Electromagnets, the motor effect and the left-hand rule – IGCSE Physics exam questions with mark scheme

1Grade 4–5Triple only4 marks

A student makes an electromagnet by wrapping insulated wire around an iron nail and connecting it to a battery.

(a) State three ways the student could make the electromagnet stronger. [3]

(b) Give one reason why iron is used for the core rather than steel. [1]

(a) any three from: increase the current (in the coil), allow use a higher-voltage battery / more cells; increase the number of turns (of wire on the coil), allow 'more coils / loops'; wind the turns closer together (more turns per unit length); allow use a thicker / larger iron core. Ignore 'bigger battery' or 'more power' on its own; ignore 'add an iron core' (it already has one)(1)
(a) a second way from the list(1)
(a) a third way from the list(1)
(b) iron is a (magnetically) soft material: it loses its magnetism when the current is switched off (so the electromagnet can be turned off); accept steel is magnetically hard / would stay magnetised (become a permanent magnet) when the current is switched off; allow iron is magnetised more easily (so the electromagnet is stronger); reject 'steel is not magnetic'(1)
Answer: (a) any three of: more current, more turns, turns closer together (thicker core) (b) iron is magnetically soft, so it loses its magnetism when the current is switched off
Examiner tip: 'More power' or 'bigger battery' is too vague: say increase the current.

2Grade 4–54 marks

The diagram shows a wire placed between the poles of two magnets. The current in the wire is into the page.

(a) State the direction of the force on the wire. [1]

(b) State two changes that would increase the size of the force on the wire. [2]

(c) The direction of the current and the direction of the magnetic field are both reversed. State the effect on the force. [1]

(a) downwards (towards the bottom of the page); allow 'down'(1)
(b) any two from: increase the current (in the wire); use a stronger magnet / stronger magnetic field; increase the length of wire in the field(1)
(b) a second change from the list; ignore 'bigger battery' unless linked to more current(1)
(c) force is in the same direction (as before); accept no change (in direction)(1)
Answer: (a) downwards (b) any two of: bigger current, stronger magnet, longer length of wire in the field (c) same direction
Examiner tip: In Fleming's left-hand rule the First finger is the Field (N to S), the seCond finger the Current and the thuMb the Motion.

3Grade 6–74 marks

A simple d.c. motor has a rectangular coil of wire that can rotate between the poles of a magnet. The coil is connected to the supply through a split-ring commutator.

Explain why the coil rotates continuously in the same direction. [4]

current in the two sides of the coil (that are in the field) is in opposite directions; allow a current-carrying wire in a magnetic field experiences a force (motor effect)(1)
so the forces on the two sides are in opposite directions (one up, one down)(1)
this produces a turning effect / moment on the coil(1)
the split-ring commutator reverses the current (in the coil) every half turn / when the coil passes the vertical, so the forces keep turning the coil the same way(1)
Answer: Opposite currents give opposite forces, a turning effect; commutator reverses the current every half turn.
Examiner tip: For the commutator mark say what it does and when: it reverses the current every half turn; 'it keeps the motor going' gains nothing.

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