Permanent & Induced Magnetism, Magnetic Forces & Fields (AQA GCSE Physics): Revision Notes
Magnets and magnetic fields
What are magnets?
Certain materials can become magnetised. They can be permanent magnets or induced magnets.
All magnets have two main properties:
Fundamental Rules of Magnetism:
- Like magnetic poles repel (push away from each other)
- Unlike magnetic poles attract (pull towards each other)
Magnetic fields
A magnetic field is the area around a magnet where it can affect other magnetic materials.
Key facts about magnetic fields:
- The magnetic field is strongest at the poles
- Field lines are drawn closer together where the field is stronger
- The magnetic field gets weaker the further you go from the poles
- Magnetic field lines always go from N to S (North to South)
When two magnets are brought close together, they exert a force on each other. This happens even when they don't touch - this is called a non-contact force.
Non-contact forces are fascinating because they demonstrate how magnetic fields can transmit forces through empty space. This principle is fundamental to understanding how magnets work and is the basis for many technological applications.
Permanent magnets vs induced magnets
Permanent magnets
- A permanent magnet produces its own magnetic field
- It stays magnetic all the time
- Can attract and repel other magnets
Induced magnets
- An induced magnet only becomes magnetic when placed in a magnetic field
- When removed from the magnetic field, an induced magnet loses most of its magnetism immediately
Critical Difference: Induced magnetism always causes attraction - never repulsion. This is a key distinction that helps identify the type of magnet you're dealing with.
Worked Example: Paper Clips and Magnetism
When you hold a permanent magnet near iron paper clips:
- The paper clips become temporarily magnetised (induced magnetism)
- They are attracted to the permanent magnet
- The paper clips can even attract other paper clips in a chain
- When you remove the permanent magnet, the paper clips lose their magnetism and fall apart
This demonstrates induced magnetism in action.
Magnetic materials
At room temperature, magnetic materials include:
- Iron
- Cobalt
- Nickel
- Steel
Uses of magnets
Magnets have many practical uses:
Permanent magnets:
- Fridge magnets
- Compasses
- Motors and generators
- Loudspeakers
- Door closers on fridges
Induced magnets:
- Electromagnets
- Circuit breakers
- Electric bells
- Magnetic relays
Plotting compasses
A plotting compass contains a small bar magnet. The compass needle points in the direction of the Earth's magnetic field.
Key facts about Earth's magnetism:
- The Earth has a magnetic field
- The compass needle shows the direction of the magnetic field at that point
- The compass needle points towards the Earth's magnetic north pole
- This proves that the Earth's core is magnetic
The Earth acts like a giant magnet with its own magnetic field. This field extends far into space and protects us from harmful solar radiation. The magnetic north pole is actually slightly different from the geographic north pole, which is why compass readings need to be adjusted in navigation.
Testing for magnetic materials
You can tell if a material is a permanent or induced magnet by testing how it behaves with other magnets.
Worked Example: Identifying Magnet Types
To test an unknown magnetic object:
Step 1: Bring a known magnet close to the object
- If it's only attracted → it's an induced magnet
- If it can be both attracted AND repelled → it's a permanent magnet
Step 2: Try to repel the object
- If you can repel it with either pole of your test magnet → it's definitely a permanent magnet
- If it's always attracted regardless of which pole you use → it's an induced magnet
Key Testing Rule: If you can repel the material with a magnet, it must be a permanent magnet. Induced magnetic materials will always be attracted by a magnet, while permanent magnets can be both attracted and repelled by other magnets.
Key Points to Remember:
- Like poles repel, unlike poles attract - this is fundamental
- Magnetic field lines always go from North to South
- Permanent magnets make their own field and can repel other magnets
- Induced magnets only work in magnetic fields and are always attracted
- Magnetic materials include iron, cobalt, nickel and steel
- The Earth itself acts as a giant magnet with its own magnetic field