## Solutions of Magnetic Effects of Electric Current Lakhmir Singh Manjit Kaur  SAQ, LAQ, MCQ, HOTS, and VSAQ Pg No. 91 Class 10 Physics

17A. A current-carrying conductor is placed perpendicularly in a magnetic field. Name the rule which can be used to find the direction of force acting on the conductor.

→ Fleming’s left hand rule can be used to find the direction of force acting on the conductor.

17B. State two ways to increase the force on a current-carrying conductor in a magnetic field.

→ The two ways are:-
(1) by increasing the flow of current.
(2) by increasing the strength of the magnetic field.

17C. Name one device whose working depends on the force exerted on a current-carrying coil placed in a magnetic field.

→ Electric motor is the device whose working depends on the force exerted by a current carrying conductor.

18. State Fleming's left-hand rule. Explain it with the help of labelled diagrams.

→ Fleming’s left hand rule says that the centre finger shows the direction of the current, the fore finger represents the direction of the magnetic field and the thumb shows the direction of force or motion.

19. What is the principle of an electric motor? Name some of the devices in which electric motors are used.

→ The principle on which the electric motor work says that if a rectangular current carrying coil is placed in a magnetic field, a force act on it which make it to rotate continuously.
Some of the devices using electric motors are:- electric fans, washing machine, mixer, grinder, etc.

20A. In a d.c. motor, why must the current to the coil be reversed twice during each rotation ?

→ To make the coil rotate continuously in the same direction, the current to coil in most of the d.c motor reversed twice during each rotation.

20B. What device reverses the current?

→ The device which reverse the current is known as commutator.

21A. State what would happen to the direction of rotation of a motor if :
(i) the current were reversed
(ii) the magnetic field were reversed
(iii) both current and magnetic field were reversed simultaneously.

→ (i) When the current were reversed then in that case the rotation of the motor would be reversed.
(ii) When the magnetic field were reversed then in that case the rotation of the motor would reverse.
(iii) When both the current and magnetic field were reversed simultaneously then in that case the rotation of the motor remains same or unchanged.

21B. In what ways can a motor be made more powerful?

→ To make a motor more powerful it is necessary to increase the number of turns on the coil in the motor.

22A. What is an electric motor? With the help of a labelled diagram, describe the working of a simple electric motor.

→ It is device used for the conversion of electrical energy in to mechanical energy.

The working of a simple electric motor.

Initially, the coil ABCD is in the horizontal position. On pressing the switch, current enters the coil through carbon brush P and commutator half ring X. The current flows in the direction ABCD and leaves via ring Y and brush Q. The direction of magnetic field is from N pole to S pole of the magnet. According to Fleming's left-hand rule, the force on sides AB and CD is in the downward and upward directions respectively. This makes the coil ABCD move in the anticlockwise direction.

When the coil reaches vertical position, then the brushes P and Q will touch the gap between the two commutator rings and current is cut off. But the coil does not stop rotating as it has already gained momentum. When the coil goes beyond the vertical position, the side CD comes on the left side and side AB comes to the right side, and the two commutator rings change contact from one brush to the other. This reverses the direction of current in the coil, which in turn reverses the direction of forces acting on the sides AB and CD of the coil. The side CD is pushed down and side AB is pushed up. Thus, the coil rotates anticlockwise by another half rotation.

The reversing of current in the coil is repeated after every half rotation due to which the coil (and its shaft) continues to rotate as long as current from the battery is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.

22B. What are the special features of commercial electric motors?

→ The special features of commercial electric motors are:-
i. the coil is wound on a soft iron core. This increases the strength of magnetic field, which makes the motor more powerful.
ii. the coil contains a large number of turns of insulated copper wire.
iii. a powerful electromagnet is used in place of permanent magnet.

Multiple Choice Questions (MCQs)-Pg-92

23. In an electric motor, the direction of current in the coil changes once in each:
A. two rotations
B. one rotation
C. half rotation
D. one-fourth rotation

→ In an electric motor, the direction of current in the coil changes once in half rotation.

24. An electron beam enters a magnetic field at right angles to it as shown in the Figure.

The direction of force acting on the electron beam will be:
A. to the left
B. to the right
C. into the page
D. out of the page

→ The direction of force acting on the electron beam will be into the page.

25. The force experienced by a current-carrying conductor placed in a magnetic field is the largest when the angle between the conductor and the magnetic field is:
A. 45°
B. 60°
C. 90°
D. 180°

→ The force experienced by a current carrying conductor placed in a magnetic field is the largest when the angle between the conductor and the magnetic field is 90°

26. The force exerted on a current-carrying wire placed in a magnetic field is zero when the angle between the wire and the direction of magnetic field is:
A. 45°
B. 60°
C. 90°
D. 180°

→ The force exerted on a current carrying wire placed in a magnetic field is zero when the angle between the wire and the direction of magnetic field is 180°.

27. A current flows in a wire running between the S and N poles of a magnet lying horizontally as shown in Figure below:

The force on the wire due to the magnet is directed:
A. from N to S
B. from S to N
C. vertically downwards
D. vertically upwards

→ A current flows in a wire running between the S and N poles of a magnet lying horizontally then the force on the wire due to the magnet is directed vertically downward.

28. An electric motor is a device which transforms:
A. mechanical energy to electrical energy
B. heat energy to electrical energy
C. electrical energy to heat energy only
D. electrical energy to mechanical energy

→ An electric motor is a device which transform electrical energy into mechanical energy.

29. A magnetic field exerts no force on:
A. an electric charge moving perpendicular to its direction
B. an unmagnetized iron bar
C. a stationary electric charge
D. a magnet

→ A magnetic field exert force on a stationary electric charge.

30. A horizontal wire carries a current as shown in Figure below between magnetic poles N and S:
Is the direction of the force on the wire due to the magnet:
A. in the direction of the current
B. vertically downwards
C. opposite to the current direction
D. vertically upwards

→ The direction of the force on the wire due to the magnet is vertically upwards.

Questions Based on High Order Thinking Skills (HOTS)-Pg-93

31. In the simple electric motor of figure given below, the coil rotates anticlockwise as seen by the eye from the position X when current flows in the coil.

Is the current flowing clockwise or anticlockwise around the coil when viewed from above?

→ Clockwise direction is observed when seen form above. this can be observed by the Fleming’s left hand rule.

32. Which way does the wire in the diagram below tend to move?

→ The direction of the wire is to move in the upward direction by the virtue of Fleming’s left hand rule.

33. H the current in a wire is flowing in the vertically downward direction and a magnetic field is applied from west to east, what is the direction of force on the wire?

→ H the current in a wire is flowing in the vertically downward direction and a magnetic field is applied from west to east, the direction of force on the wire is due south.

34. Which way does the wire in the diagram below tend to move?

→ The movement of the wire is in the downward direction, by the virtue of Fleming’s left hand rule.

35. What is the force on a current-carrying wire that is parallel to a magnetic field? Give reason for your answer.

→ The force on a current carrying wire that is parallel to a magnetic field will be zero.
This is because the magnitude of force depends on the sin of the angle between the direction of current and the direction of magnetic field, so if the current carrying wire is held parallel to the magnetic field, the force will be zero.

36. A charged particle enters at right angles into a uniform magnetic field as shown:

What should be the nature of charge on the particle if it begins to move in a direction pointing vertically out of the page due to its interaction with the magnetic field?

→ The nature of charge on the particle if it begins to move in a direction pointing vertically out of the page due to its interaction with the magnetic field, will be positively charged.

1. Name the device which converts mechanical energy into electric energy.

→ The device which converts mechanical energy into electric energy is electric generator.

2. Out of an A.C. generator and a D.C. generator:
(a) which one uses a commutator (split rings)?
(b) which one uses slip rings?

→ (a) D.C generator uses split rings.
(b) A.C generator uses slip rings.

3. Name the phenomenon which is made use of in an electric generator.

→ The phenomenon which is made use of in an electric generator is electromagnetic induction.

4. Name the rule which gives the direction of induced current.

→ The rule which gives the direction of induced current is Fleming’s left hand rule.

5. What condition is necessary for the production of current by electromagnetic induction?

→ A relative motion between the wire and the magnet is necessary for the production of current by electromagnetic induction.

6. What type of generator is used at Power Stations?

→ A.C. generator (or Alternator) is used at Power Stations.

7. What change should be made in an a.c. generator so that it may become a d.c. generator?

→ Change should be made in an a.c. generator so that it may become a d.c. generator is :-
1) replace the slip rings of an AC generator by a commutator.

8. State whether the following statements are true or false:
(a) A generator works on the principle of electromagnetic induction.
(b) A motor works on the principle of electromagnetic induction.

→ (a) True, Electromagnetic induction is necessary for the working of a generator.
(b) False, Electromagnetic induction principle is for electric generator but not for motor.

9. What is the function of brushes in an electric generator?

→ To transfer the current from coil to load is the function of brushes in the electric generator.

10. When a wire is moved up and down in a magnetic field, a current is induced in the wire. What is this phenomenon known as?

→ When a wire is moved up and down in a magnetic field, a current is induced in the wire. this phenomenon known as Electro-magnetic induction.

11. When current is 'switched on' and 'switched off' in a coil, a current is induced in another coil kept near it.

→ When current is 'switched on' and 'switched off' in a coil, a current is induced in another coil kept near it. this phenomenon known as Electromagnetic induction.

12. What is the major difference between the simple alternator and most practical alternators?

→ The major difference between the simple alternator and most practical alternators is,
Simple alternator: Magnet fixed and coil rotates; Practical alternator: Coil fixed and magnet rotates .

13. Why are Thermal Power Stations usually located near a river?

→ To obtain water for making steam for turning turbines and for cooling spent steam to condense it back into hot water for making fresh steam Thermal Power Stations usually located near a river.

14. List three sources of magnetic fields.

→ Three sources of magnetic fields are, Permanent magnets; Electromagnets ; Conductors carrying current (such as straight wire, circular coil and solenoid carrying current) .

15. Complete the following sentence :
A generator with commutator produces..........current.

→ A generator with commutator produces direct current.

16. Two circular coils A and B are placed close to each other. If the current in coil A is changed, will some current be induced in the coil B? Give reason for your answer.

→ If the current in coil A is changed then Yes, some current will be induced in the coil B because of change in magnetic field through the coil B due to change in current in coil A. This is called electromagnetic induction.

17A. Explain the principle of an electric generator.

→ when a straight conductor is moved in a magnetic field, then current is induced in the conductor is the principle on which electric generator works.

17B. State two ways in which the current induced in the coil of a generator could be increased.

→ Two ways in which the current induced in the coil of a generator could be increased are:-
(i) by rotating the coil faster
(ii) by using a coil with a larger area.

18A. What is the difference between alternating current and direct current?

→ DC flows in one direction only while AC reverses direction after equal intervals of time is the difference between alternating current and direct current and also,
(i) DC current remains same with time in its value and direction.
(ii) AC current changes with time and changes its direction every time after a certain interval of time.

18B. What type of current is given by
(i) A dry cell, and
(ii) A Power House generator?

→ (i) A dry cell gives dc current.
(ii) A Power House generator gives ac current.

19. State and explain Fleming's right hand rule.

→ Fleming’s right hand rule gives the direction of induced current produced in a straight conductor moving in a magnetic field.
According to Fleming's right hand rule: Hold the thumb, the fore finger and the centre finger of your right-hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.

20. Name and state the rule to find the direction of:
(a) current induced in a coil due to its rotation in a magnetic field.
(b) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.

→ (a) current induced in a coil due to its rotation in a magnetic field is stated by Fleming’s right hand thumb rule.
(b) Force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it is stated by Fleming’s left hand rule.

21A. In what respect does the construction of an A.C. generator differ from that of a D.C. generator?

→ The construction of an A.C. generator differ from that of a D.C. generator are:-

21B. What normally drives the alternators in a Thermal Power Station? What fuels can be used to heat water in the boiler?

→ High pressure steam normally drives the alternators in a Thermal Power Station.
Fuels can be used to heat water in the boiler are Coal; Natural gas; Oil.

22. Draw the labelled diagram of an A.C. generator. With the help of this diagram, explain the construction and working of an A.C. generator.

Construction:
AC generator consists of a rectangular coil ABCD which can be rotated fastly between the poles N and S of a strong horseshoe-type permanent magnet M. The coil is made of a large number of turns of insulated copper wire. The two ends A and D of the coil are connected to two circular pieces of copper metal called slip rings R1 and R2. As the slip rings rotate with the coil, the two fixed pieces of carbon called brushes, B1 and B2, keep contact with them. So, the current produced in the rotating coil can be tapped out through slip rings into the carbon brushes. The outer ends of carbon brushes are connected to a galvanometer to show the flow of current in the external circuit.
Working:

Let us assume the coil ABCD, which is at the start in the horizontal position, is rotated in the anticlockwise direction. The side AB of the coil moves down and side CD moves up. By virtue of this, induced current is produced in both the sides, which flows in the direction BADC (according to Fleming's right hand rule). Thus, in the first half rotation, the current in the external circuit flows from brush B1 to B2. After half revolution, sides AB and CD will interchange their positions. So, side AB starts moving up and side CD starts moving down. As a result, direction of induced current in the coil is reversed and flows in the direction CDAB. The current in the external circuit flows from brush B2 to B1.

23A. What do you understand by the term "electromagnetic induction"? Explain with the help of a diagram.

→ The term "electromagnetic induction refers to the production of electricity from magnetism is known as electromagnetic induction.

23B. Name one device which works on the phenomenon of electromagnetic induction.

→ A device which works on the phenomenon of electromagnetic induction is Electric generator.

23C. Describe different ways to induce current in a coil of wire.

→ Current can be induced in a coil of wire through these ways:-
(i) by moving the coil relative to a fixed magnet
(ii) by keeping the coil fixed and moving a magnet relative to it.

24A. What do you understand by the terms 'direct current' and 'alternating current'?

→ In direct current (DC), the electric charge (current) only flows in one direction. Electric charge in alternating current (AC), on the other hand, changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction.

24B. Name some sources of direct current and some of alternating current.

→ Some sources of direct current and some of alternating current are:-
Dynamo, solar cell, batteries. Etc. (direct current).
Hydroelectric power plant, electric generator, etc. (ac current).

24C. State an important advantage of alternating current over direct current.

→ Changes direction periodically in ac occurs but this is not possible with dc.

24D. What is the frequency of A.C. supply in India?

→ The frequency of A.C. supply in India is 60HZ.

Multiple Choice Questions (MCQs)-Pg-103

25. A rectangular coil of copper wire is rotated in a magnetic field. The direction of induced current changes once in each:
A. two revolutions
B. half revolution
C. one revolution
D. one-fourth revolution

→ A rectangular coil of copper wire is rotated in a magnetic field the direction of induced current changes once in each half revolution.

26. The phenomenon of electromagnetic induction is:
A. the process of charging a body.
B. the process of generating magnetic field due to a current passing through a coil.
C. producing induced current in a coil due to relative motion between a magnet and the coil.
D. the process of rotating a coil of an electric motor.

→ The phenomenon of electromagnetic induction is producing induced current in a coil due to relative motion between a magnet and the coil.

27. The device used for producing electric current is called a:
A. generator
B. galvanometer
C. ammeter
D. motor

→ The device used for producing electric current is called a generator.

28. The essential difference between an AC generator and a DC generator is that:
A. AC generator has an electromagnet while a DC generator has permanent magnet.
B. DC generator will generate a higher voltage.
C. AC generator will generate a higher voltage.
D. AC generator has slip rings while the DC generator has a commutator.

→ The essential difference between an AC generator and a DC generator is that AC generator has slip rings while the DC generator has a commutator.

29. When the switch Sis closed in the figure given below, the pointer of the galvanometer moves to the right.

If S is kept closed, will the pointer:
B. stay over on the right?
C. move to the left and stay there
D. move to and fro until S is opened

→ When the switch Sis closed in the figure given below, the pointer of the galvanometer moves to the right. If S is kept closed, will the pointer return to zero.

31. The north pole of a long bar magnet was pushed slowly into a short solenoid connected to a galvanometer. The magnet was held stationary for a few seconds with the north pole in the middle of the solenoid and then withdrawn rapidly. The maximum deflection of the galvanometer was observed when the magnet was:
A. moving towards the solenoid
B. moving into solenoid
C. at rest inside the solenoid
D. moving out of the solenoid

→ The north pole of a long bar magnet was pushed slowly into a short solenoid connected to a galvanometer. The magnet was held stationary for a few seconds with the north pole in the middle of the solenoid and then withdrawn rapidly. The maximum deflection of the galvanometer was observed when the magnet was moving out of the solenoid.

32. An electric generator converts:
A. electrical energy into mechanical energy
B. mechanical energy into heat energy
C. electrical energy into chemical energy
D. mechanical energy into electrical energy.

→ An electric generator converts mechanical energy into electrical energy.

33. A d.c. generator is based on the principle of:
A. electrochemical induction
B. electromagnetic induction
C. magnetic effect of current
D. heating effect of current

→ A d.c. generator is based on the principle of electromagnetic induction.

34. An induced current is produced when a magnet is moved into a coil. The magnitude of induced current does not depend on :
A. the speed with which the magnet is moved
B. the number of turns of the coil
C. the resistivity of the wire of the coil
D. the strength of the magnet

→ The magnitude of induced current does not depend on the resistivity of the wire of the coil.

35. The frequency of direct current (d.c.) is:
A. 0 Hz
B. 50 Hz
C. 60 Hz
D. 100 Hz

→ The frequency of direct current (d.c.) is 0 Hz.

36. The frequency of alternating current (a.c.) supply in India is:
A. 0 Hz
B. 50 Hz
C. 60 Hz
D. 100 Hz

→ The frequency of alternating current (a.c.) supply in India is 50 Hz.

Questions Based on High Order Thinking Skills (HOTS)-Pg-105

37. A coil is connected to a galvanometer. When the N-pole of a magnet is pushed into the coil, the galvanometer deflected to the right. What deflection, if any, is observed when:
(a) the N-pole is removed?
(b) the s-pole is inserted?
(c) the magnet is at rest in the coil?
State three ways of increasing the deflection on the galvanometer.

→ (a) when the N-pole is removed the galvanometer is deflected to the left.
(b) the S-pole is inserted the galvanometer is deflected to the left.
(c) when the magnet is at rest in the coil No deflection in galvanometer is absorved.
three ways of increasing the deflection on the galvanometer are:-
1) Increase the number of turns in the coil.
2) Use a stronger magnet.
3) Increase the speed with which magnet is pushed into the coil (or removed).

38. When the magnet shown in the diagram below is moving towards the coil, the galvanometer gives a reading to the right.

(i) What is the name of the effect being produced by the moving magnet?
(ii) State what happens to the reading shown on the galvanometer when the magnet is moving away from the coil.
(iii) The original experiment is repeated. This time the magnet is moved towards the coil at a great speed.

→ (i) The name of the effect being produced by the moving magnet is electromagnetic induction.
(ii) When the magnet is moving away from the coil the needle of the galvanometer get deflected to left.
(iii) Large deflection to right occurs more quickly when the magnet is moved towards the coil at a great speed.
State two changes you would notice in the reading on the galvanometer.
two changes you would notice in the reading on the galvanometer are:-
1) it deflects toward left.
2) And it gets deflected towards right when the magnet is moved toward the coil at a great speed.

39.If you hold a coil of wire next to a magnet, no current will flow in the coil. What else is needed to induce a current?

→ If you hold a coil of wire next to a magnet, no current will flow in the coil. Motion of the magnet into the coil is needed to induce a current.

40. The wire in Figure below is being moved downwards through the magnetic field so as to produce induced current.

What would be the effect of:
(a) moving the wire at a higher speed?
(b) moving the wire upwards rather than downwards?
(c) using a stronger magnet?
(d) holding the wire still in the magnetic field?
(e) moving the wire parallel to the magnetic field lines?

→ (a) when the current increased
(b) moving the wire upwards rather then downward will reverse the Current.
(c) the current will be increased by using a strong magnet.
(d) by holding the wire still in the magnetic field will increase the current.
(e) when the wire is moved parallel to the magnetic field then the current is zero.

41. Two coils A and B of insulated wire are kept close to each other. Coil A is connected to a galvanometer while coil B is connected to a battery through a key. What would happen if:
(i) a current is passed through coil B by plugging the key?
(ii) the current is stopped by removing the plug from the key?