Past papers- 4. magnetism and electricity

Question 1

Define potential difference (p.d.). and e.mf.

State the equation which defines electromotive force (e.m.f.) E

State what is meant by electric current.

Answer 1

work done in passing charge through / across a component
work done per unit charge

energy supplied by a source in driving charge around a complete circuit
E = W / Q

Q / t or (rate of) flow of (electric) charge / electrons

Question 2

Explain what happens in the circuit you have drawn in (c)(i) when the temperature of the thermistor is increased.

Answer 2

resistance (of thermistor) decreases (when temperature increases)

resistance of circuit decreases OR greater current (in lamp so brightness of lamp increases) OR greater p.d. across lamp
(so brightness of lamp increases)

Question 3

how to determine the direction of movement of coil

Answer 3

(induced) current, (magnetic) field direction, motion of (wire) at right angles to each other

magnetic field from left to right / N to S AND current is from A to B / positive to negative

Question 4

State and explain what happens to the coil as it reaches the vertical position. ( coil bw two magnets)

Answer 4

(at vertical) the coil stops OR (at vertical) the coil overshoots and comes back OR the coil vibrates (about the vertical)

any one from:
* (as the coil approaches vertical) the turning effect decreases
* (at vertical) the turning effect is zero
* (past vertical) the turning effect reverses / changes direction

Question 5

To operate as a motor, a split‑ring commutator and brushes are added to the parts.

Explain the effects of the split‑ring commutator and the brushes on the action of the motor

Answer 5

reverses the current

any two from:
* (brushes) ensure current is maintained
* coil rotates continuously / continues to move in the same direction
* (allows current to change direction) without wires getting tangled
* (reverses the current) every half turn / 180 degrees / OR (reverses the current) when the coil is vertical / at right angles
to the magnetic field

Question 6

Describe what is meant by a potential divider.

Answer 6

(potential divider) splits / shares / divides the e.m.f. / voltage / potential difference / p.d. (of a power source / in a circuit)

(e.m.f. is) split between (two) resistors / components (connected in series to power source)

(potential divider shares e.m.f.) in proportion to the resistances (of the resistors / components)

Question 7

Explain the principle of operation of a simple iron-cored transformer.

Answer 7

(Primary voltage causes) an alternating current in (primary coil)

(current in primary generates) changing magnetic field

iron core concentrates (magnetic) field OR iron core transfers (magnetic) field (to secondary coil)

secondary coil is in alternating / changing (magnetic) field OR secondary coil cuts (magnetic) field

e.m.f. induced (in secondary coil)

Question 8

State what is meant by a magnetic field.
Define the direction of a magnetic field

Describe what is meant by an electric field.
State what is meant by the direction of an electric field

Answer 8

1. region in which a (magnetic) pole experiences a force
   in the direction of the force on the N pole
2. region in which an electric charge experiences a force
   direction of force on a positive charge

Question 9

Explain how rotating the coil in Fig. 6.2 continuously causes the galvanometer needle to show an alternating current.

Answer 9

(as coil rotates) it cuts (magnetic) field between the magnets

This induces an e.m.f. / voltage / p.d. (in the coil)

This produces a current in the (coil transferred to the) galvanometer (via the slip rings and carbon brushes)

Direction of current flow changes with each 180 degree rotation of coil

Question 10

1. Explain how the earth wire protects the user.
2. Explain how the fuse protects the circuit.

Answer 10

if the tap becomes live or if the (live) cable touches the (metal) tap
there is a current to earth / in the earth wire (which blows the fuse)

the current (in earth wire) is large and fuse melts / blows / stops current / breaks circuit

Question 11

(b) The magnet is then held at the top of the copper tube and released. As it falls through the copper tube, an electric current is generated in the copper.

(i) Explain why there is a current in the copper.
(ii) The current in the copper produces a magnetic field of its own in the tube. The magnet falls much more slowly in the copper tube than in the plastic tube. Explain why the magnet falls more slowly in the copper tube.

Answer 11

magnetic field / magnetic field lines cut the copper / tube / it (or vv.)
electromagnetic induction occurs or e.m.f. induced

(upwards / opposing) force on magnet
force / magnetic field / e.m.f. / current opposes the change (producing it) / opposes motion or force on magnet due to
magnetic field caused by current in tube

Question 11

Suggest a practical use for light-dependent resistor

Describe a practical use for a thermistor.

Answer 11

switch on street lights (at night) / turn on security light (at night)

requiring temperature control or depending on temperature- fire alarms, to keep computers cool (by operating fan), in incubators, electronic thermometer, electronic thermostat in kettle / car engine

Question 12

Fig. 9.1 shows a magnet on the end of a spring and a coil of wire connected to a sensitive
centre-zero galvanometer. The magnet can move freely through the coil.

The magnet is pulled down and released.
Describe and explain what happens to the needle of the sensitive galvanometer.

Answer 12

any four from:

 needle oscillates (as magnet moves up and down)
 coil cuts magnetic field / magnetic field changes (as magnet moves)
 changing (magnetic) field induces voltage/current
 induced voltage/current opposes the motion/change causing it
 force, magnetic field and induced current are mutually perpendicular

Question 13

The electric starter motor in a car is switched on and off using a relay. The relay consists of a plastic case and two flexible springy strips, X and Y, which are made of
soft iron. These iron strips act as the switch when a circuit is connected between the terminals W and Z.

Switch S is now closed.
Explain what happens to the springy iron strips X and Y.

Answer 13

X and Y / they become magnetised or they / strips have poles

strips in the centre have opposite (magnetic) poles or X and Y attract

X and Y touch / close switch / activate relay / complete circuit

Question 14

Explain why copper wires with a large cross-sectional area are used for this circuit.

Answer 14

smaller resistance
less thermal energy produced or wires do not melt or large current mentioned

Question 15

State one other similarity between e.m.f. and p.d

State one other difference between e.m.f. and p.d

Answer 15

both relate to energy per unit charge

e.m.f. applies to the whole circuit / source or p.d. to one (or more) component or energy conversion to electrical for e.m.f. or from electrical for p.d.

Question 16

State what is observed on the ammeter when the wire XY is moved
(i) in the opposite direction to part (b)
(ii) in the same direction as part (b) but at a greater speed .

Answer 16

opposite current (direction) / opposite deflection (on ammeter)

greater current / deflection

Question 17

The student turns the handle more quickly in an a.c generator.

State two ways in which the e.m.f. between terminals X and Y changes.

Answer 17

(amplitude / maximum e.m.f.) increases
(e.m.f.) changes direction more often or greater frequency

Question 18

A few hours later, the student notices that the reading on the voltmeter ( thermistor in series) is greater.
Explain what can be deduced from this observation.

Answer 18

larger proportion of the e.m.f. (across thermistor) or smaller voltage across 800Ω

temperature (of thermistor) is smaller / has decreased

resistance of thermistor / circuit is large(r)

Question 19

(c) Describe a method of demagnetising a bar magnet.

Answer 19

heat to high temperature
OR hammer with magnet lying (magnetically ) E – W
OR place in coil / solenoid with a.c. and withdraw OR reduce current to 0

Question 20

The coil is rotated through 180° from the position shown. By considering the forces on the coil, explain how the split-ring commutator enables the motor to turn continuously.

Answer 20

current in coil reverses OR changes direction
force(s) (on wires in new positions) still up on L OR down on R

Question 21

At low temperature, component K has a much greater resistance than resistor R.

At high temperature, component K has a much smaller resistance than resistor R.

State and explain the effect on the lamp when the temperature changes from very low to very
high.

Answer 21

incorrect

(much) greater current in lamp OR lamp activated / lights / glows / gets brighter

resistance of thermistor / component / K reduced (compared to value at (very) low temperature)

voltage / p.d. of point X / across R increases

(larger) current in lamp

Question 22

Another piece of wire is made of the same metal as PQ. The length of the new piece of wire is twice the length of PQ. The diameter of the new piece of wire is twice the
diameter of PQ. Calculate the resistance of the new piece of wire. 2.2 Ω

Answer 22

1.1Ω

resistance proportional to length (so twice length twice resistance)

resistance inversely proportional to area (so twice diameter decreases resistance by factor of 4)

Question 23

Describe, in terms of particles, how the rod becomes negatively charged when rubbed with the cloth.

A light, conducting ball is at rest on a metal table. When the rod is brought close to the ball, as shown in Fig. 7.1, the ball jumps up towards the rod.

The ball touches the rod and falls back down to the table.
Explain why this happens

Answer 23

electrons mentioned
negative charges / electrons move from cloth or move to rod

electrons / negative charge(s) repelled to earth or ball charged by induction
ball positively charged
opposite charges attract

negatively charged (by rod) or ball discharges / becomes neutral
repelled by rod or pulled down by gravity / its weight

Question 24

A wire is connected from object A to earth.
State and explain any movement of charge

Answer 24

-ve charges (flow) from earth OR -ve charges flow to object B1
electrons flow to balance (excess) +ve charge on the object

Question 25

Explain the benefit of earthing the metal case.

Explain how a fuse protects a circuit.

correct fuse rating for this appliance and explain your answer

Answer 25

prevents electrocution OR metal case cannot become live OR metal case always at earth potential / voltage
if live wire touches metal case fuse melts

if current too high
fuse melts

fuse rating/value above but near (to) normal operating current/ 10 A OR fuse rating/value slightly higher (than) normal operating current /10A

Question 26

wire moves down and then .The power supply is switched off and the wire returns to its original position. The power supply is then switched on so that the current is in the opposite direction. State and explain what happens to the wire.

A split-ring commutator is an important feature of a d.c. motor. Suggest one reason why the d.c. motor cannot operate without a split-ring commutator.

Answer 26

wire moves up
(reversing direction of the current) reverses the direction of force

coil does not continue to rotate in the same direction

Question 27

The pan contains water. State and explain the effect of the quantity induced in part (b) on the temperature of the water
in the pan

Describe the magnetic effect of this alternating current.

Answer 27

(temperature) increased
current in base of pan due to induced e.m.f
thermal energy (produced in base of pan)

magnetic field (produced)
(magnetic field / magnetic flux / magnetic effect / magnetism) (it) alternates / changes direction / reverses

Question 28

why is iron a suitable material for the core of an electromagnet.

electromagnet inside a second solenoid Y.

(i) Describe and explain what happens in solenoid Y when solenoid X is connected to an alternating current (a.c.) power supply.

(ii) A switch and a lamp are connected in series with the terminals of solenoid Y. When the switch is closed, the lamp lights up at normal brightness. Describe and explain what happens to the current in solenoid X when the switch is
closed.

Answer 28

magnetic material OR easily magnetised
temporary magnetic material OR easily demagnetised

7(b)(i) alternating / changing / varying magnetic field (produced by X)
(electromagnetic) induction in Y
(alternating) electromotive force (e.m.f.) between terminals of Y / in Y

7(b)(ii) current in X increases
to supply the power used in Y / the lamp

Question 29

Explain why it is necessary to use thick wires to carry this current.

Answer 29

thick wires have a smaller resistance
less thermal energy generated in wires
more efficient OR less risk of fire / insulation melting

Question 30

Explain the function of the piece of iron in the transformer

Answer 30

• links magnetic fields of coils / primary and secondary
• stronger magnetic field in secondary
• better induction

Question 31

Another transformer is used in a school laboratory to step down a mains supply with a p.d. of 110V to 12V. This transformer is mounted in a metal case. State and explain an essential safety feature required for this arrangement.

Answer 31

metal case earthed
in case wire falls off / insulation fails / live(wire) touches case

Question 32

Fig. 8.1 shows a polystyrene ball covered with aluminium paint. The polystyrene ball is
suspended between two charged metal plates by an insulated thread.

The ball oscillates between the two charged plates.
Explain why the ball oscillates.

Answer 32

• ball moves towards positive plate
• ball touches positive plate
• made of conducting material so becomes positively charged
• repelled from positive plate
• touches negative plate and loses charge
• negatively charged ball attracted back to positive plate and process repeats

Question 33

diode in opposite direction of conventional current what happens?

Answer 33

(V) OR nothing OR no reading
diode does not pass current (in reverse direction)

Question 34

Electrical power is produced in a power station by an alternating current (a.c.) generator. The output of the generator has a voltage of 22000V. The electrical power is transmitted at a voltage of 400000V.

Explain why electrical power is transmitted at a voltage of 400000V and not 22000V.

Answer 34

high voltage results in low current (for same power)

• heat loss depends on current
• less power / energy loss (in transmission)
• thinner wires can be used ((thinner & cheaper))

Question 35

10 Fig. 10.1 shows a relay.

(a) The switch in circuit A is closed. Describe how this operates the motor in circuit B.

(b) The switch in circuit A is opened. The soft-iron armature is replaced with a steel armature.
The switch in circuit A is closed.
Explain what happens when the switch in circuit A is then opened.

Answer 35

iron core / coil / solenoid becomes a magnet
iron core / coil / solenoid attracts iron armature
armature pivots/moves and contacts touch / there is a current in circuit B

current in circuit B does not stop when switch in circuit A is opened
steel remains magnetised when there is no current in the coil

Question 36

A lamp and an open switch are connected in series to the output terminals of the a.c.
generator.

The switch is closed and the lamp lights up. The student has to apply a greater force on the
handle.

Explain why a greater force is needed to keep the lamp lit.

Answer 36

current in coil
current in (magnetic) field experiences a force
opposes the change causing it

energy supplied to / lost from lamp B1
student must do more work / supply more energy /
more energy needed
greater force to do more work

Question 36

A conducting sphere is mounted on an insulating stand. Explain how you would use a positively charged rod of insulating material to charge the sphere by induction.

Answer 36

bring (charged) rod close to sphere / touching sphere

earth sphere or equivalent

remove earth (connection) AND keep rod close to sphere (until earth removed)

Question 37

The potential difference (p.d.) of the supply is increased so that the current in the lamp increases.

State and explain any change in the resistance of the lamp.

Answer 37

Statement : resistance of lamp increases
Explanation : temperature of lamp increases

Question 38

what happens when fuse rating is too low or too high

Answer 38

if too low it would break / blow / melt when the appliances are operating normally

if fuse too high wouldn’t break / blow until current was too high which would be dangerous (to people /wires /appliance)

Question 39

A fuse is made out of a short length of wire.

Explain why fuses of a higher rating are made of thicker wire.

Answer 39

(Resistance inversely proportional to area so) resistance of thicker wire is lower

Fuse will melt at higher current

because heating effect = I^2R OR less heating effect (for same current)

Question 40

a.A smaller, uncharged metal sphere S is suspended by a plastic thread and brought close to the negatively charged sphere. Fig. 5.2 shows the two spheres.

b(i) By drawing on Fig. 5.2, indicate the distribution of charge on S.

b(ii) State what happens to S.

b(iii) An earth wire is then touched against S. Describe what happens in the wire and state how this affects the charge on S.

c.The metal sphere S is an electrical conductor. The plastic thread is an electrical insulator. Explain this difference by referring to the structures of the two materials.

Answer 40

5(a) four or more radial arrows /lines outside surface
at least one arrow pointing towards (centre of) sphere and none wrong

5(b)(i) positive charges on left and negative charges on right of S equal numbers

5(b)(ii) it moves towards /attracted towards the negatively charged sphere / to the left

5(b)(iii) electrons / negative charges move (along the wire) towards Earth / towards ground /down the wire S becomes positively charged

5(c) electrons mentioned free (to move)/ delocalised / mobile in metals /S or fixed in position in plastic / stand

Question 41

(a) Fig. 7.1 shows a coil of wire wound on a thin plastic cylinder. The plastic has no effect on any magnetic field. The galvanometer is extremely sensitive. A magnet is fixed to a small trolley that runs without friction on a track through the cylinder and coil.

(i) The trolley is released from point A so it runs through the coil from right to left. State and explain what is observed on the galvanometer.

(ii) The trolley is now released from point B so it runs through the coil from right to left again. State what is observed on the galvanometer and explain why it is different to your answer in (a)(i).

Answer 41

(a)(i) deflection
(then) reverse deflection / current / voltage OR greater deflection OR deflection for shorter time OR change of (magnetic) field / flux

(a)(ii) larger deflection OR deflection for shorter time
higher speed OR larger (rate of) change of magnetic field / flux

Question 42

State and explain three dangers with this arrangement.

3kW electric heater
cut in outer insulation
plug and socket lying on grass- damp grass
extension lead designed for use with a 25W lamp

Answer 42

{current / power too high OR trip hazard} AND cut (in insulation) AND plug / socket on damp / wet (grass)

overheating / fire in extension lead OR trip hazard

short circuit / shock / electrocution through cut (in insulation)

short circuit / shock / electrocution through plug on damp / wet (grass)

Question 43

A magnet and a coil are attached separately to a door and a door frame as shown in Fig. 10.1.
The purpose of the arrangement is to activate a circuit connected to an LED indicator when the door is opening or closing. This will provide a visual indication that the door is being used.

Initially, the door is closed and then it is opened.
(i) Explain why the indicator comes on and then goes off when the door is opened.

(ii) The door shuts. The indicator comes on more brightly but for a shorter time than it did in (i). Suggest and explain why this happens.

(b) A circuit breaker is recommended for use with an electric lawnmower.
State two reasons for this recommendation.

Answer 43

movement of magnet relative to coil OR induces emf / pd / current (across / in LED)
light goes off when magnet no longer directly below coil

10(a)(ii) door closes more quickly than it was opened so higher current in LED
door / magnet moving for shorter length of time

10(b) Any two from:
* quick response
* protects against electric shock
* protects against overheating
* (easily) re-settable
* avoids damage to lawnmower

Question 44

Fig. 10.1 shows a straight wire AB placed in the magnetic field between the poles of a magnet. The ends of AB are connected to a galvanometer.

When AB is moved vertically, the needle of the galvanometer shows a deflection. State three factors that affect the size of the deflection.

Answer 44

Strength of magnetic field / magnet
Speed (of movement of wire)
Length of AB / wire (within field)

Question 44

A student wants to demagnetise a permanent bar magnet. She suggests these steps:

1. Place the magnet in a long coil.
2. Switch on a large alternating current in the coil.
3. Switch off the current.
4. Remove the bar from the coil.

State and explain whether the steps will always be able to demagnetise the magnet.

Answer 44

Would not be effective OR No

With current on OR the (alternating) current should not be switched off

Magnet should be withdrawn from the coil

OR Magnet would be alternately magnetised in different directions

Would remain magnetised in the direction occurring at the moment of switching off

Question 45

Explain why the voltage of the supply to the primary coil of a transformer must be alternating.

Answer 45

To produce an alternating/changing magnetic field
so that current/voltage is induced (continuously) in the secondary coil OR secondary circuit

Question 46

When there is a current in the coil, the coil experiences a turning effect.

(i) Explain why there is a turning effect.

Answer 46

forces on AB and CD in opposite (vertical) directions

Question 47

A student is supplied with a small block of iron, a thermometer and an electrical heater of power P. There are two holes drilled in the iron block. The heater fits tightly into one hole and the student places the thermometer into the other hole.

State:
* the other equipment the student will need
* the measurements the student needs to take
* the equation used when calculating the value of the specific heat capacity of iron

Answer 47

power supply and (top-pan) balance / scales and stopwatch / timer / joulemeter

measure mass (of block) and initial and final temperature

reading from joulemeter or measure time (of heating) and (E =) Pt/VIt or c = Pt/m∆T

c = Pt/m∆T or c = E/m∆ T

Question 48

Describe how a direct current (d.c.) differs from an alternating current (a.c.).

Answer 48

(a d.c. has) constant value / magnitude or direction does not change or has only one direction

Question 49

The rate at which the coil of the a.c. generator rotates increases.

State two ways in which the alternating voltage changes.

Answer 49

greater (maximum) voltage

greater frequency or smaller time period or changes direction more often or alternates faster

Question 50

Describe

• how the steel bar can be magnetised,
• how the steel bar can then be demagnetised.

Answer 50

connect d.c. supply (to terminals / circuit) B1
switch on B1

connect a.c. supply (to terminals / circuit) M1
withdraw rod from solenoid/reduce
current

Question 51

Discuss any dangers of the electrical arrangement shown in Fig. 9.1.

weather-proof socket on wall
damp grass
electric mower
cut in insulation covered with tape
socket designed for indoor use
excess length of cable coiled up
extension cable with thin wires

Answer 51

Mention of overheating or fire seen anywhere
Mention of electric shock or electrocution seen anywhere

Fire / overheating: if thin/extension cable carries too large
a current OR because thin /extension cable has no fuse.

Fire / overheating due to extension cable being coiled (so
that escape of heat is prevented)

Electric shock /electrocution (of gardener) if unsuitable
socket lets in moisture /gets wet

Electric shock /electrocution (of gardener) if tape repair
lets in moisture / gets wet

Electric shock /electrocution if cable is cut by mower and
no circuit-breaker