78Fundamental Electrical and Electronic Principlesbodies is occupied by the electric ﬂ ux, so there are no ‘ gaps ’ in which a charged particle would not be affected. 4 The lines of force (ﬂ ux) radiate outwards from the surface of a positive charge and terminate at the surface of a negative charge. 5 The lines always leave (or terminate) at right angles to a charged surface. 6 Although the lines drawn on a diagram do not actually exist as such, they are a very convenient way to represent the existence of the electric ﬁ eld. They therefore aid the understanding of its properties and effects. 7 Since force is a vector quantity any line representing it must be arrowed. The convention used here is that the arrows point from the positive to the negative charge. It is evident from Fig. 3.3 that the spacing between the lines of ﬂ ux varies depending upon which part of the ﬁ eld you consider. This means that the ﬁ eld shown is non-uniform. A uniform electric ﬁ eld may be obtained between two parallel charged plates as shown in Fig. 3.4 . QQ Fig. 3.4 Note that the electric ﬁ eld will exist in all of the space surrounding the two plates, but the uniform section exists only in the space between them. Some non-uniformity is shown by the curved lines at the edges (fringing effect). At this stage we are concerned only with the uniform ﬁ eld between the plates. If a positively charged particle was placed between the plates it would experience a force that would cause it to move from the positive to the negative plate. The value of force acting on the particle depends upon what is known as the electric ﬁ eld strength. 3.3 Electric Field Strength (E) This is deﬁ ned as the force per unit charge exerted on a test charge placed inside the electric ﬁ eld. (An outdated name for this property is ‘ electric force ’ ).