102Fundamental Electrical and Electronic Principles A C 270 10 12 F; A 20 3 10 4 m 2 ; ε r 2.1; E 350 10 3 V/m (a) CAddACεεεεoror faradso, metre8 854026002724..111100004 32111thus, mm d.Ans (b) Dielectric strength is the same thing as electric fi eld strength, expressed in volt/metre, so EEVdVdV volt/metreand voltso, 35000 4 3044 6331111..VV Ans Note: This fi gure is the voltage at which the dielectric will start to break down, so, for practical purposes, the maximum working voltage would be specifi ed at a lower value. For example, if a factor of safety of 20% was required, then the maximum working voltage in this case would be specifi ed as 115 V. 3.18 Capacitor Types The main difference between capacitor types is in the dielectric used. There are a number of factors that will inﬂ uence the choice of capacitor type for a given application. Amongst these are the capacitance value, the working voltage, the tolerance, the stability, the leakage resistance, the size and the price. Tolerance is the deviation from the nominal value. This is normally expressed as a percentage. Thus a capacitor of nominal value 2 F and a tolerance of 10%, should have an actual value of between 1.8 and 2.2 FSince C ε A/d , then any or all of these factors can be varied to suit particular requirements. Thus, if a large value of capacitance is required, a large csa and/or a small distance of separation will be necessary, together with a dielectric of high relative permittivity. However, if the area is to be large, then this can result in a device that is unacceptably large. Additionally, the dielectric cannot be made too thin lest its dielectric strength is exceeded. The various capacitor types overcome these problems in a number of ways.