Revision notes for OCR GCSE Physics Magnets and magnetic fields. Open the guide for explanations and worked examples. Written against the OCR GCSE Physics (J249) 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, and how to show this with field-line diagrams.
- The difference between permanent magnets and induced magnets.
- How compasses give evidence that Earth has a magnetic field.
- How electric current in a wire or solenoid creates a magnetic field.
The starting point: magnets and poles
A magnet is an object that produces a magnetic field and can exert a non-contact force on magnetic materials or other magnets.
Most everyday magnets have two ends where the magnetic effect is strongest: a north pole and a south pole.
Definition
Magnetic pole
A magnetic pole is a region of a magnet where the magnetic force is strongest. Every bar magnet has a north pole and a south pole.
The basic rule is simple:
Key Idea
Pole rule
Unlike poles attract: north and south pull together. Like poles repel: north and north, or south and south, push apart.
Not all metals are magnetic. The main magnetic materials you need to know are iron, steel, nickel and cobalt.
Example
Predicting attraction or repulsion
Two bar magnets are brought close together. The left magnet has its south pole facing the right magnet’s north pole.
- Identify the two poles that are facing each other: one is south and one is north.
- Compare them using the pole rule: south and north are unlike poles.
- Therefore, the magnets attract and move towards each other if they are free to move.
Common Mistake
Bigger does not always mean stronger
A larger magnet is not automatically stronger. Magnetic strength depends on the material, how strongly it has been magnetised, and the field it produces. In diagrams, a stronger field is shown by field lines that are closer together.
Permanent and induced magnets
A permanent magnet keeps its magnetism all the time. It produces its own magnetic field without needing another magnet nearby.
Definition
Permanent magnet
A permanent magnet is a magnet that produces its own magnetic field and keeps its magnetism for a long time.
An induced magnet is different. It becomes magnetic only because it is placed in a magnetic field. For example, an iron nail can become magnetised when it is close to a bar magnet.
Definition
Induced magnet
An induced magnet is a material that becomes magnetic when it is placed in a magnetic field. Its magnetism is usually temporary.
Induced magnets are attracted to permanent magnets because the end nearest the permanent magnet becomes the opposite pole. When the permanent magnet is removed, the induced magnet may lose most or all of its magnetism.
Example
Identifying an induced magnet
An iron paperclip is picked up by a bar magnet. When the bar magnet is removed, the paperclip stops picking up other paperclips.
- The paperclip only behaved like a magnet while it was near the bar magnet.
- Its magnetism disappeared when the external magnetic field was removed.
- So the paperclip was acting as an induced magnet, not a permanent magnet.
Magnetic fields around magnets
A magnetic field is the region around a magnet where another magnet or magnetic material experiences a force.
Definition
Magnetic field
A magnetic field is a region where a magnetic material or another magnet experiences a force.
We draw magnetic fields using field lines. These are not real lines in space — they are a model that helps us show the field’s direction and strength.
Direction of field lines
Outside a bar magnet, magnetic field lines go from the north pole to the south pole.
A small plotting compass placed in the field lines up with the field. The north-seeking end of the compass needle points in the direction of the magnetic field.
Strength of the field
The field is strongest where the field lines are closest together. For a bar magnet, this is near the poles.
Further away from the magnet, the field lines are more spread out, so the magnetic field is weaker.
Example
Interpreting a field-line diagram
A point A is close to the north pole of a bar magnet, where the field lines are close together. A point B is far away, where the field lines are spread out.
- Compare the field-line spacing: the lines are closer together at A than at B.
- Closer field lines mean a stronger magnetic field.
- So the magnetic field is stronger at A, and a compass there would experience a larger turning effect.
Tip
How to investigate a magnetic field
Place a plotting compass near the magnet, mark the direction of the needle, then move the compass and repeat. Joining the arrows shows the field pattern. Iron filings show the shape of the field, but a compass is better for showing direction.
Compasses and Earth’s magnetic field
A compass contains a tiny magnet that is free to turn. Because it lines up in a particular direction, it gives evidence that Earth has a magnetic field.
A dipping compass is a compass needle that can tilt up or down as well as turn horizontally. This shows that Earth’s magnetic field has a vertical component, not just a horizontal one.
Earth behaves as if it has a giant magnet inside it. This is evidence that Earth’s core must be magnetic. The actual cause is linked to movement of magnetic material in the Earth’s outer core, but at GCSE the key point is that compass behaviour shows Earth has a magnetic field.
Common Mistake
Geographic north is not exactly magnetic north
The geographic poles are based on Earth’s rotation axis. The magnetic poles are not in exactly the same places, so compass north and true geographic north are slightly different.
Magnetic field around a current-carrying wire
An electric current is a flow of charge. When current flows through a conductor, such as a metal wire, it produces a magnetic field around the wire.
Key Idea
Current creates magnetism
A current-carrying wire has a magnetic field around it. If the current stops, the magnetic effect stops.
You can show this by passing a wire through a card and placing plotting compasses around it. When the current is switched on, the compasses line up in circles around the wire.
The field around a straight current-carrying wire forms concentric circles centred on the wire.
Direction around the wire
Use 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 around the wire.
In diagrams, a dot often means current coming out of the page towards you, like the tip of an arrow. A cross means current going into the page, like the tail feathers of an arrow.
Example
Using the right-hand grip rule
A straight wire carries conventional current out of the page.
- Point your right thumb out of the page, towards yourself.
- Look at the way your fingers curl around the wire.
- Your fingers curl anticlockwise, so the magnetic field circles anticlockwise around the wire.
What affects the field around a wire?
The strength of the magnetic field around a current-carrying wire depends on two things:
- Current: a larger current produces a stronger magnetic field.
- Distance from the wire: the field gets weaker as you move further from the wire.
There is no GCSE equation you need to use here for this spec point — it is a qualitative relationship.
Example
Comparing magnetic field strength around wires
Two identical straight wires carry current. Wire A carries 1 A and wire B carries 3 A. You compare the field at the same distance from each wire.
- The distance from each wire is the same, so distance is not causing the difference.
- Wire B has the larger current.
- Therefore, wire B produces the stronger magnetic field at that distance.
Common Mistake
When two factors change
If one wire has a larger current but you measure much further away from it, GCSE information may not be enough to decide which field is stronger. Compare one factor at a time unless the question gives extra data.
Solenoids and electromagnets
A solenoid is a coil of wire. When current flows through it, each turn of wire produces a magnetic field. These fields add together, making the total field much stronger than the field around one straight wire.
Definition
Solenoid
A solenoid is a coil of insulated wire that produces a magnetic field when current flows through it.
Inside a solenoid, the field lines are close together and nearly parallel. This means the magnetic field inside is strong and fairly uniform. Outside, the field pattern looks similar to the field around a bar magnet, with a north pole at one end and a south pole at the other.
An electromagnet is a magnet made using electric current. A solenoid with an iron core is a common electromagnet.
You can make the magnetic effect stronger by:
- increasing the current
- increasing the number of turns of wire per metre
- adding an iron core inside the solenoid
Example
Improving an electromagnet
A student wants a solenoid to pick up more steel paperclips.
- To increase the magnetic field, they can increase the current through the coil.
- They can also add more turns of wire in the same length, so the fields from more loops add together.
- Adding a soft iron core strengthens the field further, so the electromagnet can attract more paperclips.
Common Mistake
Solenoids do not store magnetism like a battery
A solenoid is magnetic because current is flowing through it. Switch off the current and the magnetic field mostly disappears, especially if it uses a soft iron core.
Exam technique
In the exam
- For magnet questions, always name the facing poles first, then apply: like repel, unlike attract.
- For field diagrams, use arrows from north to south outside a magnet, and show stronger fields with closer lines.
- For current-carrying wires and solenoids, mention both direction and strength: reversing current reverses the field direction, while increasing current makes the field stronger.
Self review
Check yourself
- Why are magnetic field lines closer together near the poles of a bar magnet?
- How can a plotting compass show the direction of a magnetic field?
- What three changes can make a solenoid’s magnetic field stronger?
Recap questions
Test yourself with 5 quick questions on this guide. Answer them all correctly to complete it.
Magnetism and magnetic fields
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