Magnets and magnetic fields (AQA GCSE Physics Combined Science): Revision Notes
Magnets and magnetic fields
What are magnets?
Magnets are special objects that can attract certain materials. They have two ends called poles - a North pole and a South pole. Understanding how magnets behave is key to learning about magnetism.
The basic rule of magnets is simple:
- Like poles repel (North repels North, South repels South)
- Unlike poles attract (North attracts South)
This happens because of an invisible magnetic field around every magnet. This field is strongest at the poles and gets weaker as you move further away.
The magnetic field around a magnet is completely invisible to our eyes, but it's very real! You can actually see evidence of it by sprinkling iron filings around a magnet - they'll arrange themselves along the invisible field lines.
Magnetic fields
Every magnet creates an invisible area of influence called a magnetic field. You can think of this field as having invisible lines that:
- Always flow from North to South poles
- Are closest together at the poles (where the field is strongest)
- Spread out further away from the magnet (where the field is weaker)
When two magnets are brought close together, they push or pull on each other without touching. This is called a non-contact force because the magnets don't need to physically touch to affect each other.
Remember: Magnetic forces can act through empty space and even through non-magnetic materials like paper, plastic, or glass. This is what makes magnetic fields so useful in many applications!
Permanent magnets vs induced magnets
There are two main types of magnets you need to know about:
Permanent magnets make their own magnetic field all the time. They stay magnetic even when no other magnets are nearby. Common permanent magnets include bar magnets and horseshoe magnets.
Induced magnets are different. These are materials that only become magnetic when they're placed in a magnetic field created by another magnet. The moment you take them away from the magnetic field, they lose most or all of their magnetism immediately.
Induced magnetism always causes attraction. For example, iron paper clips will always be attracted to a permanent magnet, even though the clips aren't permanent magnets themselves.
Worked Example: Induced Magnetism in Action
Step 1: Take an iron nail (not magnetised) and some paper clips Step 2: Touch the nail to the paper clips - nothing happens Step 3: Hold a permanent magnet near the nail Step 4: The nail becomes an induced magnet and attracts the paper clips Step 5: Remove the permanent magnet - the nail loses its magnetism and drops the clips
Magnetic materials
Only certain materials can become magnetic. At room temperature, the main magnetic materials are:
- Iron
- Cobalt
- Nickel
- Steel (which contains iron)
Remember these with the word "COIN" - Cobalt, O (not used), Iron, Nickel - plus steel. This mnemonic makes it easy to recall the main magnetic materials in exams!
Uses of magnets
Magnets have many practical uses in everyday life:
Permanent magnets are used in:
- Fridge door seals
- Compasses
- Motors and generators
- Loudspeakers
Induced magnets (electromagnets) are used in:
- Circuit breakers
- Electric bells
- Magnetic relays
- Some door closers on fridges
Plotting compasses
A plotting compass contains a tiny bar magnet (the needle) that can rotate freely. Because Earth has its own magnetic field, the compass needle always points towards magnetic North.
The compass needle shows the direction of Earth's magnetic field at any location. This proves that Earth's core contains magnetic materials and acts like a giant magnet.
Earth's magnetic North pole is actually slightly different from its geographic North pole - they're about 11 degrees apart! This is why navigators sometimes need to account for "magnetic declination" when using compasses.
How to tell permanent and induced magnets apart
Sometimes you need to work out whether a magnetic material is permanent or induced:
- Induced magnetic materials will always be attracted to any magnet you bring near them
- Permanent magnets can be both attracted to and repelled by other magnets (depending on which poles you bring together)
Key Test: If you can make a magnetic material repel another magnet, you know it must be a permanent magnet. Induced magnets can never repel - they can only attract.
Key Points to Remember:
- Like magnetic poles repel, unlike poles attract
- Magnetic field lines always go from North to South poles
- Permanent magnets make their own magnetic fields, induced magnets only become magnetic in another magnet's field
- The main magnetic materials are iron, cobalt, nickel and steel (remember "COIN")
- Compasses work because Earth has its own magnetic field
- Induced magnets always attract, permanent magnets can attract or repel