Magnetic poles
Definition
Magnetic pole
One of the two ends of a magnet, called the north pole and the south pole, where the magnetic field is strongest.
Definition
Magnetic force
The force between two magnets, or between a magnet and a magnetic material, which acts without the objects touching.
- Every magnet has two ends called poles, a north pole (N) and a south pole (S).
- The poles are where the magnetic effect of a magnet is concentrated, so the forces it exerts on other magnets are strongest at its ends.
- If a bar magnet is hung from a thread so that it can turn freely, it settles with one end pointing roughly north, and that end is named the north-seeking pole, or north pole.
- A magnet always has both kinds of pole, so cutting a bar magnet in half gives two smaller magnets, each with its own north pole and south pole.
- The force between two magnets is a non-contact force, so it acts across the gap between them even when they are not touching.
- The force acts on both magnets at the same time, with each magnet pushing or pulling on the other in the opposite direction.
- The closer the two poles are, the larger the force between them, which is why the effect is easy to feel when the magnets are almost touching and fades as they are pulled apart.
Attraction and repulsion
Definition
Like poles
Two magnetic poles of the same type, either two north poles or two south poles, which repel each other.
Definition
Unlike poles
A north pole and a south pole, which attract each other when they are brought close together.
- Whether two magnets attract or repel depends only on which poles face each other.
- Unlike poles attract, so a north pole and a south pole pull towards each other.
- Like poles repel, so two north poles push each other apart, and so do two south poles.
- Attraction is seen when the magnets move towards each other, and repulsion is seen when they move apart.
- There are four possible pairings of poles.
- North facing south gives attraction.
- South facing north gives attraction.
- North facing north gives repulsion.
- South facing south gives repulsion.
- Repulsion feels like a springy push that makes it hard to hold two like poles together.
- If one magnet is free to turn, it swings round until its unlike pole faces the other magnet and is then pulled towards it.
Practical
Investigating forces between magnets
- Aim: to find out which pairs of poles attract and which repel, using a bar magnet that is free to turn.
- Apparatus: two bar magnets with their poles marked N and S, a strip of paper folded into a stirrup, a length of cotton thread, a clamp stand and clamp, and a wooden block to rest the magnets on.
- Method:
- Fold the paper strip into a small stirrup and tie the thread to its top so that the stirrup hangs level.
- Hang the stirrup from the clamp, well away from iron and steel objects such as table legs, so that nothing else pulls on the magnet.
- Rest one bar magnet in the stirrup and wait until it stops swinging and hangs still.
- Hold the second magnet by its middle and slowly bring its north pole towards the north pole of the hanging magnet, without letting them touch.
- Watch which way the hanging magnet turns, and record whether its end moves towards or away from the magnet in your hand.
- Repeat with the other three pairings: north towards south, south towards north and south towards south.
- Repeat each pairing from a few different distances and note how quickly the hanging magnet turns as the poles get closer.
- Results: the hanging magnet swings away whenever like poles face each other and swings towards the held magnet whenever unlike poles face each other, and it turns more sharply the closer the poles are.
- Conclusion: like poles repel and unlike poles attract, and the magnets do not need to touch for the force to act.
- Watch out: a nearby steel object or a third magnet on the bench can turn the hanging magnet by itself, and bringing unlike poles too close makes the magnets jump together and stick.
- Safety: strong magnets can snap together and pinch fingers, so bring them together slowly and keep them away from phones, bank cards and computers.
Field patterns between two magnets
Definition
Magnetic field
The region around a magnet in which another magnet or a magnetic material experiences a force.
Definition
Neutral point
A point between two magnets where their magnetic fields cancel, so the overall magnetic field there is zero.
- The space around a magnet where its force acts is its magnetic field, and it is drawn using field lines with arrows on them.
- Outside a magnet, the arrows on the field lines point away from a north pole and towards a south pole.
- When two magnets are close together, their fields combine, and the shape of the combined pattern shows whether they attract or repel.
- Between unlike poles, the field lines run straight across the gap from the north pole of one magnet into the south pole of the other, linking the two magnets.
- These linking lines behave like stretched elastic bands, pulling the two magnets together.
- Between like poles, no field lines cross the gap, and the lines from each pole bend away sideways as though they are being squashed.
- This squashed pattern matches the push that drives two like poles apart.
- Midway between two like poles of equal strength there is a neutral point, often marked with a cross, where the fields of the two magnets cancel out.
- A field pattern with lines joining across the gap therefore shows attraction, while a pattern with lines turning away from the gap and a neutral point shows repulsion.
- Field patterns are drawn flat on paper, but the real field fills the space all around the magnets.
Example
Identifying the poles of an unmarked magnet
- A bar magnet has lost its labels, so its two ends are called X and Y.
- A marked magnet hangs in a paper stirrup, and end X is brought close to its north pole.
- The north pole of the hanging magnet swings away from X, which is repulsion.
- Repulsion only happens between like poles, so X must be a north pole.
- A magnet always has one pole of each kind, so end Y must be the south pole.
- Bringing Y up to the hanging north pole confirms this, because the two attract as unlike poles.
Example
Reading a field pattern between two magnets
- A diagram shows two bar magnets end to end, with no field lines crossing the gap and a cross marked halfway between them.
- The lines leaving the two facing ends bend away sideways instead of joining the magnets.
- This pattern, together with the neutral point, shows that like poles are facing each other.
- The arrows point away from both facing ends, so both facing poles are north poles.
- Like poles repel, so the two magnets push each other apart.
Exam technique
Describing attraction and repulsion
- Name the two poles first, then say whether they are like or unlike, and only then state whether they attract or repel.
- Give the direction of movement, writing that the magnets move towards each other or move apart, rather than only saying that they move.
- When you complete a field diagram, start each arrow at a north pole and end it at a south pole, and never let two lines cross.
- For two like poles, draw the lines bending away from the gap and mark the neutral point, rather than drawing lines that join the magnets.
Common Mistake
- Do not say that magnets always attract, because two like poles push each other apart.
- Do not describe magnetic poles as positive and negative, because poles are north and south and are not electric charges.
- Do not draw field lines joining two like poles, because the lines from like poles turn away from each other.
Self review
- Name the two poles of a magnet.
- What happens when two south poles are brought close together?
- What happens when a north pole is brought close to a south pole?
- Describe the field pattern between two unlike poles.
- What is found midway between two like poles on a field diagram?
