7.2 Electromagnetism: wires and solenoids
10 exam questions worth 22 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What produces a magnetic field around a straight wire?
Choose one answer.
Correct Not quite: the answer is A current in the wire
Mark scheme
B: A current in the wire
Model answer
A current in the wire (B).
Describe the shape of the magnetic field around a straight wire carrying a current.
Mark scheme
concentric circles (1); centred on the wire / in a plane at right angles to the wire (1)
Model answer
The field lines are concentric circles centred on the wire.
State two ways to increase the strength of the magnetic field around a straight wire.
Mark scheme
increase the current (1); move closer to the wire (1)
Model answer
Increase the current in the wire, or measure closer to the wire.
A wire carries a current out of the page. State the direction of the magnetic field around it.
Mark scheme
anticlockwise
Model answer
Anticlockwise.
State what happens to the magnetic field around a wire if the current is reversed.
Mark scheme
the direction of the field reverses
Model answer
The direction of the field is reversed.
Describe the magnetic field inside and outside a solenoid.
Mark scheme
inside: strong (1); and uniform / parallel lines (1); outside: the same shape as a bar magnet's field (1)
Model answer
Inside the solenoid the field is strong and uniform. Outside it has the same shape as the field of a bar magnet.
A student makes an electromagnet by winding wire round an iron nail. State three ways to make it stronger.
Mark scheme
increase the current (1); wind more turns (1); use a larger / better iron core (1)
Model answer
Increase the current, add more turns of wire, or use a larger soft iron core.
Explain why the core of an electromagnet is made of soft iron and not steel.
Mark scheme
soft iron is magnetised easily when the current is on (1); and loses its magnetism when the current is off, but steel stays magnetised (1)
Model answer
Soft iron is easily magnetised and demagnetised, so the electromagnet turns off properly when the current stops. Steel would stay magnetised.
Explain how an electromagnet on a scrap-yard crane is used to move a car.
Mark scheme
the current is switched on and the electromagnet becomes magnetic (1); it attracts the iron / steel car and lifts it (1); the current is switched off to drop the car (1)
Model answer
The current is switched on, making the electromagnet strongly magnetic, so it picks up the car. The crane moves it, then the current is switched off and the car is released.
A relay is used to switch the large current to a car's starter motor. Explain how a relay works and why it is useful.
Mark scheme
a small current in the circuit switches on an electromagnet (1); the electromagnet attracts an iron armature (1); which closes the switch in the second circuit (1); so a small current / switch can safely control a circuit with a large current (1)
Model answer
A small current switches on an electromagnet, which attracts an iron armature and closes a switch in the second circuit with the large current. This means the driver's small switch can safely control the large current of the starter motor.
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