Magnets and magnetic fields
Poles push and pull
Every magnet has a north pole and a south pole, where its magnetic effect is strongest.
- Like poles (N and N, or S and S) repel.
- Unlike poles (N and S) attract.
These are non-contact forces: the magnets do not have to touch.
Only a few materials are attracted to a magnet: iron, steel, nickel and cobalt. Copper, aluminium, brass and plastics are not.
Permanent and induced magnets
A permanent magnet makes its own magnetic field all the time.
An induced magnet is a piece of magnetic material that becomes a magnet only while it sits in a field. Hang a steel paper clip from a magnet and it picks up a second clip; take the magnet away and the clips fall apart, because the induced magnetism goes almost straight away.
The force between a magnet and an induced magnet is always attraction. So attraction proves nothing: only repulsion shows that an object is a permanent magnet.
Field lines
The magnetic field is the region round a magnet where a magnetic material or another magnet feels a force.
We draw it with field lines:
- Lines point from N to S outside the magnet: the way a north pole would be pushed.
- Lines never cross.
- Where the lines are close together the field is strong: at the poles. Further away it gets weaker.
The plotting compass and the Earth
A compass needle is a tiny bar magnet that is free to turn.
Its north end points along the field line it sits on. Move it step by step, marking the end it points to each time, and you trace out a field line.
With no magnet nearby, a compass still lines up north to south. That shows the Earth has a magnetic field, made in its core. The needle's north end points to geographic north, so the magnetic pole up there is actually a south pole.
Loading the question
This question is in Brainlag Plus