IGCSE Physics Magnetism & EM effects topical questions

Complete these magnetism basics.

1. (a)State what happens when a south pole is brought near a north pole.[1]
2. (b)Name the material used for the core of an electromagnet.[1]
3. (c)State which end of the field-line arrows the lines enter a bar magnet.[1]
4. (d)Name one use of an electromagnet.[1]

A plotting compass is placed near a bar magnet and the needle settles pointing away from the magnet’s north pole.

1. (a)State what the compass needle’s north-seeking end is showing.[1]
2. (b)State the direction of the magnetic field at that point.[1]
3. (c)State one other way to display the shape of the field around the magnet.[1]

A scrapyard crane’s electromagnet lifts a car, then drops it.

1. (a)Explain why the crane can pick up the car.[1]
2. (b)Explain why the crane can release the car again instantly.[2]

A steel paper clip jumps up to a strong magnet, while an aluminium ring lying beside it does not move at all.

1. (a)Explain why the paper clip is attracted to the magnet.[2]
2. (b)State whether aluminium is a magnetic material.[1]
3. (c)Name one other non-magnetic metal.[1]

A coil is connected to a centre-zero galvanometer. A bar magnet is pushed north-pole-first into the coil, held inside, then withdrawn.

1. (a)Describe what the meter shows during each of the three stages (in, still, out).[3]
2. (b)State three changes that would make the deflection larger.[3]

A transformer has 500 turns on its primary and 2500 turns on its secondary. The primary is connected to a 230 V a.c. supply.

1. (a)Calculate the secondary voltage.[2]
2. (b)State whether this is a step-up or step-down transformer.[1]
3. (c)The transformer works only when the primary supply is alternating. Explain why a steady direct current would not work.[2]

A doorbell push-button completes a circuit containing an electromagnet, which attracts an iron arm holding a hammer against a bell. When the arm moves, a contact opens and the arm springs back, re-closing the contact.

1. (a)Explain why the hammer strikes the bell when the push-button is pressed.[2]
2. (b)Explain why the hammer keeps striking repeatedly while the button is held.[3]

A student winds insulated wire around an iron nail to make an electromagnet for picking up steel pins.

1. (a)Describe how the student could use a small plotting compass to identify which end of the nail becomes a north pole.[2]
2. (b)State three separate changes that would make the electromagnet stronger.[3]

A bicycle dynamo is connected to an oscilloscope. When the wheel is spun steadily, the screen shows 6 complete cycles across a timebase of 0.12 s.

1. (a)Calculate the frequency of the dynamo’s output.[2]
2. (b)State and explain what happens to the trace if the cyclist pedals twice as fast.[2]
3. (c)Explain why the lamp connected to the dynamo still lights even though the current alternates.[1]

A power station generates 500 MW at 25 kV. It is stepped up to 400 kV for transmission through cables of total resistance 2.0 Ω.

1. (a)Calculate the current the cables would carry at 400 kV (ignore losses).[2]
2. (b)Calculate the power lost as heat in the cables at 400 kV.[2]
3. (c)Recalculate the cable current and lost power if the same 500 MW were sent at 25 kV instead, and state the lesson.[3]

A simple d.c. motor has a rectangular coil free to rotate between two magnetic poles. The coil is connected to a battery through a split-ring commutator and brushes.

1. (a)Explain why the coil begins to rotate when the current is switched on.[2]
2. (b)State three separate changes that would increase the turning effect.[3]
3. (c)Explain the problem that would occur with plain slip rings instead of a split-ring commutator, and how the commutator solves it.[2]

The same rotating coil between two magnet poles is connected to an outside circuit twice: once through a pair of slip rings, and once through a split-ring commutator.

1. (a)Describe the output shown on an oscilloscope when the slip rings are used.[2]
2. (b)State how the display changes when the slip rings are replaced by the split-ring commutator.[2]
3. (c)The coil now spins at the same speed but stronger magnets are fitted. State and explain what happens to the trace.[2]