CIE A Level Physics (9702) exams from 2022

Revision Notes

19.1.5 Energy Stored in a Capacitor

Calculating Energy Stored in a Capacitor

  • Recall the electric potential energy is the area under a potential-charge graph
  • This is equal to the work done in charging the capacitor to a particular potential difference
    • The shape of this area is a right angled triangle
  • Therefore the work done, or energy stored in a capacitor is defined by the equation:

Calculating Energy Stored in a Capacitor equation 1

  • Substituting the charge with the capacitance equation Q = CV, the work done can also be defined as:

Calculating Energy Stored in a Capacitor equation 2

  • Where:
    • W = work done/energy stored (J)
    • Q = charge on the capacitor (C)
    • V = potential difference (V)
    • C = capacitance (F)
  • By substituting the potential V, the work done can also be defined in terms of just the charge and the capacitance:

Calculating Energy Stored in a Capacitor equation 3

Worked example: Energy stored in a capacitor

Calculating_Energy_Stored_in_a_Capacitor_Worked_Example_-_Energy_Stored_in_Capacitor_Question, downloadable AS & A Level Physics revision notes

Step 1:            Write down the equation for energy stored, in terms of capacitance C and p.d V

Calculating Energy Stored in a Capacitor equation 2

Step 2:            The change in energy stored is proportional to the change in p.d

Calculating Energy Stored in a Capacitor Worked Example equation 1

Step 3:            Substitute in values

Calculating Energy Stored in a Capacitor Worked Example equation 2

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