ABSTRACT

At the end of this chapter, you should be able to:

• describe an electrostatic field

• define electric field strength E and state its unit

• define capacitance and state its unit

• describe a capacitor and draw the circuit diagram symbol

• perform simple calculations involving C = Q V

and Q = It

• define electric D and state its unit • define between ε0, εr and ε

• perform simple calculations involving D = Q A

, E = V d

and D E

= ε0 εr • understand that for a parallel plate capacitor, C = ε0εrA(n − 1)

d • perform calculations involving capacitors connected in parallel and in series • define dielectric strength and state its unit

• state that the energy stored in a capacitor is given by W = 1 2

CV 2 joules • describe practical types of capacitor • understand the precautions needed when discharging capacitors • appreciate the property of inductance • define inductance L and state its unit

• appreciate that e.m.f. E = −N d dt

= −L dI dt

• calculate induced e.m.f. given N , t, L, change of flux or change of current • appreciate factors which affect the inductance of an inductor • draw the circuit diagram symbols for inductors

• calculate the energy stored in an inductor using W = 1 2

LI2 joules

• calculate inductance L of a coil, given L = N I

A capacitor is a device capable of storing electrical energy. Next to the resistor, the capacitor is the most commonly encountered component in electrical circuits. Capacitors are used extensively in electrical and electronic circuits. For example, capacitors are used to smooth rectified a.c. outputs, they are used in telecommunication equipment – such as radio receivers – for tuning to the required frequency, they are used in time delay circuits, in electrical filters, in oscillator circuits, and in magnetic resonance imaging (MRI) in medical body scanners, to name but a few practical applications. Figure 41.1 shows a capacitor consisting of a pair of parallel metal plates X and Y separated by an insulator, which could be air. Since the plates are electrical conductors each will contain a large number of mobile electrons. Because the plates are connected to a d.c. supply the electrons on plate X , which have a small

the supply and will be repelled from the negative pole of the supply on to plate Y . X will become positively charged due to its shortage of electrons whereas Y will have a negative charge due to its surplus of electrons.