Electricity and magnetism, Year 7: more practice

They repel each other

提示Think about what happens to like poles.

为什么Like poles repel and unlike poles attract. This is why two north poles resist being pushed together, even though they are not touching.

Two smaller magnets, each with a north and a south pole

提示Can you ever separate one end from the other?

为什么Every piece of a magnet has both poles, however small the piece. Cutting never separates north from south.

From the north pole to the south pole

提示Lines leave one end and arrive at the other.

为什么The arrows on field lines show the direction a free north pole would be pushed. Outside the magnet, that is away from north and towards south.

Where the field lines are closest together

提示Look at how crowded the drawn arrows are.

为什么Field lines bunch up near the poles, where the magnet's push and pull is strongest, and spread out further away.

A magnetic south pole

提示Unlike poles attract.

为什么The compass needle's north end is attracted to the Earth's pole near the geographic north, so that pole behaves as a magnetic south pole.

The steel affects the compass needle

提示What is the pointer sensitive to?

为什么A compass needle is a small magnet. Steel near it becomes magnetised and its field is stronger than the Earth's weak field, so the needle points the wrong way.

Negatively charged

提示Electrons carry a particular kind of charge.

为什么Electrons are negatively charged. Gaining them leaves the rod with more negative charge than positive, so it is negatively charged.

Electrons

提示Only one kind of charged particle is free to move.

为什么Only electrons move; the protons stay fixed in the atoms. The material that loses electrons ends up positive and the one that gains them negative.

Negative

提示Which kinds of pair push apart?

为什么Only like charges repel. Attraction would not prove anything, because a charged rod also attracts an uncharged one, but repulsion does.

They move towards each other

提示Opposites do something that like charges do not.

为什么Unlike charges attract. The balloons pull together without touching, so the force is a non-contact force.