Charging a capacitor, Physics

Charging a capacitor:

In the diagram below, all of the resistance in the circuit is added together and shown as a single value R.

495_Charging a capictor.png

With S1 closed and S2 open, the capacitor will charge up.

Note that Kirchhoff's second law always applies, that is:

                   E  =  VR  +  VC

The charging sequence is as follows:

  • On closing S1, no current has yet flowed, the capacitor plates have no charge on them and hence, there is no voltage across it.
  • The whole of the applied voltage is developed across the resistor:

                   VR  =  E

  • The initial charging current is equal to the current through the resistor:  2110_Charging a capictor1.png

  • As C charges, the potential difference across it (VC) increases, opposing the applied voltage (E) so that the charging current is progressively reduced.
  • Finally the capacitor is fully charged (VC = E) and current ceases (consequently VR = O).
  • This sequence is shown graphically below.
    529_Charging a capictor2.png  

The curves are called 'exponential' curves and it can be seen that the slopes dVc/dt and dl/dt are progressively decreasing as time increases.


Posted Date: 9/13/2012 4:42:34 AM | Location : United States

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