D Electricity and Magnetismchapter 17 Electromagnetism

D Electricity and MagnetismChapter 17 Electromagnetism

17Electromagnetism

New Physics at Work (Second Edition)OxfordUniversity Press 2006

D Electricity and MagnetismChapter 17 Electromagnetism

Practice 17.1 (p.118)

5(a)This statement is correct.

(b)This statement is incorrect.

(c)This statement is correct.

7(a)It could be a bar magnet or an iron bar.

(b)Flip one of the magnets so that two like poles are facing the object. If the object keeps aligned with the magnets, it is an iron bar; otherwise, it is a bar magnet.

(Or other reasonable answers.)

10(a)

(b)

(c)

(d)

11(a)X is the south pole and Y is the north pole.

(b)The compass needle points to the left.

Practice 17.2 (p.132)

6(a)

(b)

(c)

7(a)(b)

(c)(d)

(e)(f)

8(a)The magnetic field increases.

(b)The magnetic field increases.

(c)The magnetic field increases.

(d)The magnetic field remains unchanged.

9There is no magnetic field at points P and S. The direction of the magnetic field at point Q is into the paper, and that at point R is out of the paper.

10(a)Component A should be made of soft-iron.

(b)X is the south pole and Y is the north pole.

(c)If component Ais made of flexible material, it may deform because of the attraction between its two ends X and Y, which are unlike poles. Hence, it is unwise to make it from a flexible material.

11The current flows from B to A.

Practice 17.3 (p. 143)

8(a)

(b)(i)The direction of magnetic force reverses (downwards).

(ii)The direction of magnetic force reverses (downwards).

(iii)The direction of magnetic force remains unchanged.

In these cases, the magnitude of the magnetic force is unchanged.

9(a)

(b)

(c)

(d)

10There is no resultant force acting on T.

11(a)

(b)(i)

(ii)

(c)Any three of the following:

Increase the current flowing in the coil.

Increase the number of turns in the coil.

Increase the area of the coil (within the magnetic field).

Wind the coil on a soft-iron core.

Use a stronger magnet.

12Yes, the magnetic force does work on the rider. It is because the rider moves along the force direction.

13(a)Since the commutator in this motor is a full-ring, the coil is short-circuited and hence the motor does not work.

(b)Change the full-ring commutator to a split-ring commutator and the motor can work properly.

14Any three of the following:

Increase the current.

Increase the number of turns in the coil.

Increase the area of the coil (within the magnetic field).

Increase the strength of the magnetic field.

15The advantage of using an electromagnet in a motor is that it can run on alternating current.

16The differences in structure between a simple motor and a practical motor are as follows.

The coil in a practical motor has a large number of turns wound on a soft-iron armature.

The armature of a practical motor has several coils set at different angles.

The pole pieces of the magnet are made curved.

17(a)On increasing the current, by P = I2R, the power loss by the resistance of the coil increases and the heat generated may damage the wiring of the motor.

(b)If the number of turns in the coil increases, the weight of the coil increases and this may reduce the turning effect.

(Or since the resistance of the coil increases with the number of turns, the current flowing through the coil decreases and this may reduce the turning effect on the coil.)

(c)On increasing the area of the coil inside the magnetic field, more wire could be used and this could increase the weight of the coil and reduce the turning effect on the coil.

Revision exercise17

Multiple-choice (p.148)

Section A

Section B

9(HKCEE 2004 Paper II Q34)

10(HKCEE 2005 Paper II Q22)

Conventional(p.150)

Section A

1The balance reading decreases.(1A)

Consider the coils P and Q in the above figure.The current flows in the same direction along these coils.

On side T, the magnetic field acting on Y due to the current flowing along Q is towards the right. (1A)

By the Fleming's left-hand rule, there is a force pulling P towards Q.(1A)

Similarly, there is a force pulling Q towards P. The case is the same on side S.(1A)

If a large current flows along the spring, the attractive force would be large enough to pull the coils upwards. (1A)

Hence, the spring is less stretched and the reading decreases.

2(a)(i)Since the electromagnet is connected to a battery, a current always flows through the coil of the electromagnet. (1A)

Hence, the electromagnet always attracts the iron at the door and the door is always locked. (1A)

(ii)Add a resistor in series with the battery.

Andthe resistance of R2R1R3 < the coil of electromagnet.(1A)

When the switch is not pressed, the contact is open and current flows through R1, R3 and the electromagnet. The attraction between the electromagnet and the iron at the door keeps the door closed. (1A)

When the switch is pressed, the contact is closed and the equivalent resistance of R2, R3 and electromagnet is much smaller than that of R1. Hence, little current flows through the electromagnet. The attraction is much reduced and the door can be opened. (1A)

(b)Any one of the following:(1A)

Increase the number of turns of the coil of the electromagnet.

Use more cells.

It is because the above method can increase the magnetic field of the electromagnet. A stronger attractive force results. (1A)

3(a)The rod moves towards A.(1A)

(b)The rod moves towards B.(1A)

(c)The rod moves towards A.(1A)

(d)The rod moves towards A, but the motion is slower.(1A)

(e)The rod moves towards A, but the motion is faster.(1A)

4(a)Commutator is to reverse the current through the coil.(1A)

(b)The direction of the coil’s rotation is anticlockwise.(1A)

(c)Reverse the polarities of the battery(1A)

so that the direction of current through the coil is reversed.(1A)

Reverse the poles of the magnet(1A)

so that the direction of the magnetic field is reversed.(1A)

The two ways above must be carried out separately, or the effect will be cancelled out by each other.

5(a)(i)Core: iron core(1A)

Wire: thick wire of 0.4 m long(1A)

Cell: 9 V(1A)

(ii)

(Solenoid wound on a core and connected to a cell.)(1A)

(Correct labels.)(1A)

(iii)The magnetic field of a solenoid increases with the current in the solenoid. To obtain the largest current, by V = IR, the voltage of the cell should be the largest and the resistance of the wire should be the smallest. (1A)

Hence, a 9-V cell should be used.

Since the resistance of the wire decreases with increasing diameter, (1A)

a thick wire (0.4-m thick wire) should be used.

A soft-iron core is used because soft-iron can enhance the magnetic field of a solenoid and its magnetism can be switched on and off easily. (1A)

(For effective communication.)(1C)

(b)(i)The device is a switch.(1A)

(ii)Yes, we can turn on and off the electromagnet without a switch.(1A)

When both ends of the wire touch the terminals of the cell, the electromagnet is on. When one end of the wire does not touch the cell terminal, the electromagnet is off. (1A)

6(a)When a current flows through the coil, the coil attracts the soft-iron block and the coil moves to the right. (1A)

The larger the current through the coil, the more the coil moves.(1A)

The pointer attached on the coil can show the size of the current after calibration.(1A)

(b)(i)The ammeter does not work anymore.
(1A)

It is because the copper block is not attracted by a current-carrying coil.(1A)

(ii)The ammeter does not work well / The ammeter does not work anymore.(1A)

The steel block does not demagnetize easily. This makes the ammeter inaccurate. (1A)

(c)Increase the number of turns of the coil. (1A)

Insert a soft-iron core inside the coil.(1A)

(d)Replace the soft-iron block with a magnet.(1A)

One of the poles of the magnet should point to the left and the other to the right.(1A)

When the current through the coil flows in one direction, the coil is attracted to the left; when the current reverses, the coil is repelled right. (1A)

7(a)(Soft) iron (1A)

(b)Soft-iron can be easily magnetized(1A)

and it loses magnetism easily.(1A)

8(a)(i)

(1A)

(ii)Earth wire is not needed in the flex of the food mixer.(1A)

(b)

(4 1A)

Section B

9(a)When the coil is vertical, the ends of the coil do not touch the wires connecting the battery.(1A)

Hence, no current flows along the coil and the coil does not rotate.(1A)

To set the coil to rotate, a push should be given to the coil.(1A)

(Or the coil should be set in horizontal position initially.1A)

(b)

As shown in the above figure, the split-ring S touches carbon brush R and split-ring U touches carbon brush T. Split-rings S and U are moving in clockwise direction. (1A)

When the coil is vertical, the carbon brushes do not touch the split-rings and the coil continues to move due to its inertia. (1A)

Then the split-ring U touches carbon brush R and split-ring S touches carbon brush T, as shown in the following figure. The current flowing in the coil changes direction. By Fleming's left-hand rule, the coil continues to rotate in clockwise direction. (1A)

Hence the motor can rotate continuously with split-rings and brushes.

(For effective communication)(1C)

(c)Any one of the following:(1A)

A practical motor has coils consisting of a large number of turns.

The coils of a practical motor are wound on a soft-iron core.

Practical motor has coil sets at different directions.

The poles of the magnet of a practical motor are curved.

(d)Replace the permanent magnet with an electromagnet. (1A)

In addition, the current flowing in the electromagnet and the coil should come from the same source. (1A)

It is because when the current changes direction, the poles of the electromagnet and the direction of the current in the coil reverse at the same time. The coil can then rotate smoothly. (1A)

10(a)The coil initially rotates in a clockwise direction.(1A)

It then stops in the vertical position.(1A)

(b)Force acts on a current-carrying wire in a magnetic field.(1A)

The coil turns and shoots through the vertical position by inertia.(1A)

The turning force reverses direction when turning through the vertical position.(1A)

The coil finally stops in a position where the turning force is zero.(1A)

(For effective communication.)(1C)

(c)(i)The coil rotates in a clockwise direction.(1A)

(ii)The coil rotates in ananticlockwise direction.(1A)

(iii)The coil rotates in a clockwise direction.(1A)

(d)Commutator can be added to make the rotation continuous.(1A)

11(a)At normal situation, the current through the electromagnet is small. The magnetic force produced is not large enough to attract the iron strip. (1A)

When the current exceeds the safety limit (a fault develops), the magnetic force produced by the electromagnet is large enough to attract the iron strip. (1A)

As a result, the circuit becomes open and no current passes through the circuit.(1A)

(For effective communication.)(1C)

(b)Statement 1 is correct.(1A)

Copper is not a magnetic material, so it cannot be attracted by a magnet.(1A)

Statement 2 is incorrect.(1A)

Reversing the polarity in the electromagnet (due to the a.c.) does not affect the attraction to the iron strip. (1A)

12(a)When the bell push is closed, a current flows through the circuit.(1A)

The electromagnet becomes magnetized and pulls the hammer across to hit the gong.(1A)

The circuit is thus open and no current flows through the electromagnet. Its magnetic force disappears and the hammer springs back to the original position. (1A)

The process is repeated while the switch is pressed.(1A)

(For effective communication.)(1C)

(b)The statement is incorrect.(1A)

The frequency of the sound produced depends on the properties of the gong and the hammer, not the frequency of the power supply. (1A)

13(a)(i)The magnetic fieldproduced by the current in wire(1A)

and the magnetic field due to the permanent magnet,(1A)

interfere to produce a force, as stated in Fleming’s left-hand rule.(1A)

(ii)The electrical energy in the coil(1A)

changes to kinetic energy of the diaphragm (then to sound energy).(1A)

(b)By the vibrationsof diaphragm, it compresses and expands surrounding air.(1A)

This produces a longitudinal wave.(1A)

(c)(i)Given the same force,(1A)

larger mass leads to less acceleration, by F = ma.(1A)

(ii)The sound produced is distorted and the amplitude is smaller (lower pitch).(1A)

14(HKCEE 2005 Paper I Q10b)

Physics in articles (p. 154)

(a)‘Powerful electromagnets’ are devices that can generate a large magnetic field from a current.(1A)

(b)For a normal coil, voltage has to be applied across the coil to produce a magnetic field and this consumes energy (P = I2R). (1A)

Since a current can continue to flow inside a superconductor without applying voltage, energy can be saved if superconducting wire is used in an electromagnet. (1A)

(c)Any one of the following: (1A)

Superconductivity of metals appears only at an extremely low temperature and practical electromagnets are usually used in places where such a low temperature is hard to maintain.

Wires connecting the electromagnet give out heat when current flows through them. This affects the temperature of the electromagnet. Thus it is difficult to use superconducting wires in practical electromagnets.

New Physics at Work (Second Edition)OxfordUniversity Press 2006