More on Power

• Potential difference is energy tranferred per charge passed
• When an electrical charge goes through a change in potential difference, then energy is transferred
• Energy is supplied to the charge at the power source to 'raise' it through a potential
• The charge gives up this energy when it 'falls' through any potential drop in components elsewhere in the circuit. The formula for this is:

  E = QV

  Energy transferred (J) = Charge flow (Q) x Potential difference (V)
• A battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows around it, because the charge is raised up 'higher' at the start
• Example

  The motor in an electric toothbrush is attached to a 3 V battery. 140 C of charge passes through the circuit as it is used. Calculate the energy transferred

  > E =QV = 140 x 3 = 420 J

  This energy is transferred to the kinetic energy store of the motor, as well as to the thermal energy stores of the surroundings
• Power also depends on current and potential difference
• As well as energy transferred in a given time, the power of an appliance can be found with:

  P = VI

  Power (W) = Potential difference (V) x Current (A)
• Example

  A 1.0 kW hair dryer is connected to a 230 V supply. Calculate the current through the hair dryer. Give your answer to 2 significan figures

  • Rearrange the equation for current

  > I = P / V

  • Make sure your units are correct

  > 1.0 kW = 1000 W

  • Then put in the numbers you have into the equation

  > I = 1000 / 230 = 4.34... = 4.3 A (to 2 s.f.)
• You can also find the power if you don't know the potential difference. To do this do:

  P = I^2 x R

  Power (W) = Current (A) x Resistance (Ω)