Magnetic fields — AQA GCSE Physics
Test yourself on Magnetic fields with AQA GCSE practice questions.
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Magnetic fields explained
The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material.
Read the full explanation
The magnetic materials are iron, steel, cobalt and nickel; copper, aluminium, brass and plastic feel no force at all, so most metals are not magnetic. The definition is written in terms of force for a reason: the field is not something you can see, and you detect it by placing a test object, such as a plotting compass, and watching for a force or a turning effect. The field exists in three dimensions around the magnet whether or not anything is there to feel it, is strongest at the poles, and gets weaker with distance.
The force between a magnet and a magnetic material is always one of attraction.
An unmagnetised piece of iron has randomly arranged domains and no poles of its own. Put it in a magnetic field and it becomes an induced magnet with the opposite pole facing the magnet, so it is pulled towards it. Turn the magnet round and the induced poles turn round too, so the result is attraction again. There is no arrangement that produces repulsion between a magnet and a magnetic material, which is why the specification says the force is always one of attraction. Non-magnetic metals such as aluminium and copper feel no force at all, neither attraction nor repulsion. Observing repulsion therefore tells you that both objects must be permanent magnets.
The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet.
How strong the field is depends on where you are. The field lines are packed closest together at the poles, so that is where the field is strongest, and they spread out as you move away, so the field gets weaker with distance from the magnet. The weakening is sharp rather than proportional, so doubling the distance does far more than halve the field. A plotting compass turns most decisively near a pole and iron filings gather most thickly there. The field around a bar magnet is never uniform, which is why 'the field is uniform away from the poles' is a wrong answer; a uniform field is found instead between a pair of opposite magnadur poles or inside a solenoid.
The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point.
Direction needs a convention, and this is it: the direction of the magnetic field at a point is the direction of the force that would act on a north pole placed at that point. The practical version is a plotting compass, whose north seeking pole is free to turn; it settles along the field, so the needle points the way the field points. That is why arrows are put on field lines, and the arrows on a field line diagram represent the direction of the field, not its strength. At any point on a curved line the field direction is along the line, in the sense the arrow shows, so a compass placed anywhere on that line lies as a tangent to it.
The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.
Outside a magnet, field lines run from the north seeking pole round to the south seeking pole. This follows from the direction convention: a north pole placed just outside the north end of a magnet would be pushed away from it and pulled towards the south end, so that is the way the arrows go. Inside the magnet the lines continue from south back to north, so every field line is a closed loop, although GCSE diagrams normally show only the part outside. Field lines never cross and never stop in mid air, and they are closest together at the poles. The statement that the lines point from south to north outside the magnet is a standard wrong answer.
A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.
A magnetic compass is a small bar magnet balanced on a pivot so that it can turn freely. With no other magnet nearby it lines up with the Earth's magnetic field, so its north seeking pole points roughly towards geographic north, and that is where the name comes from. Bring a magnet or a current carrying wire close and the much stronger local field wins, so the needle swings to follow that instead; a change in the direction of the needle is the evidence that a magnetic field has been produced. The fact that a compass works anywhere on the planet is evidence that the Earth generates its own field, produced by motion in its liquid iron and nickel core.
Your focus
- Check a given answer to 'What is a magnetic field?' against the three parts it needs: a region, a second magnet or magnetic material, and a force.
- Name four magnetic materials and one metal that is not magnetic.
- Describe how you could test whether a magnetic field is present in a region.
Show all 18 objectives
- Explain why a magnet and an iron bar attract whichever way round the magnet is held.
- Design a test that uses the always attraction rule to identify a permanent magnet.
- Predict the force between a magnet and a named non-magnetic metal.
- Predict how the force on a paperclip changes as it is moved from a pole of a bar magnet round to the middle of the bar.
- Interpret the spacing of field lines as a measure of field strength.
- Identify which statements about a bar magnet's field are false and explain why.
- Explain why every arrow on a field line diagram reverses when the magnet is turned end for end.
- Explain what the arrows drawn on magnetic field lines represent.
- Use a compass reading to state the direction of the field at a marked point.
- Draw the field of a bar magnet with the arrows in the correct direction.
- Identify which end of an unlabelled bar magnet is the north pole from the direction of the arrows on its field lines.
- Explain why two magnetic field lines can never cross.
- Predict the direction a plotting compass needle points at named positions around a bar magnet.
- Explain how a compass can show that a magnetic field exists around a wire.
- State the evidence a compass provides that the Earth has a magnetic field.
Magnetic fields exam tips
Marking Points
- one mark for defining the magnetic field as the region around a magnet where a force acts
- one mark for stating that the force acts on another magnet or on a magnetic material
- one mark for naming magnetic materials, such as iron, steel, cobalt or nickel
- one mark for stating that the field is there whether or not a second object is present to experience the force
- one mark for stating that the force between a magnet and a magnetic material is always attraction
- one mark for explaining it by induced magnetism, with the opposite pole induced nearest the magnet
- one mark for stating that reversing the magnet still gives attraction
- one mark for concluding that observed repulsion means both objects are permanent magnets
- one mark for stating that the magnetic field is strongest at the poles of the magnet
- one mark for stating that the field gets weaker as the distance from the magnet increases
- one mark for linking the spacing of the field lines to the strength of the field
- one mark for naming where a uniform field is found instead, between opposite magnadur poles or inside a solenoid
- one mark for stating that the field direction is the direction of the force on a north pole placed at that point
- one mark for stating that the arrows on field lines represent the direction of the magnetic field
- one mark for stating that the north seeking pole of a compass needle lines up with the field direction
- one mark for reading off the field direction at a marked point from a field diagram
- one mark for stating that field lines outside a magnet run from the north pole to the south pole
- one mark for drawing an arrowhead in that direction on every line
- one mark for stating that magnetic field lines never cross
- one mark for stating that the lines continue from south to north inside the magnet, so each line is a closed loop
- one mark for stating that a compass contains a small bar magnet that is free to rotate
- one mark for stating that the needle lines up with the direction of the magnetic field it is in
- one mark for stating that the Earth has a magnetic field, so a compass away from other magnets points north
- one mark for stating that a change in the direction of the needle shows that a magnetic field is present
Examiner Tips
- 💡Learn the definition as one sentence: the region around a magnet where a force acts on another magnet or a magnetic material.
- 💡Name at least one magnetic material if the question mentions materials.
- 💡Do not swap in the electric field definition; check whether the question says magnet or charge.
- 💡The word 'always' carries the mark, so write it.
- 💡Justify with induced poles rather than simply naming iron as magnetic.
- 💡Remember non-magnetic metals feel no force, rather than a repulsive one.
- 💡For tick-two questions, find both correct statements before moving on.
- 💡Answer both parts: where the field is strongest and how it changes with distance.
- 💡Show varying strength in a drawing by varying the spacing of the lines.
- 💡Quote the convention using a north pole, since that is what the specification states.
- 💡If a question asks what the arrows represent, the one word answer is direction.
- 💡Use a compass as the practical way to find the direction at a point.
- 💡Put an arrowhead on every line you draw, not just the first one.
- 💡Keep the pattern symmetrical above and below the magnet.
- 💡Name the pole the lines leave and the pole they enter in a written answer.
- 💡The credited sentence is that the changing direction of the compass needle shows a field has been produced.
- 💡Say the needle aligns with the field, rather than that it is pulled by it.
- 💡Mention the core when a question asks what the compass tells us about the Earth.
Common Mistakes
- defining the field as the region where the field lines are drawn, with no mention of force
- stating that all metals are magnetic materials
- giving the definition of an electric field, a region where a charged object experiences a force
- describing the field as something the magnet touches or gives off
- predicting repulsion when the magnet is turned round
- saying an iron bar has permanent poles of its own before the magnet arrives
- claiming a magnet repels a non-magnetic metal such as aluminium, when it exerts no force on it
- using attraction as evidence that the second object is a magnet
- saying the field strength falls in direct proportion to the distance
- describing the field around a bar magnet as uniform
- drawing evenly spaced field lines all round a bar magnet
- assuming the field stops where the drawn lines stop
- defining the direction using the force on a south pole
- saying the arrows on field lines show the strength of the field
- drawing field lines with no arrowheads at all
- assuming the field points towards the magnet everywhere around it
- drawing the arrows from south to north outside the magnet
- leaving arrowheads off some of the lines in a field pattern
- drawing lines that cross one another or meet at a single point
- drawing lines that stop part way across the gap between two poles
- saying the needle points exactly at the geographic north pole
- saying the compass is attracted towards north rather than aligning with the field
- attributing the Earth's field to iron ore in the crust rather than the liquid core
- expecting the needle to keep pointing north when a strong magnet is placed beside it