s they all have the same type of charge they tend to repel one another and stand up. In this way it can be seen that a force is exerted between them. Examples like these are quite dramatic and result because the voltages that are involved are very high and can typically be many kilovolts. However, even the comparatively low voltages that are found in electronic circuits exhibit the same effects although to a much smaller degree. The electric field radiates out from any item with an electric potential as shown in Figure 2.1. The electrostatic potential falls away as the distance from the object is increased. Take the example of a charged sphere with a potential of 10 volts. At the surface of the sphere the 18 Newnes Guide to Radio and Communications Technology Field Lines of common potential lines around the charged body Figure 2.1 Field lines and potential lines around a charged sphere Distance from centre of charged sphere Figure 2.2 Variation of potential with distance from the charged sphere Radio waves and propagation 19 electrostatic potential is 10 volts. However, as the distance from the sphere is increased, this potential starts to fall. It can be seen that it is possible to draw lines of equal potential around the sphere as shown in Figure 2.1. The potential falls away as the distance is increased from the sphere. It can be shown that the potential falls away as the inverse of the distance, i.e. doubling the distance halves the potential. The variation of potential with the distance from the sphere is shown in Figure 2.2. The electric field gives the direction and magnitude of the force on a charged object. The field intensity is the negative value of the slope in Figure 2.2. The slope of a curve plotted on a graph is the rate of change of a variable. In this case it represents the rate of change of the potential with distance at a particular point. This is known as the potential gradient. It is found that the potential gradient varies as the inverse square of the distance. In other words doubling the di