IGCSE Physics 0625 — Topic 4: Electricity and Magnetism — Paper 2 (MCQ) Cambridge Challenge 2

Question 1

A student tests two metal bars, X and Y. One end of bar X is attracted by BOTH poles of a known magnet. One end of bar Y is attracted by the north pole of the magnet but repelled by its south pole.

What can be concluded?

AX is certainly a magnet, and Y is certainly unmagnetised magnetic material such as pure iron BY is certainly a magnet, but X could be either unmagnetised magnetic material or a weak magnet held the wrong way Cboth bars are certainly permanent magnets because both interact with the known magnet Dneither bar can be a magnet, because a real magnet would repel both poles of another magnet

Question 2

The north pole of a strong bar magnet is held close to end P of an unmagnetised soft iron rod PQ, without touching it. The rod attracts steel tacks at end Q.

Which poles are induced in the rod?

ABoth ends become south poles because the rod is soft iron and cannot hold two different poles BNo poles are induced, because the tacks at Q are attracted by the bar magnet itself, not by the rod CEnd P becomes a south pole and end Q becomes a north pole DEnd P becomes a north pole and end Q becomes a south pole

Question 3

A steel bar has become permanently magnetised and must be demagnetised using a solenoid.

Which procedure demagnetises the bar?

APlace the bar in the solenoid, pass alternating current, and switch the current off while the bar is still inside BPlace the bar in the solenoid, pass a large steady direct current, and slowly withdraw the bar CPlace the bar in the solenoid and switch a direct current on and off rapidly several times DPlace the bar in the solenoid, pass alternating current, and slowly withdraw the bar while the current still flows

Question 4

A long straight vertical wire passes through a horizontal card. A plotting compass on the card, 2 cm from the wire, points along the circular field line. Two changes are then made together: the current in the wire is reversed, and the compass is moved to 4 cm from the wire.

What happens to the compass needle?

AIt reverses direction, and it sits in a field that is weaker than before BIt reverses direction, and it sits in a field that is stronger than before CIt keeps the same direction, because moving further away cancels the effect of reversing the current DIt points directly at the wire, because the field lines point radially inwards towards the current

Question 5

A horseshoe magnet has flat north and south pole faces close together and parallel.

Which statement describes the magnetic field in the gap between the pole faces?

AThe field is uniform: the field lines are parallel, equally spaced, and run from the north face to the south face BThe field lines run from the south face to the north face because the field is confined inside the gap CThe field is zero in the gap because the effects of the two opposite poles cancel each other out DThe field is strongest exactly halfway between the faces and weaker close to each pole face

Question 6

An electromagnet lifts a load. Its coil has 200 turns and carries a current of 3.0 A. The coil is rewound with 400 turns of the same wire on the same core, and the current is reduced to 1.5 A.

How does the lifting strength compare with the original?

AIt falls almost to zero, because an electromagnet needs both more turns and more current together BIt is roughly doubled, because the number of turns matters far more than the size of the current CIt is roughly the same, because doubling the turns is compensated by halving the current DIt is roughly halved, because the size of the current matters far more than the number of turns

Question 7

Two bar magnets lie on a line with their north poles facing each other, 8 cm apart. Magnet 1 is much stronger than magnet 2. There is a neutral point between the poles where the resultant magnetic field is zero.

Where is this neutral point?

ACloser to magnet 1, because the stronger magnet dominates the region of space nearest to itself BExactly halfway between the two poles, because the two fields point in opposite directions there CCloser to magnet 2, because the weaker magnet’s field can only balance the stronger field near to magnet 2 itself DThere is no neutral point, because fields from two north poles can never cancel each other

Question 8

A manufacturer chooses core materials for two products: the core of an electromagnet in a scrapyard crane, and the magnet of a compass needle.

Which pair of materials is correct, and why?

ABoth should be soft iron, because soft iron is the only material that can be magnetised by a current in a coil BCrane core: soft iron, because it loses its magnetism when the current stops; compass needle: steel, because it keeps its magnetism permanently CBoth should be steel, because steel is harder wearing and keeps its magnetism in both applications DCrane core: steel, because it becomes more strongly magnetised than iron; compass needle: soft iron, because it responds quickly to the Earth’s field

Question 9

A polythene rod is rubbed with a dry woollen cloth. The rod becomes negatively charged.

Which statement explains what has happened?

ANegative charge flows from the rod to the ground through the student’s hand and body BProtons transfer from the rod to the cloth, leaving the rod with an overall negative charge CElectrons transfer from the cloth to the rod, leaving the cloth with an equal positive charge DElectrons are created on the rod by the energy supplied through the rubbing action

Question 10

A negatively charged rod is brought near a small, uncharged ball of aluminium foil hanging on a nylon thread. The ball swings towards the rod, touches it, and then springs away.

Why does the ball spring away after touching?

AOn contact the ball gains a positive charge from the rod, and the rod then attracts the surrounding air instead BOn contact the ball gains some of the rod’s negative charge, and two negatively charged objects repel CThe thread twists during the first swing and the stored twist in the nylon pushes the ball back out DThe rod loses all its charge to the ball at the moment of contact, so there is no longer any force at all

Question 11

An electric field exists around a positive point charge.

Which statement about this field is correct?

AThe field lines point radially away from the charge, and they show the direction of the force on a small positive test charge BThe field lines form closed circles around the charge, in the same way as the field around a current-carrying wire CThe field lines point radially away from the charge, and they show the direction of the force on a small negative test charge DThe field lines point radially towards the charge, because field lines always point towards the object producing them

Question 12

A charge of 300 C passes through an electric motor in 4.0 minutes.

What is the current in the motor?

A1200 A B1.25 A C75 A D0.80 A

Question 13

Wire P has length L and diameter d. Wire Q is made of the same material, with length 2L and diameter 2d.

What is the resistance of Q compared with the resistance of P?

AThe same resistance as P BTwice the resistance of P CHalf the resistance of P DOne quarter of the resistance of P

Question 14

A student measures the current in a component at two potential differences:

at 2.0 V the current is 0.40 A; at 6.0 V the current is 0.75 A.

What do these results show?

AThe resistance falls from 8.0 Ω to 5.0 Ω, so the component behaves like a thermistor being heated BThe resistance rises from 5.0 Ω to 8.0 Ω, so the component behaves like a filament lamp CThe current is proportional to the potential difference, because both quantities increased in the same experiment DThe resistance stays constant at 5.0 Ω, so the component obeys Ohm’s law throughout

Question 15

Which statement correctly defines the electromotive force (e.m.f.) of a battery?

AThe force with which the battery pushes each electron along the connecting wires of the circuit BThe electrical work done on each unit of charge passing through only the external part of the circuit CThe maximum current the battery can supply before its stored chemical energy is used up completely DThe electrical work done by the battery on each unit of charge that it drives round the complete circuit

Question 16

A kettle is rated 1.8 kW at 230 V.

What is the current in the kettle’s element when it operates normally?

A0.13 A B128 A C7.8 A D414 A

Question 17

An electric shower is rated 7.5 kW. It is used for 12 minutes every day for 30 days. Electricity costs 30 cents per kWh.

What is the total cost?

A$13.50 B$67.50 C$0.45 D$4.50

Question 18

A 12 V electric motor draws a current of 1.5 A while lifting a 2.0 kg load through a height of 3.0 m in 10 s. The gravitational field strength g = 9.8 N/kg.

What is the efficiency of the motor?

A98% B49% C67% D33%

Question 19

In a simple circuit a lamp is connected to a cell by two copper wires.

Which row describes the motion of charge in the external circuit?

AElectrons flow from the positive terminal towards the negative terminal, in the same direction as conventional current BPositive ions drift along the copper wires from the positive terminal, carrying the conventional current CElectrons and positive charges flow in the same direction round the circuit, sharing the current equally between them DElectrons flow from the negative terminal towards the positive terminal, opposite to the conventional current

Question 20

The current in a semiconductor diode is measured for a range of potential differences applied in both directions.

Which statement matches the behaviour of the diode?

AIts current is directly proportional to the p.d. in the forward direction, starting from the origin of the graph BIt conducts in one direction only, with a current that stays constant no matter how the forward p.d. is increased CIt conducts only when connected one way round, and even then only once the p.d. exceeds a small threshold value DIt conducts equally well in both directions provided the applied p.d. is made large enough in each direction

Question 21

Three resistors of 2.0 Ω, 4.0 Ω and 6.0 Ω are connected in series across a 24 V supply.

What is the potential difference across the 6.0 Ω resistor?

A4.0 V B24 V C8.0 V D12 V

Question 22

A 4.0 Ω resistor and a 12 Ω resistor are connected in parallel.

What is the combined resistance of the pair?

A3.0 Ω B16 Ω C8.0 Ω D0.33 Ω

Question 23

A 6.0 Ω resistor is connected in series with a parallel pair of identical 4.0 Ω resistors. The combination is connected across a 16 V battery of negligible internal resistance.

What is the current in EACH 4.0 Ω resistor?

A4.0 A B0.50 A C2.0 A D1.0 A

Question 24

A potential divider consists of a 10 kΩ resistor in series with resistor R across a 9.0 V supply. The output, taken across R, must be 3.0 V.

What value of R is required?

A5.0 kΩ B30 kΩ C3.3 kΩ D20 kΩ

Question 25

A thermistor is connected in series with a fixed resistor and an ammeter across a battery. During the experiment the temperature of the thermistor falls.

Which row shows what happens?

AThermistor resistance rises, the current falls, and the share of the supply p.d. across the thermistor rises BThermistor resistance rises, the current falls, and the share of the supply p.d. across the thermistor falls CThermistor resistance falls, the current rises, and the share of the supply p.d. across the thermistor falls DThermistor resistance rises, the current rises, and the share of the supply p.d. across the thermistor stays the same

Question 26

A light meter consists of an LDR in series with a milliammeter and a cell. The meter is carried from a dark room into bright sunshine.

What happens and why?

AThe meter reading stays the same, because the cell fixes the current at a value set by its e.m.f. alone BThe meter reading falls to zero, because in bright light an LDR stops conducting almost completely CThe meter reading falls, because light raises the resistance of the LDR and so the current decreases DThe meter reading rises, because light lowers the resistance of the LDR and so the current increases

Question 27

Two identical lamps are connected in parallel across a battery of negligible internal resistance. A third identical lamp is then connected in parallel with them.

What happens to the total resistance of the circuit and the current supplied by the battery?

AThe total resistance falls and the battery current rises, because the new lamp provides an extra path for current BThe total resistance falls but the battery current also falls, because the current is now shared between three lamps instead of two CThe total resistance rises and the battery current falls, because the battery must now push charge through more lamps DBoth the total resistance and the battery current stay the same, because each lamp still has the full supply p.d. across it

Question 28

A circuit has two parallel branches connected to a battery. Branch 1 contains a diode in its forward direction in series with lamp P. Branch 2 contains a diode in its reverse direction in series with lamp Q.

What is observed, and what is observed when the battery connections are then reversed?

AAt first only lamp Q lights; after the battery is reversed, both lamps light together BBoth lamps light at first, and after the battery is reversed neither lamp lights at all CAt first only lamp P lights, and after the battery is reversed lamp P lights again because a diode does not depend on the battery direction DAt first only lamp P lights; after the battery is reversed, only lamp Q lights

Question 29

A room heater is rated 2.9 kW at 230 V. Fuses rated 3 A, 5 A, 13 A and 30 A are available.

Which fuse should be fitted in the heater’s plug?

A5 A B30 A C3 A D13 A

Question 30

Four mains appliances with a combined power of 7 kW are all plugged into one extension lead designed for a maximum of 13 A at 230 V.

Why is this dangerous?

AThe extension lead’s resistance rises sharply with each extra plug, cutting the current until nothing works BThe combined voltage across the extension lead rises far above 230 V, breaking down the cable insulation CThe appliances share the current between them, so each one receives too little current to work correctly DThe combined current is about 30 A, well above the lead’s rating, so its cable overheats and may start a fire

Question 31

A house circuit can be protected by a fuse or by a circuit breaker.

Which statement gives an advantage of the circuit breaker?

AIt allows a much larger current to flow before acting, so it interrupts the supply far less often BIt works even when no current is flowing in the circuit, unlike a fuse which needs a current to operate CIt switches off very quickly and can be reset at the flick of a switch, instead of needing a replacement part DIt permanently disconnects the circuit after a fault, so the wiring cannot be used again until it is rewired

Question 32

A hairdryer must never be used near a filled bath, even though its case is plastic.

Which statement explains the danger?

AThe steam from the bath condenses inside the hairdryer and increases its power output beyond the rated value BWater on skin greatly lowers the body’s electrical resistance, so contact with a live conductor drives a much larger and possibly fatal current through the body CWet conditions make the plastic case of the hairdryer conduct electricity as well as any metal case would DWater raises the resistance of the human body, so the mains voltage rises to compensate and becomes more dangerous

Question 33

A bar magnet is used with a coil connected to a sensitive centre-zero meter.

Which action produces the LARGEST deflection of the meter?

AMoving the coil and the magnet together at high speed in the same direction BHolding the magnet stationary at the centre of a coil that has many turns CThrusting the magnet quickly into a coil that has many turns DThrusting the magnet quickly into a coil that has had its turns reduced to one

Question 34

A transformer has 1150 turns on its primary coil and 60 turns on its secondary coil. The primary is connected to the 230 V mains.

What is the secondary voltage?

A4.4 kV B12 V C60 V D300 V

Question 35

An ideal (100% efficient) step-down transformer has a turns ratio of 20 : 1. The current in the primary coil is 0.50 A.

What is the current in the secondary coil?

A10 A B0.50 A C200 A D0.025 A

Question 36

A cable of total resistance 4.0 Ω carries a current of 5.0 A to a remote building. Engineers then double the transmission voltage, halving the current for the same delivered power.

What happens to the power wasted as heat in the cable?

AIt rises, because a higher transmission voltage always forces more power into the cable BIt falls from 100 W to 25 W — a quarter of its original value CIt falls from 100 W to 50 W — half of its original value DIt stays at 100 W, because the cable’s resistance has not been changed

Question 37

A straight wire lies horizontally between the poles of a magnet. The magnetic field points horizontally from the north pole on the left across to the south pole on the right. The current in the wire flows horizontally into the page.

What is the direction of the force on the wire?

AVertically upwards BHorizontally, from the south pole towards the north pole CVertically downwards DThere is no force, because the current is perpendicular to the field

Question 38

An a.c. generator produces a maximum e.m.f. of 6.0 V at a frequency of 25 Hz. The coil is then rotated twice as fast.

What is the new output?

AA maximum e.m.f. of about 12 V, still at a frequency of 25 Hz BA maximum e.m.f. of about 12 V at a frequency of 50 Hz CA maximum e.m.f. of about 3.0 V at a frequency of 50 Hz, since the faster coil spends less time in the field DA maximum e.m.f. still of 6.0 V, but now at a frequency of 50 Hz

Question 39

A rectangular coil carrying a current sits in the field of a permanent magnet, forming a simple d.c. motor.

Which change increases the turning effect on the coil?

ASwapping the connections to the supply so that the current flows the opposite way round the coil BWinding the coil with a greater number of turns while keeping the same current CAdding an extra resistor in series with the coil while keeping the same supply DTurning the coil so that its plane lies at right angles to the magnetic field lines

Question 40

A d.c. motor uses a split-ring commutator, but an a.c. generator uses two separate slip rings.

Why are slip rings the correct choice for the a.c. generator?

AThey keep each end of the coil permanently connected to its own brush, so the alternating e.m.f. generated in the coil reaches the circuit unchanged BThey reverse the connections to the circuit every half turn, so the output current always leaves by the same terminal CThey rub against the coil with less friction than a commutator, so the coil can be rotated much faster DThey step up the size of the induced e.m.f. in the same way that the turns of a transformer step up a voltage

Paper 2 — Multiple Choice

Section A: Magnetism

Section B: Electric Charge and Electrical Quantities

Section C: Circuits

Section D: Electrical Safety

Section E: Electromagnetic Effects