Poles of a magnet — AQA GCSE Physics
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Poles of a magnet explained
Every magnet has two poles, north seeking and south seeking, and they are where the magnetic force is strongest.
Read the full explanation
On a field diagram the poles are easy to spot because the field lines are packed closest together there, and in a practical the iron filings pile up thickest at the two ends of a bar magnet. Move away from a pole and the lines spread out, so the field and the force both get weaker with distance. The two ideas are examined together: the field is strongest at the poles and weakens further from the magnet. You cannot get a single pole on its own, because snapping a bar magnet in half simply gives two shorter magnets, each with a north and a south.
When two magnets are brought close together they exert a force on each other.
Bring two magnets close and each one exerts a force on the other. The two forces are equal in size and opposite in direction, and they act on different objects, so the magnet clamped to the bench feels exactly the same size of force as the one that jumps. Whether the pair attract or repel is decided by the poles facing each other, not by which magnet is stronger. The size of the force grows quickly as the separation is reduced and as stronger magnets are used. The force needs no contact and passes straight through air, paper or a hand, which is why a magnet held under a table can drag a paperclip across the top.
Two like poles repel each other.
Two north poles pushed together repel, and so do two south poles. Sketch the field and you can see why: the lines leaving both north poles bend away from each other, none of them run from one magnet to the other, and there is a neutral point on the line between the magnets where the two fields are equal and opposite and cancel. Repulsion is worth remembering for a second reason: it is the only sure test for a magnet. A magnet attracts a magnetic material such as an iron nail as well as another magnet, so attraction proves nothing, but only another magnet can be repelled. The force gets stronger the closer the like poles are pushed.
Two unlike poles attract each other.
A north pole and a south pole attract each other. The field pattern shows it clearly: the lines leave the north pole of one magnet, cross the gap and enter the south pole of the other, and they are tightly packed in the gap because the field there is strong. The closer the magnets, the larger the force. Attraction is the more common observation but the less useful one, because a magnet also attracts an unmagnetised magnetic material such as an iron nail, which becomes an induced magnet. So if you are asked to decide whether an unknown bar is a magnet or just a piece of iron, attraction cannot settle it; look for repulsion instead.
Attraction and repulsion between two magnetic poles are examples of non-contact force.
Two magnets interact without ever touching, so magnetic attraction and repulsion are non-contact forces, in the same family as the gravitational force and the electrostatic force between charges. The interaction is carried by the magnetic field that fills the space around each magnet: the field of one magnet reaches the other and exerts a force on it, and the second magnet's field does the same in return. Contact forces such as friction, tension, air resistance and the normal contact force require the surfaces to touch. On a free body diagram, a magnetic force is drawn as an arrow from the object in the direction the force acts, exactly like any other force, because force is a vector.
A permanent magnet produces its own magnetic field.
A permanent magnet is made from a hard magnetic material such as steel, alnico or neodymium, and it produces its own magnetic field all the time. Its north and south poles are fixed, and they are there whether or not another magnet is anywhere near. That is the crucial contrast with an induced magnet, which is only magnetic while it sits inside somebody else's field. Because a permanent magnet has its own fixed poles, it can repel as well as attract, which an induced magnet can never do. It is not indestructible: heating it strongly, hammering it or dropping it repeatedly jumbles the domains and demagnetises it, but simply moving it away from other magnets does not.
An induced magnet is a material that becomes a magnet when it is placed in a magnetic field.
Hold a piece of iron near a bar magnet and the iron becomes a magnet itself, for exactly as long as it stays there. Only magnetic materials do this: iron, steel, cobalt and nickel. The external field lines up the magnetic domains inside the material, so it develops its own north and south poles, and it keeps them only while it stays in that field. That is why a chain of iron nails can hang from a bar magnet, each nail becoming an induced magnet that induces magnetism in the next one down. The mark scheme accepts 'when it is close to another magnet' for being placed in a field, but 'a temporary magnet' on its own is not enough for the mark.
Induced magnetism always causes a force of attraction.
When a magnetic material is placed in a field, the domains inside it line up so that the pole induced nearest the magnet is always the opposite pole. A north pole induces a south pole facing it, so the two are pulled together. Turn the magnet round and the induced poles swap as well, so it is attraction once again: there is no way to arrange a magnet and a piece of iron so that they repel. This is why a magnet picks up a steel paperclip whichever end you offer it, and it is the reason repulsion is the definitive test for a magnet. The attraction is mutual, so the paperclip pulls on the magnet just as strongly.
When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.
Take the external field away and an induced magnet loses most or all of its magnetism, and it does so quickly. Without the field to hold them, the domains fall back into a random arrangement and the poles disappear, which is why a chain of nails collapses the instant the bar magnet is lifted clear. The mark scheme allows 'when there are no magnets nearby' for removing the field. How complete the loss is depends on the material: soft iron loses essentially all of it at once, while steel is harder and keeps a good deal, which is why steel is used for permanent magnets and iron for electromagnet and transformer cores that must switch off cleanly.
Students should be able to describe: the attraction and repulsion between unlike and like poles for permanent magnets the difference between permanent and induced magnets.
This pulls the section together, and answers need both halves. For poles: like poles repel, unlike poles attract, and the force grows as the magnets get closer. For magnet types: a permanent magnet produces its own field all the time with fixed poles and can repel, whereas an induced magnet is magnetic only while it is in a field, its poles are set by that field, the force is always attraction, and it loses most or all of its magnetism quickly once the field is removed. In a table of pole pairings every row is judged separately, with AQA awarding one of the two marks for three out of four rows correct.
Your focus
- Identify the poles on a field line diagram from the spacing of the lines.
- State how the strength of a magnet's field varies with distance from it.
- Explain why cutting a bar magnet in half does not produce an isolated pole.
Show all 30 objectives
- Complete a table predicting attraction or repulsion for each pairing of poles.
- Explain why a fixed magnet experiences the same size of force as the magnet that moves.
- Describe how the force between two magnets changes as they are moved apart.
- Predict the force between two stated poles of two magnets.
- Justify why repulsion, and not attraction, proves that an unknown bar is a magnet.
- Sketch the field between two like poles, showing where the fields cancel.
- Draw the field pattern between a north pole and a south pole with correctly directed arrows.
- Explain why attraction is not sufficient evidence that an object is a magnet.
- Predict the direction of the force when unlike poles are brought close together.
- Classify a list of forces as contact or non-contact, placing the magnetic force correctly.
- Explain in terms of the magnetic field why two magnets interact without touching.
- Name two other non-contact forces and one contact force.
- State what distinguishes a permanent magnet from an induced magnet.
- Describe two ways in which a permanent magnet can be demagnetised.
- Identify from a description whether an object is a permanent magnet, an induced magnet or an electromagnet.
- Decide which of several answers to 'What is an induced magnet?' would score, given that 'a temporary magnet' on its own is ruled insufficient.
- Explain why a chain of iron nails can hang from a single bar magnet.
- Name the materials that can be induced and one metal that cannot.
- Explain why an induced magnet is always attracted to the magnet that induced it.
- Predict what happens when a bar magnet near an iron nail is reversed end for end.
- Use the always attraction rule to decide which of two bars is a permanent magnet.
- Use the loss of induced magnetism to explain why the iron arm of an electric bell springs back as soon as the circuit is broken.
- Explain why iron rather than steel is chosen for an electromagnet core.
- Predict the behaviour of a chain of nails when the magnet holding them is pulled away.
- Complete a table of pole pairings, deciding attraction or repulsion for each row.
- Compare permanent and induced magnets by where their field comes from and whether it persists.
- Decide from a described observation whether an object is a permanent magnet or a magnetic material.
Poles of a magnet exam tips
Marking Points
- one mark for stating that the magnetic field is strongest at the poles
- one mark for stating that the magnetic field gets weaker further from the magnet
- one mark for identifying the poles on a field diagram as where the field lines are closest together
- one mark for stating that cutting a bar magnet in half gives two magnets, each with a north and a south pole
- one mark for stating that both magnets experience a force, not only the one that moves
- one mark for stating that the two forces are equal in size and opposite in direction
- one mark for stating that the force is larger when the magnets are closer together
- one mark for identifying the interaction as attraction or repulsion from the poles shown
- one mark for stating that two north poles, or two south poles, repel each other
- one mark for identifying repulsion as evidence that both objects are magnets
- one mark for describing the field between like poles as bending away, with a neutral point where the fields cancel
- one mark for stating that the repulsive force increases as the like poles are pushed closer together
- one mark for stating that a north pole and a south pole attract each other
- one mark for drawing field lines that run from the north pole of one magnet into the south pole of the other
- one mark for stating that the force increases as the two magnets are brought closer together
- one mark for explaining that attraction alone does not prove the second object is a magnet
- one mark for classifying magnetic attraction and repulsion as non-contact forces
- one mark for explaining that the objects need not touch because the force acts through the magnetic field
- one mark for naming another non-contact force, such as the gravitational or electrostatic force
- one mark for contrasting with a named contact force such as friction or air resistance
- one mark for stating that a permanent magnet produces its own magnetic field
- one mark for stating that the field is present whether or not another magnet is nearby
- one mark for stating that the poles of a permanent magnet are fixed, so it can repel as well as attract
- one mark for contrasting with an induced magnet, which is magnetic only while it is in an external field
- one mark for defining an induced magnet as a material that becomes a magnet when placed in a magnetic field; 'when it is close to another magnet' is allowed
- one mark for the alternative definition, that it loses most or all of its magnetism when removed from the field
- one mark for stating that the induced magnet is attracted to the magnet that induces it, whichever pole is presented
- one mark for naming a magnetic material, such as iron, that can be induced
- one mark for stating that induced magnetism always produces a force of attraction
- one mark for explaining that the pole induced nearest the magnet is always the opposite pole
- one mark for stating that reversing the magnet reverses the induced poles, so the force is still attraction
- one mark for using this to argue that any observed repulsion shows the second object is a permanent magnet
- one mark for stating that an induced magnet loses most or all of its magnetism when removed from the magnetic field; 'when there are no magnets nearby' is allowed
- one mark for stating that the loss happens quickly
- one mark for stating that the domains return to a random arrangement so the poles disappear
- one mark for applying it, such as explaining why a chain of nails drops when the magnet is taken away
- two marks for a table of pole pairings with all four rows correct; one mark if three rows are correct
- one mark for stating that a permanent magnet produces its own magnetic field at all times
- one mark for stating that an induced magnet is magnetic only while it is in a magnetic field
- one mark for stating that induced magnetism always causes attraction and is lost quickly when the field is removed
Examiner Tips
- 💡Read the spacing of field lines as the strength of the field: closest lines mean strongest field.
- 💡Tick-two questions need both correct statements; one right and one wrong scores one.
- 💡Answer the where and the how: strongest at the poles, weaker with distance.
- 💡Name the two poles facing each other before you name the force.
- 💡Use the phrase 'equal in size and opposite in direction, acting on different magnets'.
- 💡In a table of pole pairs, read each row separately rather than looking for a pattern.
- 💡Learn one sentence: repulsion is the only test that proves an object is a magnet.
- 💡When sketching like poles, leave a gap with a neutral point rather than joining the lines up.
- 💡Say which poles are facing before you say what the force does.
- 💡Arrowheads carry marks: every line needs one, pointing north to south outside the magnet.
- 💡Pair this with induced magnetism whenever a question asks 'is it a magnet?'.
- 💡Draw the lines in the gap closer together to show the stronger field there.
- 💡Give one example from each class when a question asks you to compare contact and non-contact forces.
- 💡On a free body diagram, draw each force as a labelled arrow starting at the object.
- 💡Say 'through the magnetic field' to explain how the force acts at a distance.
- 💡Difference questions need both sides written out, not just the permanent magnet.
- 💡Use the phrase 'produces its own magnetic field' word for word.
- 💡Mention that only a permanent magnet can repel, as your test for telling them apart.
- 💡Write the full sentence: becomes a magnet when it is placed in a magnetic field.
- 💡Add the removal half of the definition if the question is worth more than one mark.
- 💡Use the chain of nails as your standard example.
- 💡The word 'always' is examined, so state it explicitly.
- 💡Justify with induced poles rather than simply saying iron is magnetic.
- 💡Link straight to the repulsion test when a question asks you to identify a magnet.
- 💡Both 'quickly' and 'removed from the magnetic field' can earn credit, so include both.
- 💡Use the chain of nails as the worked example for the applied mark.
- 💡Contrast soft iron with steel if the question asks about a choice of material.
- 💡Check each row of a table independently; three right out of four still scores something.
- 💡A difference question wants a statement about each type, written as a comparison.
- 💡If asked to describe, give the behaviour, not just the name of the material.
Common Mistakes
- ticking 'the magnetic field is uniform away from the poles', which is false for a bar magnet
- drawing field lines that cross or converge to a single point
- believing a single isolated north pole can be produced by cutting a magnet
- assuming the force is strongest at the middle of the bar because that is the bulk of the magnet
- claiming the stronger magnet exerts the larger force on the weaker one
- saying only the magnet that moves experiences a force
- assuming any two magnets brought together must attract
- forgetting that the force grows sharply as the separation is reduced
- explaining repulsion by saying the magnets are made of the same material
- using attraction as proof that an unknown bar is a magnet
- drawing field lines that run from one north pole into the other north pole
- assuming there is no field at all in the gap between two like poles
- drawing the arrows the wrong way round, from south to north outside the magnets
- concluding that an object is a magnet because it was attracted to one
- forgetting that each magnet pulls on the other with an equally sized force
- leaving gaps in the field lines between the two poles
- calling the force a contact force once the two magnets have snapped together
- listing 'magnetism' as a force rather than the magnetic force
- classifying air resistance, upthrust or tension as non-contact forces
- writing that the field is the force, rather than the field carrying the force
- asserting that a permanent magnet can never lose its magnetism
- describing a permanent magnet simply as one that is always attracting things
- confusing a permanent magnet with an electromagnet, which needs a current
- saying a permanent magnet has no field until something magnetic is brought near it
- answering only 'a temporary magnet', which the mark scheme rules insufficient
- saying the material becomes magnetic by touching the magnet rather than by being in its field
- claiming that any metal, including copper or aluminium, can be induced
- describing the induced magnet as having only one pole
- expecting a magnet to repel an iron bar when it is turned round
- saying the paperclip has no poles at all while it is being attracted
- claiming induction can produce repulsion at very short range
- forgetting that the induced magnet attracts the permanent magnet just as strongly
- writing only that it stops being a magnet, with no reference to removing the field
- treating iron and steel as behaving identically once the field is removed
- claiming the nails stay magnetised permanently because they have touched a magnet
- saying the magnetism fades over hours rather than almost at once
- filling in a table of pole pairs by pattern instead of reading each pair of poles
- defining an induced magnet only as temporary, which is not enough for the mark
- describing the difference by material alone rather than by behaviour in a field
- forgetting that a permanent magnet is also attracted to the induced magnet it creates