The magnetic field
Why a compass points north
- A compass needle swings round and settles pointing north.
- It is feeling the Earth's magnetic field.
- A magnetic field is any region where a moving charge feels a force.
What a magnetic field is
- A magnetic field acts on moving charges and currents — not on charges at rest.
- A still charge feels nothing; a moving one can feel a sideways push.
Practice
A magnetic field is a region where:
Magnetic forces act only on moving charges (and currents) — a charge at rest feels nothing.
What makes one
- Moving charges (a current in a wire) make a magnetic field.
- So do permanent magnets — from tiny atomic currents inside them.
Practice
Which of these produce a magnetic field?
Moving charges and currents make magnetic fields; a permanent magnet does so via atomic currents. A still charge makes only an electric field.
Field lines
- Outside a magnet, field lines run from N to S (and S to N inside, forming closed loops).
- Lines never cross; closer lines mean a stronger field.
Practice
Outside a magnet, the field lines run:
Lines leave the N pole and enter the S pole outside, forming closed loops through the magnet.
Practice
Magnetic field lines never cross.
If they crossed, the field would point two ways at once — impossible. Closer lines just mean a stronger field.
Practice
Field lines drawn closer together mean a ____ field.
Line spacing shows strength — closer lines, stronger field.
You've got it
Key idea
- a magnetic field acts on moving charges and currents (not charges at rest)
- it is made by currents and permanent magnets
- field lines go N to S outside, never cross; closer = stronger