Revision notes for Edexcel GCSE Physics Magnets and magnetic fields. Open the guide for explanations and worked examples. Written against the Edexcel GCSE Physics (1PH0) specification, so the content matches what's examinable rather than general Physics background.
Magnets and magnetic fields
What you'll learn
- How magnetic poles attract and repel.
- The difference between permanent, temporary and induced magnets.
- How field-line diagrams and plotting compasses show magnetic fields.
- How currents in wires and solenoids create magnetic fields.
1. Magnetic poles
A magnet is an object that produces a magnetic field and can exert a force on certain materials without touching them.
The strongest magnetic effects are found at the ends of a magnet. These ends are called poles.
Definition
Magnetic pole
A magnetic pole is one end of a magnet where the magnetic force is strongest. A magnet has a north-seeking pole, labelled N, and a south-seeking pole, labelled S.
Like and unlike poles
Magnetic poles follow a simple rule:
Key Idea
Pole rule
Unlike poles attract: N attracts S.
Like poles repel: N repels N, and S repels S.
A magnet always has both a north pole and a south pole. If you cut a bar magnet in half, you get two smaller magnets, each with its own N and S pole.
2. Magnetic materials
A magnetic material is a material that is attracted by a magnet and can be magnetised. For GCSE, the key examples are:
- iron
- steel
- cobalt
- nickel
Different magnetic materials are useful in different situations.
| Material | Typical behaviour | Common GCSE use |
|---|---|---|
| Soft iron | Magnetises easily and loses magnetism easily | Temporary magnet cores in electromagnets |
| Steel | Harder to magnetise, but keeps its magnetism | Permanent magnets, compass needles |
| Cobalt | Can be used in strong permanent magnet alloys | Strong permanent magnets |
| Nickel | Magnetic metal, used in alloys and coatings | Magnetic materials and specialist alloys |
Permanent and temporary magnetic materials
A permanent magnet keeps its magnetism even when it is not near another magnet or current.
A temporary magnet only behaves like a magnet while it is being magnetised. Soft iron is often used for temporary magnets because it can switch its magnetism on and off easily.
Example
Choosing a material for an electromagnet core
You need a core for an electromagnet in a door lock. It must become magnetic when the current is on, but stop being magnetic when the current is off.
- The core must be made from a magnetic material, because non-magnetic materials would not become strongly magnetised.
- The magnetism must disappear when the current stops, so a temporary magnetic material is needed rather than a permanent one.
- Soft iron is the best choice because it magnetises and demagnetises easily. Steel would be a poor choice because it could stay magnetised and keep the lock stuck.
3. Permanent and induced magnets
An induced magnet is not normally a magnet, but becomes magnetised when it is placed in a magnetic field.
Definition
Induced magnet
An induced magnet is a magnetic material that becomes a magnet because it is in a magnetic field. It usually loses its magnetism when the field is removed.
For example, an iron nail can be attracted to a bar magnet even though the nail was not originally a magnet. The bar magnet induces magnetism in the nail.
The end of the induced magnet nearest the permanent magnet becomes the opposite pole, so it is attracted.
Example
Explaining attraction by induction
A north pole of a bar magnet is brought near an unmagnetised iron nail.
- The magnetic field from the bar magnet passes through the iron nail and induces magnetism in it.
- The end of the nail nearest the north pole becomes an induced south pole.
- The facing poles are now N and S, so they attract and the nail moves towards the magnet.
Common Mistake
Induced magnets are not permanent magnets
Do not say the iron nail “was already a magnet”. In this situation, the nail becomes magnetised because of the nearby magnetic field.
4. Magnetic fields
A magnetic field is the region around a magnet, magnetic material or current-carrying wire where magnetic forces can be felt.
Definition
Magnetic field line
A magnetic field line is an imaginary line used to show the shape and direction of a magnetic field. The arrow shows the direction a north pole would be forced to move.
In a field-line diagram:
- arrows show the direction of the magnetic field
- lines that are close together show a stronger field
- lines that are far apart show a weaker field
- field lines do not cross
Some diagrams label magnetic field as BBB. Its SI unit is the tesla (T), but in this part of the topic you mainly describe field patterns, not calculate with BBB.
Field around a bar magnet
Outside a bar magnet, magnetic field lines go from the north pole to the south pole.
The field is strongest near the poles, where the lines are closest together.
Common Mistake
Field lines are not physical strings
Field lines are a drawing tool. They show direction and strength, but there are not real blue lines floating around the magnet.
Uniform magnetic fields
A uniform magnetic field has the same strength and direction everywhere in that region.
It is drawn using:
- straight field lines
- parallel field lines
- equally spaced field lines
You can find an almost uniform field inside a long solenoid, and also between two flat opposite magnetic poles.
Example
Comparing field strength from a field-line diagram
A diagram shows field lines close together near a magnet’s pole and spread out farther away.
- Close field lines mean the magnetic field is stronger near the pole.
- Spread-out field lines mean the magnetic field is weaker farther from the magnet.
- A compass placed at any point would point along the field-line arrow at that point, with its north-seeking end in the arrow direction.
5. Plotting compasses
A plotting compass is a small compass used to map a magnetic field. Its needle is a tiny magnet.
The north-seeking end of the compass needle points in the direction of the magnetic field at that point.
How to map a bar magnet’s field
- Place the bar magnet on paper and draw around it.
- Put a plotting compass near one pole.
- Mark the direction of the north-seeking end of the needle.
- Move the compass along that direction and mark the new direction.
- Join the marks to make a field line.
- Repeat from different starting points to build the full field pattern.
Tip
Good plotting compass technique
Keep the magnet fixed in one place. Move the compass, not the magnet, or your field-line pattern will not match the magnet’s position.
6. The Earth’s magnetic field
A compass points roughly north-south even when there is no bar magnet nearby. This is evidence that the Earth has its own magnetic field.
The Earth behaves as if it has a magnetic field produced by its core. The north-seeking end of a compass points towards geographic north because that region is near a magnetic south pole.
Example
Using a compass as evidence for Earth’s field
A compass is placed on a desk far away from any obvious magnets, and it still settles pointing roughly north-south.
- A compass needle is a small magnet, so it turns when it is in a magnetic field.
- If there are no nearby laboratory magnets, the field causing the compass to turn must come from something much larger.
- Since this happens across the Earth, it is evidence that the Earth has a magnetic field, linked to its core.
Common Mistake
Geographic north is not a magnetic north pole
The north-seeking end of a compass is attracted towards the region near geographic North. That region behaves like a magnetic south pole.
7. Magnetic field around a current-carrying wire
An electric current can create a magnetic field.
A conductor is a material that allows charge to flow through it. A metal wire is a common conductor.
When a current flows through a long straight conductor, the magnetic field forms circles around the wire.
The direction is found using the right-hand grip rule:
- point your right thumb in the direction of the conventional current
- your curled fingers show the direction of the magnetic field
Showing the magnetic effect of a current
You can show this using a wire, a power supply and plotting compasses.
- Place a long straight wire through a piece of card.
- Put plotting compasses around the wire.
- Switch on the current.
- The compass needles turn and line up tangentially to circles around the wire.
- Reverse the current and the compass directions reverse.
Common Mistake
Wire heating
A large current can heat the wire. In a school demonstration, use the apparatus as instructed, keep the current on briefly, and do not short-circuit a power supply.
What affects the field strength?
For a long straight conductor:
- a larger current gives a stronger magnetic field
- the field is stronger closer to the wire
- the field is weaker farther from the wire
Example
Predicting the field around a straight wire
A wire has its current increased, and a compass is moved closer to the wire.
- Increasing the current makes the magnetic field stronger.
- Moving closer to the wire also makes the magnetic field stronger.
- Since both changes increase the field strength, the compass needle would turn more strongly from its original direction.
Common Mistake
Use conventional current for the grip rule
The right-hand grip rule uses conventional current direction, from positive to negative around the circuit. Do not switch to electron-flow direction unless the question specifically asks for it.
8. Solenoids and electromagnets
A solenoid is a coil of wire with many turns.
An electromagnet is a magnet made using an electric current. A solenoid with a soft iron core is a common electromagnet.
Definition
Solenoid
A solenoid is a long coil of wire. When current flows through it, it produces a magnetic field similar to the field of a bar magnet.
Inside a solenoid, the magnetic fields from the individual turns point in the same direction. They add together to make a strong, almost uniform field along the centre.
Outside the solenoid, the fields from different parts of the coil partly cancel, so the field is weaker.
A solenoid has a north pole and a south pole, just like a bar magnet. If the current is reversed, the poles reverse.
Example
Explaining field addition in a solenoid
A solenoid is made by wrapping many turns of wire around a soft iron core.
- Each turn of wire produces its own magnetic field when current flows through it.
- Along the centre of the solenoid, the fields from the turns point in the same direction, so they add together.
- Outside the solenoid, many of the fields oppose each other and partly cancel, so the outside field is weaker than the field inside.
Key Idea
Why electromagnets are useful
Electromagnets are useful because their magnetism can be switched on and off, and their strength can be changed by changing the current.
Exam technique
In the exam
- For pole questions, compare the two facing poles: like repel, unlike attract.
- For field-line diagrams, state both direction and strength: arrows show direction, line concentration shows strength.
- For current-carrying wires, use the right-hand grip rule and mention that increasing current or moving closer gives a stronger field.
Self review
Check yourself
- Why is an iron nail attracted to both the north pole and the south pole of a permanent magnet?
- What would a plotting compass show near a bar magnet’s north pole?
- Why is the magnetic field inside a solenoid stronger than the field outside it?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
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