E-Activity 17

The relay switch

Sometimes it is useful to be able to control the current flowing in one circuit by using a second circuit. This is especially true if the current flowing in the first circuit is large. This can be done by using a relay switch.
In figure 6, when switch A is closed, the iron core of the coil becomes magnetized and attracts the iron armature. As the end of the armature (B) is attracted, its other end pushes the wires at C together, turning the second circuit on.
If switch A is opened, the electromagnet loses its magnetism, the armature returns to its original position and the second circuit is turned off.

The electric bell

When the button is pressed, current flows around the circuit and the electromagnet at E becomes magnetized. The striker arm is attracted towards it and the striker hits the bell. While this is happening, a gap appears at the contacts. The circuit is now incomplete, the electromagnet loses its magnetism and the striker arm moves back to its original position.
The whole process then begins again. The bell continues to ring as long as the button is pressed.

Electromagnetism

When a current flows through a wire, a circular magnetic field is created around it. This field can be seen using either iron filings or compasses.
The field only exists when the current is flowing. If it is turned off, the field disappears if the current flows in the opposite direction, the field changes direction.
Field strength
The field created when current flows in a single piece of wire can be quite weak, but its strength can be increased by
• Increasing the current through the wire,
• Increasing the number of pieces of wire, i.e. making the wire into a coil.

Field shape
The magnetic field around a long coil of current-carrying wire is the same shape as that around a bar magnet.
Electromagnets
If a piece of iron is inserted down the centre of a long coil of wire, this increases the strength of its magnetic field. This combination of iron core and coil is known as an electromagnet. When current flows around the coil, the iron becomes magnetized. When current stops flowing, the iron loses its magnetism
Electromagnets are very useful because
• They can be turned on and off,
• They can be made stronger and weaker.

Magnetic shielding

When the magnet below the wooden table in figure 14 is moved, the footballer above is made to move. If you tried to play this game on a table with a steel or iron top, moving the magnet would have no effect on the footballer. Magnetism is able to pass through non-magnetic materials such as wood, but cannot pass through magnetic materials such as iron or steel. Magnetic materials are often used to shield objects such as sensitive electrical circuits from stray magnetic fields.

Magnets and Magnetism

Making a magnet
If a steel rod is stroked 15-20 times with one end of a bar magnet, the domains within the rod can be made to line up so that they are all pointing in the same direction. The rod is then magnetized.
Magnetic fields
When an object made of iron or steel is placed close to a magnet, it is attracted towards it. This happens because the object is inside the magnet’s magnetic field.
Discovering the shape of a magnetic field
1. Using iron filings
A piece of paper is placed over the magnet. Iron filings are gently sprinkled over the paper. The pattern formed by the filings shows the shape of the magnetic field.
2. Using a compass
A magnet is placed on top of a piece of paper and drawn around. A compass is placed next to the magnet, a circle drawn around it and the direction of the compass needle marked inside the circle. The compass is then moved so that its tail is next to the compass point just drawn. A circle is again drawn around the compass and its direction recorded. This process is repeated all over the paper. The compass needles on the paper will show the shape and direction of the magnetic field.
Magnetic field pattern around a bar magnet
Both methods show that the field pattern around a bar magnet is as given in figure 11Where the field is strong, the lines are drawn close together. Where the field is weak, the lines are drawn well apart.
The earth’s magnetic field
If a bar magnet is suspended so that it is free to rotate, it will eventually come to rest with its north pole pointing northwards and its south pole pointing south wards. The magnet is therefore behaving like a simple compass. Magnets and compasses do this because they are inside the earth’s magnetic field.
It is this magnetic field which has allowed travelers to navigate using a compass.

The domain theory of magnetism

We believe that magnetic materials such as iron and steel have inside them small molecular magnets. These small magnets are contained in tiny cells called domains. Within each domain all the molecular magnets point in the same direction.