1 Road Safety And Automobile Association Patrol In Great Britain

ELECTROMAGNETISM


Magnetic fields can be produced by electric currents. The current produces a magnetic field around the conductor. If a small magnetic compass is placed near the wire, and if it is moved along in the. direction in which the needle points, then the compass will trace out a circle around the wire. The magnetic field appears to be arranged in a circular manner about the wire.
If the current is switched off, the compass needle will not be able to detect any magnetism near the wire {except the earth's field). The magnetic field was due to the current itself. If the current is switched off, the magnetic field ceases to exist. If a current is passed along the wire again, a magnetic field again exists around the wire. Because the magnetism is produced by an electric current it is said to be electromagnetism.
If the conductor, say a copper wire, is coiled into a long coil and a current is switched on, the shape of the magnetic field becomes different. The dashed lines, which show the shape of the field, are called magnetic flux lines. The arrows show the direction in which the north-seeking pole of a compass needle would point if it were placed in that position.
If the current is switched off, the magnetic field ceases to exist.
It shows the shape of a magnetic field around a permanent bar magnet. If this magnet is moved over to the coil in it, the two magnetic fields will affect each other. If the N pole of the magnet is taken close to the N pole of the coil field, the magnet and the coil will repel each other.
If the S pole of the magnet is placed near the N pole of the coil field, the magnet and the coil will attract each other.
The magnet can exert a force on the coil, although they are not touching each other. Looked at the other way round, the coil can exert a force on the magnet, even though they don't touch each other.

Topic # 14

LIGHT AND LIGHTING


Light is electromagnetic radiation that is detected by the eye. In wavelengths, it ranges from about 400 nanometers to about 700 nanometers. Figure 1 shows this. The eye sees different wavelengths as different colors, the shortest wavelengths giving sensations of violet and blue, and the longest wavelengths giving sensations of red.
The graph in figure 1 shows that the sensitivity of the eye is different for different wavelengths. It is most sensitive to wavelengths of about 555 nm, for most people; and this is in the green-yellow part of the spectrum.
Light enables us to see our surroundings. When we are at work we obtain information through our senses, mainly sight, hearing, and touch. Of the information which we receive, about 80% comes through sight. Light and lighting are therefore very important to us. We see objects either by the light, which they give out, for example fires and electric lamps, or by the light, which they reflect. Most objects around us do not give out light. Most of the objects around us are seen by us because they reflect light into our eyes. The light has to come from somewhere. During the daytime it usually comes from the sun; in very cloudy weather and at night, the light will have to come from a specially made source, like a candle, a paraffin lamp, domestic electrical lamp bulb, or an electric street light. Because we depend very much on our sight, and because the sun does not always provide us with light, we depend very much on lamps and lighting systems, which have been made by people. Lighting engineers devise and make lamps and lighting arrangements. Homes, offices, supermarkets, factories, hospital operating theatres, sports grounds, city streets and motorways each need different sorts of lighting. By finding out how visible radiation can be produced, how it behaves, and how it can be controlled, lighting engineers are able to provide suitable lighting for all of these needs. The engineers also develop new types of lighting to meet new needs.

Topic # 15