yes. If the force is F, it is given by the expression, F=qvB
Where, q = Amount of charge
y = Velocity of charged particle perpendicular to the magnetic field.
B = Magnetic field

Question 42.
Can we generalize the equation for magnetic force on charge when there Is an angle ‘q’ between the directions of field ‘B’ and velocity ‘V’?

Answer:
Then force F is given by the formula F = qvB Sin θ.

Question 43.
What Is the magnetic force on the charge moving parallel to a magnetic field?

Answer:
When the charge moves parallel to the magnetic field, the value of θ becomes zero. In the equation F=qvB sin θ. since θ=0, sinθ=0 the value of force F also becomes zero.

Question 44.
What is the direction of magnetic force acting on a moving charge?

Answer:
By applying the “right-hand’ rule we can guess the direction of magnetic force acting on a moving charge is direction of the thumb.

Page 219

Question 45.
What is the direction of force acting on a negative charge moving in a field?

Answer:

  1. At first find the direction of force acting on a positive charge, by applying right-hand rule for positive charge.
  2. Then reverse the direction and this new direction is the direction of force acting on a negative charge.
Question 46.
What happens when a current-carrying wire ¡s placed in a magnetic field?

Answer:
The current-carrying wire experiences magnetic force when it is kept in magnetic field.

Think And Discuss

Question 1.
How does a tape recorder which we use to listen songs / speeches work?

Answer:

  1. The tape recorder which we use to listen songs/speeches and to record the voices works on the principle of electromagnetic induction.
  2. it consists of a piece of plastic tape coated with ferromagnetic substance, iron oxide which is magnetised more in some parts than in others due to varying currents produced by our voice.
  3. When the tape moves past as a small coil of wire (it is called head of the tape recorder.), the magnetic field produced by the tape changes which leads to generation of current in the small coil of wire.
  4. These varying currents are fed into a loud speaker where they are reproduced as sounds.
Question 2.
How could we use the principle of electro-magnetic induction in the case of using ATM card?

Answer:
When the magnetic strip of ATM card coated with iron oxide moves past as a small coil of wire, the magnetic field changes which leads to generation of current and it leads to the operation of the card.

Activity 1

Question 1.
With the help of an activity show the current-carrying wire produces magnetic field.
(or)
How do you prove magnetic field develops around a current-carrying conductor?

Answer:

  1. Take a thermocol sheet and fix two thin wooden sticks of height 1cm which have small slit at the top
    of their ends.
  2. Arrange a copper wire of 24 gauge which passes through these slits and make a circuit in which the elements1 at 3 volt battery, key and copper; wire are connected in series.
  3. Now keep a magnetic compass below the wire.
  4. Bring a bar magnet close to the compass.
  5. Take a bar magnet far away from the circuit and switch on the circuit. Observe the changes In compass.
  6. The compass needle deflects.
  7. This deflection Is due to the magnetic field produced by current-carrying wire.

Activity 2

Question 2.
Show that the magnetic field around a bar magnet is three dimentional and its strength and direction varies from place to place.

Answer:

  1. Take a sheet of white paper and place it on the horizontal table.
  2. Place a bar magnet in the middle of the sheet.
  3. Place a magnetic compass near the magnet. It settles to a certain direction:
  4. Use a pencil and put dots on the sheet on either side of the needle. Remove the compass. Draw a small line segment connecting the two dots Draw an arrow on it from South Pole of the, needle to North Pole of the needle.
  5. Repeat the same by placing the compass needles at various positions on the paper. The compass needle settles in different directions at different positions.
  6. This shows that the direction of magnetic field due to a bar magnet varies from place to place.
  7. Now take the compass needle to places far away from magnet, on the sheet and observe the orientation of the compass needle in each case.
  8. The compass needle shows almost the same direction along north and south at places far from the magnet.
  9. This shows that the strength of the field varies with distance from the bar magnet.
  10. Now hold the compass a little above the table and at the top of the bar magnet.
  11. We observe the deflection in compass needle. Hence we can say that the magnetic field is three dimensional i.e., a magnetic field surrounds its source.
  12. From the above activities we can generalize that a magnetic field exists in the region surrounding a bar magnet and is characterized by strength and direction.

Activity 3

Question 3.
Explain the process of tracing magnetic field lines.

Answer:

  1. Place a white sheet of paper on a horizontal table.
  2. Place a compass in the middle of it. Put two dots on either side of the compass needle. Take it out.
    Draw a line connecting the dots which shows the North and South of the earth.
  3. Now place a bar magnet on the line drawn in such a way that its north pole points towards the Magnetic field lines geographic north.
  4. Now place the compass at the north pole of the bar magnet. Put a dot at the north pole of the compass needle.
  5. Now remove the compass and place it at the dot. It will point in other direction. Again put a dot at the north pole of the compass needle.
  6. Repeat the process till you reach the south pole of the bar magnet.
  7. Connect the dots from ‘N’ of the bar magnet to S’ of the bar magnet, with a’ free hand ctirve. You will get a curved line.
  8. Now select another point from the north pole of the bar magnet. Repeat the process for many points taken near the north pole.
  9. You will get different curves as shown in the figure.
  10. These lines are called magnetic field lines. They are Imaginary lines.

Activity 4

Question 4.
Describe an activity to find magnetic field due to straight wire carrying current. What Is its direction?

Answer:
1. Take a wooden plank and make a hole as shown in figure, Place this plank on the table.
2. Now place a retort stand on the plank as shown In figure.
3. Pass a 24 gauge copper wire through hole of the plank and rubber knob of the retort stand in such a way that the wire be arranged In a vertical position and not touch the stand.
4. Connect the two ends of the wire to a battery via switch.
5. Place 6 to lo compass needles in a circular path around the hole so that its centre coincides with the hole.
6. Use a 9V battery in the c!rcuet. Swtch on current and it flows through the wire.
7. We notice that they are directed as tangents to the circle.
8. The magnetic field around the wire is circular ¡n shape. This can be verified by sprinkling iron filings around the wire when current flows in the wire.
9. The direction of magnetic field around the wire will be as shown in the following figure.

10. The direction of the magnetic field around the current-carrying wire can be determined by right-hand thumb rule ie., grab the current-carrying wire with your right hand in such a way that thumb is in the direction of current, then the curled fingers show the direction of magnetic field as shown in the figure.

Activity 5

Question 5.
Trace the magnetic field due to circular coil.

Answer:
1. Take a thin wooden plank covered with white paper and make two holes on its surface as shown in the figure.
2. Pass insulated copper wire (24 guage) through the holes and wind the wire 4 to 5 times through holes
such that it looks like a coil.
3. The ends of the wire are connected to terminals of the battery through a switch. Now switch on the
circuit.
4. Place a compass needle on the plank at the centre of the coil. Put dots on either side of the compass needle.
5. Again place compass at one of the dots put other dots further.
6. Do the same till you reach the edge of the plank.
7. Now repeat this for the other side of the coil from the centre. Then draw a line joining the dots. We will get a field line of the circular coil.
8. Do the same for the other points taken In the between the holes. Draw corresponding lines. We will get field lines of the circular coil.
9. The direction of the field due to coil is s determined by using right-hand rule, which states that, when you curl your right-hand fingers in the direction of current, thumb waves the direction of magnetic field.

Activity 6

Question 6.
Find the magnetic field due to a solenoid.
(or)
Write the experimental procedure and observations of the experiment that is to be performed to observe the magnetic field formed due to solenoid.

Answer:

  1. Take a wooden plank covered with a white paper.
  2. Make equidistant holes on its surface as shown in the figure.
  3. Pass copper wire through the holes. This forms a coil.
  4. Join the ends of the coil to a battery through a switch.
  5. Switch on the circuit. Current passes through the coil.
  6. Now sprinkle Iron filings on the surface of the plank around the coil. Give-a small jerk to it, An orderly pattern Of iron filings is seen on the paper.
  7. The long coil is called solenoid. The field of solenoid is shown in the figure.
  8. This activity proves that current-carrying solenoid forms magnetic field.

Activity 7

Question 7.
Explain magnetic force on moving charge and current carrying wire.

Answer:
(a) TV Screen Activity:

  1. Take a bar magnet and bring it near the TV screen.
  2. Then the picture on the screen is distorted.
  3. Here the distortion is due to the motion of the electrons reacting the screen are affected by the magnetic field.
  4. Now move the bar magnet away from the screen.
  5. Then the picture on the screen stabilizes.
  6. This must be due to the fact that the magnetic field exerts a torce on moving charges. This force Is called magnetic force.
  7. The magnitude of the force is F = Bqv where B Is magnetic induction, q’ is the charge and y Is the velocity of the charged partide.

(b) Procedure:

  1. Take a bar magnet near to the TV screen. We observe that electrons are affected by the field produced by the bar magnet. Then the picture Is disturbed.
  2. Move the bar magnet away from screen. Then you will get a clear picture.
  3. The force on the moving charge is given by F = Baqv.
  4. Right hand rule is used when velocity and field are perpendicular to each other

Fore-finger —- Direction of current (V).
Middle finger —- Direcrtion of field
Thumb —- direction of force
This rule is applicable to positive charge.

Activity 8

Question 8.
Explain the result of magnetic force applied on a current-carrying wire by an experiment.
(or)
Why the current carrying straight wire which is kept In a uniform magnetic field, perpendicularly to the direction of the field bends aside P Explain this process with a diagram showing the direction of forces acting on the wire?

Answer:
1. Take a wooden plank. Fix two long wooden sticks on It. These wooden sticks are split at their top ends.
2. A copper wire is passed through these splits and the ends of the wire are connected to a battery
of 3V, through a switch.
3. Close the switch to make the circuit. Current passes through the wire.
4. Now bring a horseshoe magnet near the copper wire as shown in the figure. Observe the deflection of the wire.
5. change polarities of the horseshoe magnet. Again observe the deflection. Repeat this by changing the direction of current in the circuit.
6. When current passes through wire, it produces a magnetic field and this field overlaps with the field by horseshoe magnet and gives a non-uniform field.
7. The field in between north arid south poles of horseshoe magnet is shown in the figure.

8. Let us make a wire passing perpendicular to the paper. Let the current pass through It. It produces magnetic field as shown in the figure.

9. The resultant field will be as shown below.
10. We can see that the direction of the field lines due to wire in upper part (of circular lines) coincides
with the direction of field lines of horseshoe magnet.

11. The direction of field lines by wire in lower part (or circular lines) Is opposite to the direction of the
field lines of horseshoe magnet. The net field in upper part is strong and in lower part it is weak. Hence a non-uniform field is created around the wire.
12. Therefore the wire tries to move to the weaker field region.

Activity 9

Question 9.
Take wooden base. Fix a soft iron cylinder on the wooden base vertically. Wind copper wire around the soft Iron as shown in the figure. Now take a metal ring which is slightly greater in radius than the radius of the soft iron cylinder and insert it through the cylinder on the wooden base. Now explain the behaviour of metal ring when the ends of copper wire are connected to (i) AC source and (ii) DC source.

Answer:
When the ends of copper wire are connected to AC source:

  • Connect the two ends of copper wire to AC source and switch on the current.
  • The metal ring Levitates due to net force acting on it is zero according to Newton’s second law.

Reason:

  1. AC changes both its direction and magnitude in regular intervals.
  2. Due to the magnetic field produced by current in the coil, one end of the coil behaves like North pole and the other end behaves like south pole for certain time interval.
  3. For the next interval, the coil changes its polarties.
  4. Assume that the current flows in clockwise direction in the solenoid as viewed from the top. Then the upper end becomes a south pole.
  5. An upward force Is applied on the ring only when the upper side of the ring becomes north pole.
  6. It Is only possible when there exists anti clock wise current viewed from the top in the ring.
  7. After certain intervals, solenoid changes its polarities, so that the ring should also change its polarities in the same intervals.
  8. This is the reason why the metal ring is levitated.

When the ends of copper wire are connected to a DC source:

  1. Now connect the ends of copper wire to a DC source.
  2. When the current is allowed to flow through the solenoid, It behaves like bar magnet. So the flux is linked to the metal ring when the switch is on.
  3. At that instant there is a change in flux linked with the ring. Hence the ring rises up.
  4. Thereafter, there Is no change In flux linked with coil, hence it falls down.
  5. If the switch is off, the metal ring again lifts up and falls down. In this case also there is change
    in flux linked with ring when the switch is off.

Important Question

TS 10th Class Physical Science Important Questions Chapter 10 Electromagnetism
1 Mark Questions

Question 1.
What is electromagnetic induction?

Answer:
Mechanical energy can be converted Into electrical energy by moving a magnet inside a coil.

Question 2.
Mention two uses of solenoid.

Answer:
It is used in electric bells, fans and motors.

Question 3.
Mention applications of electromagnetic Induction.

Answer:
It is used in devices which convert mechanical energy Into electrical energy.

Question 4.
What is Maxwells right-hand screw rule?

Answer:
The direction of current is the direction in which the tip of the screw advances and direction of ration of the screw gives the direction of magnetic lines of force.

Question 5.
Where are the electromagnets used?

Answer:
In electric generators and televisions.

Question 6.
What is electromagnet?

Answer:
When current carrying conductor is wind over a magnetic material like soft Iron it gets magnetized.

Question 7.
What are different types of power stations?

Answer:
Electrical energy is produced in different power stations from mechanical energy of water, heat energy, and nuclear energy.

Question 8.
What is the frequency of the AC. supplied in your house?

Answer:
It is approximately 50 Hz.

Question 9.
What type of current Is generated in electric power station?

Answer:
Alternating current.

Question 10.
Which instruments detect small currents?

Answer:
Galvanometer.

Question 11.
What is a transformer?

Answer:
It Is a device which increases or decreases the voltage.

Question 12.
What is the relation between tesla and ampere meter?

Answer:
1 Tesla = 1 NA-1 m-1.

Question 13.
If a copper rod carries a direct current, then where will the magnetic field in the conductor?

Answer:
It will be both inside and outside the rod.

Question 14.
Who proposed that a magnetic field is present in a current-carrying wire?

Answer:
Hans Christian Oersted proved that electricity and magnetism were related phenomena and also proved that a current-carrying wire possesses magnetic field.

Question 15.
When a magnetic compass is kept near a current-carrying wire, what happens? Why?

Answer:
When a magnetic compass is kept under a current-carrying wire the needle of compass deflects. This deflection is due to the magnetic field, possessed by current-carrying wire.

Question 16.
How is a magnetic field characterized?

Answer:
The magnetic field exists in the region surrounding a bar magnet and is characterized by Its strength and direction.

Question 17.
What is the direction of magnetic field lines?

Answer:
Magnetic field lines start from North pole of bar magnet and end at South pole. These are Imaginary lines.

Question 18.
What is the use of magnetic field lines?

Answer:
Magnetic field lines help us
To understand the nature of this field.
The tangent drawn to the field line at a point gives the direction of the field.

Question 19.
Why the magnetic field drawn is non-unIform?

Answer:
The magnetic field drawn with the help of magnetic field lines is non-uniform because the strength and direction both change from point to point.

Question 20.
When will be the magnetic field uniform?

Answer:
The magnetic field will be uniform if both strength and direction are constant throughout the field.

Question 21.
Define magnetic flux. Mention its S.I. units.

Answer:
The number of magnetic lines passing through the plane of area ‘A’ perpendicular to the field is called magnetic flux, denoted by 4 the S.J. unit of magnetic flux is weber.

Question 22.
What Is magnetic flux density?

Answer:
The magnetic flux through unit area, which Is perpendicular to the magnetic field is known as magnetic flux density (B) or magnetic field induction.
B = Φ/A The unit of (B) is "Weber /m2 or "Tesla.

Question 23.
Write the formula for magnetic flux passing through an area A., with an angle q.

Answer:
The flux through the plane of area A, with an angle θ, is given by Φ = BA cos θ.

Question 24.
What Is the flux through the plane taken parallel to the field?

Answer:
The flux passes parallel to the field. Then θ = 0°
∴ Φ =BA cosθ = BA cos 00⇒ Φ = BA

Question 25.
What do you conclude about magnetic field lines after conducting experiment with solenoids?

Answer:
Magnetic field lines are closed loops.

Question 26.
Calculate the force experienced by a charge moving in a magnetic field and perpendicular to the field?

Answer:
Let the charge q move with a velocity ‘y’ perpendicular to the magnetic field ‘B’. The value of magnetic force on the moving charge can be found experimentally as
F = qvB.

Question 27.
What is the magnetic force on moving charge when there is an angle q between the directions of field ‘B’ and velocity ‘V’?

Answer:
When there is an angle between direction of field and velocity, the magnetic force experienced by charge is given by
F = qvB sin θ

Question 28.
What is the magnetic force on the charge moving parallel to a magnetic field?

Answer:
When charge moves parallel to the magnetic field the value of O becomes 0.
F = qvB sin θ = qvB sin 0° = O
Thus the charge experiences no force.

Question 29.
What is the direction of magnetic force acting on a moving charge?

Answer:
Keep your right-hand fingers along the direction of velocity F of moving charge and next curl your fingers towards the direction of magnetic field, then the thumb gives the H direction of magnetic force.


Question 30.
What is the direction of force acting on a negative charge moving in a field?

Answer:
First find the direction of force acting on a positive charge moving In a field, using right-hand rule, and then reverse its direction. This new direction is the direction of force acting on the negative charge.

Question 31.
Write the Faraday’s law of electromagnetic induction.

Answer:
Whenever there is a continuous change of magnetic flux linked with a closed coil, a current is generated in the coil.

Question 32.
Write Lenzs law.

Answer:
Lenz’s law states that "the Induced current will appear in such a direction that it opposes the changes in the flux In the coil".

Question 33.
How did a metal detector work?

Answer:
During security check, people are made to walk through a large upright coil of wire which produces a weak A.C. magnetic field. If we are carrying any significant quantities of Iron, the magnetic flux linked with the large coil changes and the Induced current generated in coil triggers an alarm.

Question 34.
What Is the principle of generator? (AS 1)

Answer:
Generator works on the principle of electromagnetic induction. It converts mechanical energy into electrical energy.

Question 35.
What Is the difference between AC generator and DC generator?

Answer:
In an AC generator, the ends of coil are connected to two slip rings.
In a DC generator, the ends of coil are connected to two half-slip rings.

Question 36.
What are the characteristics of AC current?

Answer:
AC current changes the direction of charge flow periodically.
AC possesses certain frequency.

Question 37.
How would you demonstrate the presence of an induced emf in the absence of a Galvanometer?

Answer:
Replace the galvanometer by a small bulb, such as the one we find in a small torch light. The relative motion between the coil and magnet will cause the bulb to glow and thus demonstrate the presence of an induced current.

Question 38.
On what factors the magnetic induction at the center of the coil depends?

Answer:
It depends on current, number of turns, and radius of the coil.

Question 39.
Name the Instrument on which Ampere’s law Is used.

Answer:
Tangent gaivanometer.

Question 40.
Which is more dangerous AC or DC?

Answer:
AC is more dangerous.

Question 41.
In DC motor what type of magnetic field is produced by concave cylindrical magnets?

Answer:
Radial

Question 42.
What Is the use of galvanoscope?

Answer:
It detects the flow of current. It acts as current detector.

Question 43.
State two serious hazards of electricity.

Answer:
If a person touches the live wire he gets severe shock which may prove fatal.
Short-circuIting can cause a spark which may lead to fire In a building.

Question 44.
Why earthing of electrical appliances is recommended?

Answer:
To protect the user from any accidental electrical shock caused due to leakage of current.

Question 45.
Why Is a spark produced at the place of short circuit? Why Is the spark is of white colour?

Answer:
The resistance of circuit decreases and a sudden flow of large current heats up the live wire and vaporizes the metal. This causes spark. The metal of wire becomes white hot and naturally emits white light.

Question 46.
Is the magnetic field formed ¡n a solenoid uniform or non-uniform?

Answer:
It is non-uniform.

Question 47.
Is an electromagnetic field a vector or a scalar?

Answer:
It is a vector quantity.

Question 48.
In an electric bell which is attracted towards electromagnet?

Answer:
Armature.

Question 49.
If the strength of the electric current increases, does the magnetic field Increase or decrease?

Answer:
The magnetic field also increases.

Question 50.
If the current ¡n the coil Is in an anti-clockwise direction then what would be the face of the coil?

Answer:
It behaves as north pole.

Question 51.
If the current In the coil is in clockwise direction then what would be the face of the coil?

Answer:
It behaves as south pole.

Question 52.
What Is the shape of magnetic lines due to straight current-carrying conductor?

Answer:
They are concentric circles.

Question 53.
State two ways by which speed or rotation of rotation of electric motor can be Increased.

Answer:
By Increasing strength of the current,
By increasing number of turns in the coil.

Question 54.
What happens If an Iron piece is dropped between two poles of strong magnet?

Answer:
Eddy current Is produced in it. These eddy currents oppose the motion of the piece of Iron. So It falls as it is moving through a viscous iiquid.

Question 55.
In what form the energy ¡n a current-carrying coil is stored?

Answer:
It is stored In the form of magnetic field.

Question 56.
What is Solenoid?

Answer:
A solenoid is a long wire wound In a close-packed helix.

Question 57.
What Is the pattern of field lines Inside a solenoid around when current-carrying solenoid?

Answer:
Parallel to each other.

Question 58.
List any two properties of magnetic field lines.

Answer:
Inside the magnet they start from south pole and end at north pole whereas outside the magnet they start at north pole and end at south pole.
Two magnetic lines of force never intersect each other.

Question 59.
Why does the picture appear distorted when the bar magnet is brought close to the screen of a television?

Answer:
Picture on a television screen is due to motion of the electrons reaching the screen. These electrons are affected by magnetic field of bar magnet.

Question 60.
What is meant by electromagnetic induction?

Answer:
Whenever there Is continuous change of magnetic flux linked with a closed coil, current is generated in the coil Is called electromagnetic induction.

Question 61.
What is the use of slip ring In AC motor?

Answer:
Uses of slip rings: Slip rings are used to change the direction of current in the coil continuously.

Question 62.
Correct the diagram according to Lenz law and draw It again.

Answer:

2 Marks Questions

Question 1.
How can we find the direction of magnetic field due to straight wire carrying Current?

Answer:
The direction of magnetic field due to straight wire carrying current can be determined by right-hand thumb rule.


If you grab the current-carrying wire with your right hand, In such a way that thumb is In the direction of current, then the curled fingers show the direction of magnetic field.

Question 2.
How can we find the direction of magnetic field due to coil or solenoid, carrying current?

Answer:
The direction of the field due to coil or solenoid carrying B current can be determined by using right-hand rule. When you curl your right-hand fingers in the N direction of current, thumb gives the direction of magnetic field."


Question 3.
Explain the working of induction stove.

Answer:
An induction stove works on the principle of electromagnetic Induction.
A metal coil is kept Just beneath the cooking surface. ¡t carries alternating current (AC). So that AC produces an alternating magnetic field.
When you keep a metal pan with water on it, the varying magnetic field beneath it crosses the bottom surface of the pan and an EMF is Induced in it.
Since pan has a finite resistance, the flow of Induced current In It produces heat in it and this heat Is conducted to the water.
That’s why we call this stove as Induction stove.

Question 4.
How does the tape of a tape recorder reproduce voice?

Answer:
The tape recorder which we use to listen songs or record voices works on the principle of electromagnetic induction.
It consists of piece of plastic tape coated with iron oxide and is magnetised more in some parts than in others.
When the tape is moved past as a small coil of wire (head of the tape recorder), the magnetic field produced by the tape changes, which leads to generation of current in the small coil of wire.
This small current will be converted into sound by speaker of varying strengths.
Thus the voice will be reproduced from the tape of a tape recorder.

Question 5.
What is the direction of magnetic force when velocity of charge is perpendicular to the magnetic field?
(Or)
State Right-hand rule with a labelled diagram.

Answer:
If the forefinger points towards the direction of velocity of charge or current, middle finger points to the direction of the field (B), then thumb gives direction of force when three fingers are stretched in sud a way that they are perpendicular to each other.


This rule is applicable to positive charges.

Question 6.
Current is flowing in a wire as shown In figure. Consider a square frame near to It. If suddenly current is stopped in wire then what is the direction of induced current In square frame?

Answer:
According to right-hand thumb rule, the magnetic field is into the plane of the book. Suddenly current is stopped means magnetic field is decreasing. According to Lenz’s law to oppose it in the square frame the induced current generates in clockwise direction ( S-Pole) and magnetic field is Into the paper.



Question 7.
Which energy we get from an electric motor? Write two daily life applications of the electric motor?

Answer:
Mechanical energy.
Applications of electric motor:
Grinders
Fans
Pump motors
Refrigerators

Question 8.
Take a long cylindrical copper tube. Hold It so that It Is perpendicular to the horizontal. A stone and a bar magnet are left freely, so that bar magnet passes through the tube and stone outside the tube. Which one reaches the earth first? Guess why It Is so? Give proper reasons.

Answer:
Stone will reach the ground quickly.
Because when magnet moves through the copper cylinder, assume the copper cylinder Is a combination of copper ring
When the magnet moves through the rIngs, current is in the rings.
It opposes the movement of the magnet according to Lenz’s law.


Question 9.
Why do not two Magnetic lines of force Intersect each other?

Answer:
The tangent at any point on a magnetic field line gives the direction of Magnetic field at that point. 1f two magnetic field lines across each other than at that point of intersection, there will be two tangents. Hence there will be two directions of the magnetic field at the point of intersection. This Is not possible. Hence no two magnetic field lines can intersect each other Let r be the radius of the circular path. We know that centripetal force = mv2/r

4 Marks Questions

Question 1.
A coil is hung as shown in the figure. A bar magnet with north pole facing the coil Is moved perpendicularly.
a) How does the magnetic flux passing through the coil change?
b) State the direction of the flow of the current induced in the coil, keeping the direction of bar magnet In views.
c) Draw the diagrams showing the magnetic field formed due to bar magnet at the surface of the coil and the magnetic field formed due to Induced current.
d) Explain the reason for induced current.

Answer:
a) A bar magnet with north pole facing the coil is moved perpendicularly, the magnetic flux is increases when passing through the coil.
b) The direction of the flow of the current induced in the coil, keeping the direction of bar magnet is anti-clockwise due to north pole.

c) Φ = 0
Plane of coil Is parallel to ‘B’
b) Electromagnetic induction is the reason for induced current.

Question 2.
Conductor of length moves perpendicular to its length with the speed V. Length of the conductor is perpendicular to the magnetic field of the conductor. Let us assume that electrons could move freely in the conductor and the charge of an electron is ‘e’.
a) What is the magnetic force acting on electron in the conductor.
b) In which direction does the above force act.
c) What effect does this force have on motion of electrons.

Answer:
a) Magnetic field acting on the electron inside the conductor is = Fπ = e(V ×B) = BeV
This field acts from P to Q.
b) Consider in the field P and Q are ends of a conductor. Q acts as negative end and ‘P’ acts as positive end then flow passes from P to Q means downwards.
c) The force on electrons shows an effect creates a potential difference at the ends of the roads.
∴ BeV=eE ⇒ E=BV


Question 3.
How can we apply conservation of energy for electromagnetic induction I?

Answer:
We know when a bar magnet is pushed towards a coil with its north pole facing the coil an induced current is set up in the coil.
The north of the pole magnet induces north polarity in the coil, as a result there will be a repulsive force between coil and magnet.
Hence we need to do work to overcome this force.

This work done on the magnet Is converted into electrical energy in the coil, as a result, an induced current is produced In an anticlockwise direction with respect to north pole of the bar magnet.
In this way, conservation energy takes place in electromagnetic induction.

Question 4.
Draw the diagram of magnetic field lines when current passes through the solenoid and label the parts.

Answer:

Question 5.
Draw the magnetic field lines to form around the bar magnet.

Answer:

Question 6.
A coil of Insulated copper wire Is connected to a galvanometer. What will happen If a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary Inside the coil?

Answer:

i) When a bar magnet is pushed into the coil, Induced current flows through the coil due to the phenomenon of electromagnetic induction. This Induced current is indicated by the deflection of the needle of the galvanometer as shown in figure (a).
ii) When a bar magnet is withdrawn from inside the coil, again induced current flows through the coil due to the phenomenon of electromagnetic induction. In this case, the direction of induced current Is opposite to the direction of the current In case (i) as shown In figure (b).
iii) When the bar magnet is held stationary inside the coil, there is no change in magnetic field around the coil. Hence, no Induced current flows through the coil. Therefore, galvanometer shows no deflection as shown in figure (c).

Question 7.
Derive Faraday’s law of Induction from law of conservation of energy.

Answer:
Let us arrange an apparatus as shown In figure.
It consists of a pair of parallel bare conductors which are spaced ‘I’ meters apart in uniform magnetic field of R.
We can hold another bare conductor in such a way that it is in contact with the two parallel wires.
A galvanometer is connected to the ends of parallel conductors to complete an electric circuit.
Now if the cross wire placed across parallel conductors is moved to the left, galvanometer needle will deflect in one direction.
If the cross wire is moved to the right, its needle deflects In a drectlon opposite to the previous deflection.
A current will set up ifl the circuit only when there is an EMF in the circuit. Let this EMF be ε.
According to principle of conservation of energy this electric energy must come from the work that we have done in moving the cross wire.
If we ignore friction, the work done by this applied force = Fs (where is the distance moved by cross conductor)
The force applied on the cross wire by the field B is F = BIl → (1)
The work done by us in moving the cross wire converts into electrical energy. So the work done is given by
W = FS
Substitute (1) W = FS= BIls → (2)
∆Φ = Bls → (3)
From (2) and (3)
W = (∆Φ)I
Let us divide both sides by ∆t
W∆t=I ∆Φ/∆t → (4)
ε = ∆Φ/∆t
Electric power, P = εI → (5)
Electric power P = I ∆Φ/∆t

Divide (2) by ∆t
W/∆t=Fs/∆t=BIls/∆t ...... (6)
Here s∆t gives the speed of the cross wire, let it be v.
Electric power
P= W/∆t=Fv = BI/v ......... (7)
Power is also given as force times velocity. From (5) and (7), εI = BI/v, ε = B/v

Question 8.
Draw a neat diagram of electric motor. Name the parts.

Answer:

Question 9.
Answer the following questions by observing diagram.
(i) Which device function of working does the figure gives?

Answer:
Electro motor
(ii) What is the angle made by AB and CD with magnetic field?

Answer:
AB and CD are perpendicular to the magnetic field. (90 with the magnetic field)

(iii) What are the directions of magnetic forces on sids AB and CD?

Answer:
The force on AB acts perpendicular to the magnetic lines of force and into the page.
The force on CD acts perpendicular to the magnetic lines of force and out of the page.
(iv) What is the net force acting on the rectangular coil?

Answer:
The net force is Zero.
Solved Problems

Question 1.
A charged particle ‘q’ is moving with a speed ‘y’ perpendicular to the magnetic field of Induction B. Find the radius of the path and the time period of the particle.

Solution:
Let us assume that the field is directed into the page as shown in figure. Then the force experienced by the particle is F = qvB, We know that this force is always directed perpendicular to velocity. Hence the particle moves along a circular path and the magnetic force on a charged particle acts like a centripetal force.

Lets r be the radius of the circular path
We know that centripetal force = mv2/r
qvB =mv2/r
Solving this equation, we get; r = mv / Bq
Time period of the particle; T = 2πr/v
Substituting in above equation, we get T = 2πm/Bq

Question 2.
The magnetic flux inside a coil of 400 turns changes for each single turn with time as shown in figure. Determine the maximum induced emf generated In the coil. Is there any change in induced EMF from t = 0.1 second to 0.3 second?

Solution:
From the given graph, the increase in magnetic flux through one turn of coil ¡n 0.1 second is 0.00 1 Wb. According to Faraday’s law, the maximum induced emf generated in the coil is given by.
ε = N∆Φ/∆t
Substituting the values, we get 400 (0.001/0.1) = 4V
From graph, there ïs no change In magnetic flux through coil from t = 0.1 s to 0.3s.
Hence no emf is generated.


Question 3.
Find the length of the conductor which is moving with a speed of 10 m/s in the direction perpendicular to the direction of magnetic field of induction 0.8 T, if it induces an emf of 8V between the ends of the conductor.

Solution:
Given that B = 0.8T V = 10 m/s and ε = 8V
Using ε= BIv
8 = 0.8 (l) (10)
l (length of the conductor) = 8/0.8×10 = 8/8 = 1 m

Question 4.
A circular coil of radius 10 cm, 500 turns, and resistance 2W is placed with its plane perpendicular to the horizontal component of the earth‘s magnetic field. It is rotated about its vertical diameter through 180° in 0.25 sec. Estimate the magnitudes of the EMF and current induced In the coil. (Horizontal component of the earth‘s magnetic field at the place
is 3.0 x 10-5 T)

Solution:
Earth’s magnetic field ‘B’ = 3.0 x 10-5 T
Area of the coil = πr²= π x 10-2m² (radius = 1o cm = 10-1 m → r² = 10-2m²)
Initial flux through the coil
Q(initial) = BA cos θ = (3.0 x 10-5) x (π x 10-2) x cos 180° = 3π x 10-7wb
(since the plane of coil is perpendicular to the magnetic field, the angle is zero)
Final flux after rotation
Q(fial) = BA cos θ = 3.0 x 10-5 x π x 10-5 x cos 180° = - 3π x 10-7wb
Total flux = (3π x 10-7) - (3π x 10-7) = 6π x 10-7 wb
The value of estimated emf is
ε = N . ∆Φ/∆t=500 × 6π×10?7/0.25 = 3.8 × 10-3 V
The value of estimated current is
I = ε/R = 3.8×10?3/2Ω
= 1.9 × 10-3 V

Question 5.
Calculate the flux passing through an area of 50 cm2 when it is placed in a magnetic field of induction 1.5 T such that normal to the plane of the area makes an angle 60° with magnetic field.

Solution:
Φ = BA Cos
Φ = 1.5 x 50 x 10-4 x cos 60°
Φ = 75 x 10-4 x √3/2
Φ = 37.5 x 1.732 x 10-4
Φ = 64.95 x 10-4 wb

Question 6.
A proton is sent into a transverse magnetic field of Induction 2T with a speed of 3.6 x 106 m/s. What is the force experienced by it?

Solution:
Magnetic field induction B = 2T
Speed of proton y = 3.6 x 106 m/s
Charge on proton q = 1.6 x 10-19 c
Force experienced by proton: F = qvB
= 1.6 x 10-19 x 3.6x 106 x 2
∴ F = 115.2 x 10-13
F = 1.152 x 10-11 N.

Question 7.
A magnetic field Is changing at a rate of 3.5 T/s. If the coil’s loo turns has calculate the emf induced in the coil. (Area of cross-section of the coil is 150cm2)

Solution:
Area of the coil : A = 150 cm2 = 150 x 10-4m2
Rate of change of magnetic field = ∆B/∆t = 3.5 T/s
No. of turns in the coil : N = 100
Induced emf: ε =N ∆ϕ/∆t =N ∆(BA)/∆t = NA ∆B/∆t
ε = 100 x 150 x 10-4 x3.5
ε = 1.5 x 3.5 = 5.25
E = 5.25 V

Question 8.
A charged particle is moving at a speed of 3 x 104 m/s along a magnetic field of induction 3 x 10-6T. Calculate the force on the particle If Its charge is 3 mc.

Solution:
Speed of the particle : y = 3 x 104 m/s
Magnetic field induction : B = 3 x 10-6 T
Charge on the partIcle : q = 3 x 10-3 C
Forte on the particle: F = qvB sin θ = 3 x 10-3 x 3 x 104 x 3 x 10-6 x sin θ (θ = O)
F= 0 newtons.
Do You Know?
The relation between direction of induced current, magnetic field and force can also be explained by Fleming’s left-hand rule. Stretch the. left-hand thumb, middle finger and forefinger ¡n such a way that they are mutually perpendicular to each other. The forefinger indicates the direction of magnetic field, the middle finger indicates the direction of current and thumb indicates direction of force. By using the Flemings left-hand rule we can explain the working of electric motor. (Page 224)
Do You Know? (Non-Textual)
1. Hans Christian Oersted (1777 - 1851):
He Is one of the leading Scientists of 19th Century, played a crucial role in understanding electromagnetism. He gave lectures which were quite popular among the public and also learnt a lot during the tours. During one such lecture In April 1820, Oersted carried out an experiment that was never performed before. He placed a compass needle underneath a wire and then turned on electric current. The needle of the compass showed deflection.
Oersted recognized the significance of what he had Just done. Earlier, It was believed that electricity and magnetism were two different unconnected sciences. Oersted had demonstrated that they were Interconnected. Through this observation he showed that electricity and magnetism were related phenomena. Some scientists, influenced by this experiment, continued to work in the modem field of "electromagnetism".
Their research resulted In several new scientific theories and various vital Inventions like the dynamo and the electric motor. With this a new technology prospered, leading to inventions such as radio, television, and fiber optics. The unit of magnetic field strength is named Oersted in his honour. Oersted was made a foreign member of "Royal Swedish Academy of Sciences in 1822.
2. Michael Faraday:
Michael Faraday was born on 22nd September 1791 in a very poor family in London. He worked as a news paper boy at the age of 13 years. In 1821, he published his work on electromagnetic induction. Encouraged and helped by Sir Humphry Davy, Faraday began his own experiments. He was famous for inventing Dynamo to generate electricity. He introduced new scientific technology like anode, cathode, anion, cation, electrode etc.
3. George Simon Ohm (1787 - 1854):
George Simon Ohm ¡s a German Physicist and a Professor at Munich. Ohm was led to his law by an analogy between the conduction of heat and the electric current. The electric field is analogous to the temperature gradient, and the electric current is analogous to the heat flow.
4. You might have seen that, during security check, people are made to walk through a large upright coil of wire which produces a weak AC (alternating) magnetic field. If we are carrying any significant quantities of iron, the magnetic flux linked with the large coil changes and the induced current generated in coil triggers an alarm.

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