Magnetism

Magnetism A magnet is an object that has the ability to attract iron and has an external magnetic field. Most familiar magnets are permanent magnets made of steel or some other alloy. Electromagnets are temporary magnets – they lose their magnetism immediately the direct electric current is switched off. Permanent magnets hold their magnetism for a long time and do not depend on continuous electrical input for their magnetism.

Magnetism is a Natural Phenomenon The Earth is a magnet – it attracts iron and has an external magnetic field. Some types of rocks are magnets e.g. magnetite. During the slow cooling of magma the iron minerals that crystallize out early are turned facing the same direction under the influence of the Earth’s magnetism. Magnetism has been detected in other planets. Some creatures use magnets for navigation e.g. pigeons.

Forces of Attraction and Repulsion

Like magnetic poles repel each other Unlike magnetic poles attract each other.

(i) Show Attraction and Repulsion Between Magnets

(a) Hang a bar magnet by a thread at its middle.

(b) Give it a gentle push at one end to spin it and allow it to come to rest.

(c) Note that it comes to rest pointing roughly in a north-south direction.

(d) Mark the end that points to the north.

(e) Repeat the action many times noting the direction of rest and the position of the marked end.

Results: when the magnet comes to rest it is always pointing north-south,

ii) Test a Variety of Materials for Magnetism

Bring the north pole of a bar magnet towards the object. Record if it is repelled, attracted or not affected.

Then bring the south pole of a bar magnet towards the same part of the object. Record if it is repelled, attracted or not affected. If the object is not affected by the magnetic poles of the magnet then the material is not a magnetic material. If the object is attracted by the north and south poles it is composed of a magnetic substance. If the object is repelled by either the north or south pole of the magnet then the object is a magnet.

A Magnetic Field is the space around a magnetic where a magnetic force can be detected.

To plot the outline of magnetic field lines, using a compass, around a Magnet.

1 Place a magnet in the centre of a large sheet of paper, draw the outline of the magnet!

2 At a point on the side near a pole make a mark. From this point place the compass

3 Place a dot at the point on the edge of the compass that is where the needle points!

4 Repeat the previous 2 steps placing the back of the needle on the point last made

Earth’s Magnetic Field

The Earth is a magnet because it has a magnetic field. The pattern of the magnetic field of the Earth is similar to that of a bar magnet. It would appear that there is a bar magnet just slightly at an angle to the north-south rotational axis of the Earth. In Ireland the angle is 7° west of north.

The Magnetic Compass is an instrument that indicates direction A magnetic compass is a small magnet free to turn horizontally and indicates direction by responding to the earth’s magnetic field.

Magnetism Has Many Useful Applications

1. Electric motors and generators (dynamos) use permanent magnets. 2. Door bells use electromagnets.

3. Loudspeakers, telephone earpieces and the ESB meter use permanent magnets.

4. Magnets are used to remove steel splinters from wounds and eye in industrial mishaps.

5. Very strong magnets are used for body scans e.g. MRI scanning equipment.