Electromagnetism — AQA GCSE Physics
Test yourself on Electromagnetism with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Electromagnetism explained
A current is moving charge, and moving charge always produces a magnetic field.
Read the full explanation
Around a long straight wire the field lines are circles centred on the wire, lying in a plane at right angles to it, and they are closest together near the wire. The direction comes from the right hand grip rule: point the right thumb along the conventional current and the curled fingers give the direction of the field, so reversing the current reverses the field. To show the field exists, pass the wire vertically through a horizontal card. Move a plotting compass around the wire and the changing direction of the needle shows a field has been produced, or sprinkle iron filings on the card and tap it so they line up into circles.
The strength of the magnetic field depends on the current through the wire and the distance from the wire.
Two things set the strength of the field around a straight wire: the size of the current and the distance from the wire. Increase the current and the field at any given point is stronger, which is why the field lines are drawn more densely for a larger current. Move away from the wire and the field gets weaker, which is why the concentric circles are drawn further apart further out. Nothing else on the list of tempting answers works: reversing the current only reverses the direction, and heating the wire does not strengthen the field. You can test each factor by holding a compass at a fixed distance and varying the current, then holding the current fixed and moving the compass.
Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.
A solenoid is a long coil of wire. Each turn carries the same current the same way round, so each produces its own circular field, and where the turns sit side by side those fields point the same way inside the coil and add together, while outside the coil neighbouring contributions largely cancel. The result is a much stronger field inside for exactly the same current, which is why shaping a wire into a solenoid is one of the two changes AQA credits for strengthening the field around a current carrying wire, the other being increasing the current. Winding more turns into the same length strengthens the field again, because there are more contributions to add.
The magnetic field inside a solenoid is strong and uniform.
Inside a solenoid, away from the two ends, the field lines run parallel to the axis, evenly spaced and all pointing the same way. That is what uniform means: the same strength and the same direction everywhere in that region. It is also a strong field, because the contributions of all the turns add there. Near the ends the lines begin to spread out, so the uniform region is the middle of the coil. This is the contrast with a bar magnet, whose field is never uniform, and it is why solenoids are used where a predictable field is needed, from relays to the magnets in a hospital scanner.
The magnetic field around a solenoid has a similar shape to that of a bar magnet.
Step back from a solenoid and its external field looks just like that of a bar magnet: lines emerge from one end, loop round through the surrounding space and enter the other end, so the coil behaves as though it had a north pole at one end and a south pole at the other. Which end is which depends on the direction of the current around the turns, so reversing the current swaps the poles, something a bar magnet cannot do. You can find the north end with a plotting compass, or by the right hand grip rule applied to the turns. The internal and external lines join up: each line is a closed loop through the coil and back round outside.
Adding an iron core increases the strength of the magnetic field of a solenoid.
Slide an iron core inside a solenoid and the magnetic field gets much stronger for the same current. The core sits in the coil's field and becomes an induced magnet, so its own field adds to the coil's and the total field outside is far greater. Take the core out and the field is weaker again. Iron is the required material because it is magnetically soft: it is easily magnetised and easily demagnetised. This means it strengthens the field while the current flows and loses its magnetism the moment the current stops. Steel is rejected because it is magnetically hard and keeps its magnetism, leaving the electromagnet permanently on.
An electromagnet is a solenoid with an iron core.
Wind a coil of wire around an iron bar, pass a current through it, and you have an electromagnet. Because the magnetic field only exists while there is a current in the coil, it can be switched on and off. Its strength can also be changed by altering the current, changing the number of turns, or adding an iron core, none of which is possible with a permanent magnet. That switchability is what makes electromagnets useful in devices like relays, electric bells, circuit breakers, and scrapyard cranes, where a load must be picked up and then released. Examiners often ask you to identify the core or explain how to increase the field strength.
Students should be able to: describe how the magnetic effect of a current can be demonstrated draw the magnetic field pattern for a straight wire carrying a current and for a solenoid (showing the direction of the field) explain how a solenoid arrangement can increase the magnetic effect of the current.
Three skills are examined together here. To demonstrate the magnetic effect of a current, pass the wire through a card and either move a plotting compass around it, watching the needle change direction, or sprinkle iron filings and tap the card so they form circles. To draw the patterns, show concentric circles round a straight wire, widening with distance, and parallel evenly spaced lines inside a solenoid that loop round outside like a bar magnet's; put an arrowhead on every line and mark the current direction. To explain the solenoid, say that each turn contributes a field and that the contributions add inside the coil, so the same current produces a much greater magnetic effect.
(Physics only) Students should be able to interpret diagrams of electromagnetic devices in order to explain how they work.
This is physics only, and it rewards reading the diagram rather than reciting a device. Relays, electric bells and circuit breakers all use the same chain: closing the switch drives a current through the electromagnet, the iron core becomes magnetised, the electromagnet attracts an iron arm or armature, and the moving arm does something mechanical. In a car ignition circuit the arm pushes contacts together, completing the starter motor circuit so that a current passes through the starter motor and it turns, which lets a small current switch a very large one. Write the steps in the order the diagram shows, because each linked step earns credit.
Your focus
- Describe two ways of demonstrating the field around a current carrying wire, giving the observation in each.
- Draw the field pattern around a straight wire with the correct direction marked.
- Predict the effect on the field of reversing the current in the wire.
Show all 27 objectives
- Compare the field strength at two points at different distances from the same current carrying wire and justify the comparison.
- Choose, from a list including direction and temperature, the change that strengthens the field.
- Describe how the field strength varies with distance from a current carrying wire.
- Explain why coiling a wire into a solenoid strengthens the magnetic field it produces.
- Give two changes that increase the strength of the field around a current carrying wire.
- Predict the effect on the field of adding more turns at the same current.
- Identify on a solenoid field diagram the region where the lines are parallel and equally spaced, and say what that shows about the field.
- Draw the field inside and outside a solenoid with consistent arrow directions.
- Compare the field inside a solenoid with the field around a bar magnet.
- Draw the complete field pattern of a solenoid, both inside and outside the coil.
- Compare the external field of a solenoid with that of a bar magnet.
- Explain how the north and south ends of a solenoid can be swapped.
- Predict how the strength of an electromagnet's field changes when its metal core is removed and the current is kept the same.
- Justify the choice of iron rather than steel for the core of an electromagnet.
- Explain in terms of induced magnetism why the core strengthens the field.
- Define an electromagnet as a solenoid with an iron core.
- State three ways of increasing the strength of an electromagnet.
- Explain one advantage of an electromagnet over a permanent magnet.
- Describe a demonstration of the magnetic effect of a current, including what is observed.
- Draw and direct the field patterns of a straight wire and of a solenoid.
- Explain how a solenoid arrangement increases the magnetic effect of a given current.
- Explain, step by step from a diagram, how a relay uses one circuit to switch another.
- Identify the electromagnet, the armature and the contacts on a device diagram.
- Predict the effect of increasing the current or moving the armature further from the electromagnet.
Electromagnetism exam tips
Marking Points
- one mark for describing moving a plotting compass around the wire
- one mark for stating that the changing direction of the compass needle shows a magnetic field has been produced
- one mark for the alternative of sprinkling iron filings onto the card and tapping it to reveal the pattern
- one mark for stating that the field around a straight wire is circular and reverses when the current reverses
- one mark for stating that increasing the current increases the strength of the magnetic field
- one mark for stating that the field is weaker at greater distances from the wire
- one mark for choosing 'increase the current in the wire' rather than changing its direction or its temperature
- one mark for describing a test in which only one of the two factors is changed at a time
- one mark for stating that shaping the wire into a solenoid increases the strength of the magnetic field
- one mark for explaining that the fields due to the individual turns add together inside the coil
- one mark for stating that increasing the number of turns increases the field strength for the same current
- one mark for giving increasing the current as the other change that strengthens the field
- one mark for stating that the magnetic field inside a solenoid is strong
- one mark for stating that the field inside is uniform, meaning the same strength and direction throughout
- one mark for drawing evenly spaced parallel lines inside the coil, all pointing the same way
- one mark for contrasting it with the field around a bar magnet, which is not uniform
- one mark for stating that the field outside a solenoid has the same shape as the field of a bar magnet
- one mark for identifying a north pole at one end of the solenoid and a south pole at the other
- one mark for stating that reversing the current swaps the poles
- one mark for drawing the external lines running from the north end round to the south end
- stating that adding an iron core increases the strength of the magnetic field
- stating that removing the core makes the field weaker at the same current
- explaining that the core becomes an induced magnet so its field adds to that of the coil
- justifying iron as the material because it is easily magnetised and demagnetised
- defining an electromagnet as a solenoid, or coil of wire, with an iron core
- stating that the magnetic field is present only while there is a current in the coil
- naming three ways of changing its strength: increasing the current, adding more turns to the coil, or adding an iron core
- explaining that an electromagnet can be switched on and off, unlike a permanent magnet
- one mark for a demonstration that names the apparatus, such as a plotting compass moved around the wire or iron filings on card
- one mark for stating the observation that shows the field is there, such as the needle changing direction
- one mark for a straight wire field drawn as concentric circles with the direction shown
- one mark for a solenoid field drawn with parallel lines inside and a bar magnet pattern outside
- one mark for explaining that the fields of the individual turns add, so the magnetic effect is greater
- one mark for stating that closing the switch causes a current in the electromagnet
- one mark for stating that the iron core of the electromagnet becomes magnetised
- one mark for stating that the electromagnet attracts the iron arm or armature
- one mark for stating what the moving arm does, such as pushing the contacts together to complete the second circuit
- one mark for the consequence, such as a current passing through the starter motor so that it turns
Examiner Tips
- 💡A two mark demonstration question needs the action and the observation, so write both.
- 💡Mark the current direction on any field diagram you draw.
- 💡Name the apparatus precisely: a plotting compass, a card, iron filings.
- 💡In a multiple choice item the distractors are usually direction and temperature; rule both out.
- 💡Name the factor and the direction of the change: increase the current, so the field is stronger.
- 💡Keep one variable fixed when you describe a test.
- 💡A 'give two changes' question wants two genuinely different ideas, so do not repeat yourself.
- 💡Keep current, number of turns and core as three separate factors.
- 💡Explain by adding fields, which is the reason the mark scheme is looking for.
- 💡Show uniform by drawing equally spaced parallel lines, with arrows all the same way.
- 💡State both properties, strong and uniform, when a question asks about the inside of a solenoid.
- 💡Begin the drawing inside the coil and then continue the lines round the outside.
- 💡Draw the parallel lines inside first, then curve them round outside to the other end.
- 💡Label the N and S ends of the solenoid on your diagram.
- 💡If the current direction is given, use it to decide which end is north before labelling.
- 💡Core questions want the specific material (iron) and the reason; the reason mark usually depends on the material mark.
- 💡Write both halves of the reason: easily magnetised and easily demagnetised.
- 💡A complete definition needs all three parts: a coil, an iron core, and a current.
- 💡State the main advantage over a permanent magnet as being able to switch the magnetic field on and off.
- 💡Mark the current direction on every field diagram you draw.
- 💡Give the observation as well as the apparatus in any demonstration answer.
- 💡Draw the solenoid pattern in two stages, inside then outside, so the lines join up.
- 💡Annotate the diagram as you read it, then write the steps as a numbered chain.
- 💡Start at the switch and follow the current; every linked step is worth a mark.
- 💡If asked how to increase the force on the armature, reduce the distance or increase the current.
Common Mistakes
- saying the wire itself becomes magnetised rather than producing a field only while a current passes
- drawing the field lines as straight lines running along the wire
- leaving out the tapping step, without which the iron filings do not form the pattern
- saying the compass is attracted to the wire rather than aligning with the field
- saying that reversing the direction of the current makes the field stronger
- offering the temperature or the resistance of the wire as a factor
- saying the field is strongest at the ends of the wire, as though it were a bar magnet
- confusing the distance from the wire with the length of the wire
- saying the coil concentrates the current rather than adding the fields of the turns
- arguing the field is stronger because the longer wire has a greater resistance
- claiming that adding more turns increases the current in the circuit
- naming the iron core when the question asks only about shaping the wire
- drawing lines inside the solenoid that curve or converge towards the centre
- describing uniform as simply meaning all in one direction, while drawing unevenly spaced lines
- saying the field is strong only on the axis and weak elsewhere inside the coil
- leaving the inside of the solenoid blank when asked to draw the field pattern
- drawing the external field as circles around the coil, as though it were a straight wire
- leaving the arrowheads off the lines outside the solenoid
- saying the poles of a solenoid are fixed in the way a bar magnet's are
- drawing the inside and outside lines as two separate systems that never join
- saying the core increases the current in the coil, rather than increasing the magnetic field strength
- choosing steel, nickel, or cobalt for the core; AQA specifically requires iron for electromagnets
- claiming the core stores the magnetism so the electromagnet stays on after the current stops, instead of demagnetising
- giving only 'easily magnetised' and leaving out 'and easily demagnetised' when explaining the choice of iron
- defining an electromagnet as any coil of wire, omitting the essential iron core
- saying an electromagnet is magnetic even when no current flows, rather than losing its magnetism
- claiming an electromagnet cannot be switched off, confusing it with a permanent magnet
- specifying a steel core for an electromagnet, which would remain magnetised
- drawing field patterns without marking the direction of the current
- drawing the lines inside a solenoid as loops rather than straight and parallel
- describing the iron filings method without the tapping step
- explaining the solenoid by saying there is simply more wire
- describing what the device is for instead of following the sequence shown in the diagram
- saying the electromagnet attracts the contacts directly rather than attracting the iron arm
- leaving out the magnetisation of the iron core
- failing to say that a small current in one circuit is switching a much larger current in another