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Capacitance

  • Capacitors are used to store charge.
  • The capacitance of a capacitor is the value that tells you how good a capacitor is at storing charge

Capacitance = Charge stored on a capacitor / Voltage across the capacitor

  • Capacitance (C) = Q / V
  • Q = Coulombs (c)
  • V = Volt
  • Coulomb / Volt = Farad (F)

Capacitors store Electrical Potential Energy:

  • PEelectric = QV

But, Eof a capacitor = 1/2 QV

  • Not all of the charges drop through the voltage V
  • As charge decreases, the voltage across the capacitor decreases
  • Only the first charge drops through voltage V

 

Here we have two plates with charge of -6, and +6; denoted with the plus and minus symbols.

1

  • The electrons are going to move to the positive plate
  • Now, on the first go, the positive plate has 6/6 = 100%

2

  • On the third one, the positive plate only has 4/6 at the ‘time of departure’, which is 66%

3

  • The fifth one has 2/6 = 33%, and so on:

4

  • Thus, E = 1/2 QV

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