What is Capacitance (C)

Chapter 3.6 Capacitance (C)

Fundamental Electrical and Electronic Principles Third Edition Book
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Fundamental Electrical and Electronic Principles Third Edition Book

  • Electric Fields and Capacitors 83 Worked Example 3.3 Q Two parallel plates separated by a dielectric of thickness 3 mm acquire a charge of 35 mC when connected to a 150 V source. If the eff ective csa of the fi eld between the plates is 144 mm 2 , calculate (a) the electric fi eld strength and (b) the fl ux density. A d 3 10 3 m; Q 35 10 3 C; V 150 V; A 144 10 6 m 2 (a) EEAnsVd volt/metreso kV/m 115030506 (b) DQAD coulom /metreso C/m b23623504402431111.Ans 3.6 Capacitance ( C ) We have seen that in order for one plate to be at a different potential to the other one there is a need for a charge. This requirement is known as the capacity of the system. For a given system the ratio of the charge required to achieve a given p.d. is a constant for that system. This is called the capacitance (C) of the system i.e. faradCQV(3.7)or coulombQVC(3.8) From equation (3.7) it may be seen that the unit for capacitance is the farad (F). This is defi ned as the capacitance of a system that requires a charge of one coulomb in order to raise its potential by one volt. The farad is a very large unit, so in practice it is more usual to express capacitance values in microfarads ( F), nanofarads (nF), or picofarads (pF). Worked Example 3.4 Q Two parallel plates, separated by an air space of 4 mm, receive a charge of 0.2 mC when connected to a 125 V source. Calculate (a) the electric fi eld strength between the plates, (b) the csa of the fi eld between the plates if the fl ux density is 15 C/m 2 , and (c) the capacitance of the plates. A d 4 10 3 m; Q 2 10 4 C; V 125 V; D 15 C/m 2