Fundamental Electrical and Electronic Principles, Third Edition

Electric Fields and Capacitors 107Assignment Questions1 Two parallel plates 25 cm by 35 cm receive acharge of 0.2 μ C from a 250 V supply. Calculate(a) the electric flux and (b) the electric fluxdensity.2 Th e flux density between two plates separatedby a dielectric of relative permittivity 8 is1. 2 μ C/m2. Determine the potential gradientbetween them.3 Calculate the electrical field strength betweena pair of plates spaced 10 mm apart when ap.d. of 0.5 kV exists between them.4 Two plates have a charge of 30 μ C. If theeffective area of the plates is 5 cm 2 , calculatethe flux density.5 A capacitor has a dielectric 0.4 mm thick andoperates at 50 V. Determine the electric fieldstrength.6 A 10 0 μ F capacitor has a p.d. of 400 V across it.Calculate the charge that it has received.7 A 4 7 μ F capacitor stores a charge of 7.8 mCwhen connected to a d.c. supply. Calculate thesupply voltage.8 Determine the p.d. between the plates ofa 470 nF capacitor if it stores a charge of0.141 mC.9 Calculate the capacitance of a pair of plateshaving a p.d. of 600 V when charged to 0.3 μ C.10 The capacitance of a pair of plates is 40 pFwhen the dielectric between them is air. If asheet of glass is placed between the plates(so that it completely fills the space betweenthem), calculate the capacitance of the newarrangement if the relative permittivity of theglass is 6.11 A dielectric 2.5 mm thick has a p.d. of 440 Vdeveloped across it. If the resulting flux densityis 4.7 μ C/m 2 determine the relative permittivityof the dielectric.12 State the factors that affect the capacitance ofa parallel plate capacitor, and explain how thevariation of each of these factors affects thecapacitance.Calculate the value of a two plate capacitorwith a mica dielectric of relative permittivity 5and thickness 0.2 mm. The effective area of theplates is 250 cm2 .13 A capacitor consists of two plates, each ofeffective area 500 cm2, spaced 1mm apart inair. If the capacitor is connected to a 400 Vsupply, determine (a) the capacitance,(b) the charge stored and (c) the potentialgradient.14 A paper dielectric capacitor has two plates,each of effective csa 0.2 m2. If the capacitanceis 50 nF calculate the thickness of the paper,given that its relative permittivity is 2.5.15 A two plate capacitor has a value of 47 nF. If theplate area was doubled and the thickness ofthe dielectric was halved, what then would bethe capacitance?16 A parallel plate capacitor has 20 plates, each50 mm by 35 mm, separated by a dielectric0.4 mm thick. If the capacitance is 1000 pFdetermine the relative permittivity of thedielectric.17 Calculate the number of plates used in a0.5 nF capacitor if each plate is 40 mm square,separated by dielectric of relative permittivity6 and thickness 0.102 mm.18 A capacitor is to be designed to have acapacitance of 4.7 pF and to operate with a p.d.of 120 V across its terminals. The dielectric is tobe teflon ( ε r 2.1) which, after allowing fora safety factor has a dielectric strength of25 kV/m. Calculate (a) the thickness of teflonrequired and (b) the area of a plate.19 Capacitors of 4 μ F and 10 μ F are connected(a) in parallel and (b) in series. Calculate theequivalent capacitance in each case.20 Determine the equivalent capacitance whenthe following capacitors are connected (a) inseries and (b) in parallel(i) 3 μF, 4 μ F and 10 μ F(ii) 0.02 μ F, 0.05 μ F and 0.22 μ F(iii) 20 pF and 470 pF(iv) 0.01 μ F and 220 pF.21 Determine the value of capacitor which whenconnected in series with a 2 nF capacitorproduces a total capacitance of 1.6 nF.22 Th re e 15 μ F capacitors are connected in seriesacross a 600 V supply. Calculate (a) the totalcapacitance, (b) the p.d. across each and (c) thecharge on each.23 Three capacitors, of 6 μF, 8 μ F and 10 μ Frespectively are connected in parallel across a60 V supply. Calculate (a) the total capacitance,