7.1 Permanent and induced magnetism, magnetic forces and fields — Practise with help

1. A contact force acts when two objects touch, for example friction or a push. A non-contact force acts without touching, for example gravity or a magnetic force.
2. $a = \dfrac{F}{m} = \dfrac{12}{3.0} = 4.0\,\text{m/s}^2$.

If question 1 went wrong, look again at Section 2.1. If question 2 went wrong, rearrange $F = ma$ before you put the numbers in.

The two poles are the same type. Decide whether two same-type poles push or pull.

Name the two poles as like poles, then say what like poles do.

The magnets repel each other. A south pole and a south pole are like poles, and two like poles repel. The force acts without contact, so the magnets push apart before they touch. This is a model answer for you to compare with your own wording.

Think about what the field of the bar magnet does to the magnetic parts inside the steel.

Use the phrase induced magnet. Say what the field does to the nail, and what kind of force induced magnetism causes.

The steel nail becomes an induced magnet when it is placed in the field of the bar magnet. The field lines up the magnetic parts inside the steel. Induced magnetism always causes a force of attraction, so the nail is pulled towards the magnet. When the magnet is taken away, the nail loses most of its magnetism.

Ask whether aluminium is a magnetic material.

Name the four magnetic materials, then say whether aluminium is one of them.

Aluminium is not a magnetic material. The magnetic materials are iron, steel, cobalt and nickel. A magnet attracts a magnetic material and also attracts another magnet, but it does not attract aluminium, so the can is not pulled.

The needle is a small bar magnet. A bar magnet lines up only when it is in a magnetic field.

Say that the Earth must have a magnetic field, then say what the field must come from.

The compass needle is a small bar magnet, and it lines up only when it is in a magnetic field. The needle points north wherever it is used on the Earth's surface, so the Earth must have a magnetic field. The field must come from magnetic material deep inside the Earth, which is evidence that the core of the Earth is magnetic.

A field line is found one compass position at a time. The compass has to be moved forwards along the line.

Start the compass near one pole. Mark where the needle points. Move the compass so its south end sits at the mark.

Put the magnet on paper and mark its outline. Put the compass near one pole and mark the two ends of the needle. Move the compass so that its south end sits where the north end was, and mark the new position of the north end. Repeat until the marks reach the other pole, then join the marks with a smooth curve. Start from a different place to plot another line.

Look at how the field is described around a magnet.

Name the parts of the magnet where the field lines are closest together, then say what close lines mean.

The field is strongest at the poles, which are the two ends of the magnet. The field lines are closest together there. The field gets weaker as the distance from the magnet increases, so it is weakest far from the magnet.

One metre is 100 cm. Decide whether the number in metres should be larger or smaller than 8.0.

Divide 8.0 by 100.

$8.0\,\text{cm} = \dfrac{8.0}{100}\,\text{m} = 0.080\,\text{m}$.

The magnetic force is the resultant force on the ball. Use Newton's second law.

Write $a = \dfrac{F}{m}$, then put in $F = 0.40$ N and $m = 0.10$ kg.