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Energy & Power in Circuits

Energy (E) is measured in joules (J).

Power (P) is measured in watts (W).  Power is the rate at which energy is transferred. One watt is equal to one joule per second ( 1 W = 1 J s-1 ).

Energy electricity equation

  • E = energy in joules, J
  • V = potential difference in volts,V
  • I = current in amperes, A
  • t = time in seconds, s

Power electrcity equation

  • P = power in watts, W
  • V = potential difference in volts, V
  • I = current in amperes, A

Power dissipated by electricity

  • P = power in watts, W
  • I = current in amperes, A
  • R = resistance in ohm’s,  W


Q1) How much energy is dissipated by a resistor if a potential difference of 9.0V is applied to it for 331 seconds and a current of 0.23 A flows through it?

Q1 energy

Q2) What is the current flowing through a bulb if it has a power of 100W when the potential difference supplied to it is 230V?

Q2 current

Q3) What is the power dissipated by a 32 ohm resistor when a current of 1.4 A flows through it?

Q3 power