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Physics geometrical configuration (shape, size, separation) of the system of two conductors. [As we shall see later, it also depends on the nature of the insulator (dielectric) separating the two conductors.] The SI unit of capacitance is 1 farad (=1 coulomb volt ) or 1 F = 1 C V . A capacitor with fixed capacitance is symbolically shown as -- ||--, while the one with variable capacitance is shown as . Equation (2.38) shows that for large C, V is small for a given Q. This means a capacitor with large capacitance can hold large amount of charge Q at a relatively small V. This is of practical importance. High potential difference implies strong electric field around the conductors. A strong electric field can ionise the surrounding air and accelerate the charges so produced to the oppositely charged plates, thereby neutralising the charge on the capacitor plates, at least partly. In other words, the charge of the capacitor leaks away due to the reduction in insulating power of the intervening medium. The maximum electric field that a dielectric medium can withstand without break-down (of its insulating 6roper–1) is called its dielectric strength; for air it is about 3 × 10 Vm . For a separation between conductors of the order of 1 cm or so, this field corresponds to a potential difference of 3 × 10 V between the conductors. Thus, for a capacitor to store a large amount of charge without leaking, its capacitance should be high enough so that the potential difference and hence the electric field do not exceed the break-down limits. Put differently, there is a limit to the amount of charge that can be stored on a given capacitor without significant leaking. In practice, a farad is a very big unit; the most common –6 –9 –12 units are its sub-multiples 1 F = 10 F, 1 nF = 10 F, 1 pF = 10 F, etc. Besides its use in storing charge, a capacitor is a key element of most ac circuits with important functions, as described in Chapter 7. 2.12 T HE P ARALLEL P LATE C APACITOR A parallel plate capacitor consists of two large plane parallel conducting plates separated by a small distance (Fig. 2.25). We first take the intervening medium between the plates to be vacuum. The effect of a dielectric medium between the plates is discussed in the next section. Let A be the area of each plate and d the separation between them. The two plates have charges Q and –Q. Since d is much smaller than the linear dimension of the plates (d << A), we can use the result on electric field by an infinite plane sheet of uniform surface charge density (Section 1.15). Plate 1 has surface charge density = Q/A and plate 2 has a surface charge density – . Using Eq. (1.33), the electric field in different regions is: Outer region I (region above the plate 1), FIGURE 2.25 The parallel plate capacitor. σ σ 68 E = 2ε − 2ε = 0 (2.39) 0 0 Reprint 2026-27