Activity: Electrical power


  • Derive the formula P = IV
  • Measure the electrical power for different examples
  • Estimate power loss in cables.
  • Have an idea of magnitudes of domestic electrical power usage

Power converted by the battery

  • What is the definition of EMF?
  • How much chemical energy is converted to electrical PE when charge q flows in the battery?
  • If charge q flows in time t what is the power delivered by the battery?

You should now understand why the power delivered by the battery is given by:

P = εI

Power dissipated in a resistor

  • What is the definition of potential difference across a resistor?
  • How much electrical energy is converted to heat when charge q flows?
  • If charge q flows in time t what is the power dissipated in the resistor?

So power dissipated is give by:

P = IV

From Ohm's law V = IR.

  • Substitute for V to give an equation in terms of I and R.
  • Substitute for I to give an equation in terms of V and R.

Measuring power

Electric kettle

An electric kettle converts electrical energy into heat.

  • Use a temperature sensor to measure the rate of temperature rise for an electric kettle and calculate the power delivered to the water.
  • How does your value compare to the makers value?

Light bulb

A light bulb converts electrical energy into light but also produces heat.

  • Use the special "light bulb in water" apparatus to measure how much energy is converted to heat per unit time.
  • Measure the current through and pd across the light bulb and calculate the power dissipated.
  • Calculate the % of electrical energy converted to light per second.

Electric motor

An electric motor converts electrical energy to mechanical energy

  • Devise a method for measuring the power of a battery power drill. Discuss the method with your teacher and if possible perform the experiment.

Heat loss in cables

Ideal wires have zero resistance, real ones don't

  • Measure the resistance of the cable connecting the kettle to the mains supply. The cable must be disconnected from the mains supply when you do this.
  • Calculate the heat loss per second in the cable when the kettle is in use.

If too much heat is generated in a cable it will get hot and the insulator will melt.


Different electrical devices need different amounts of current.

  • Use the "power meter" to measure the power of different devices.
  • Is there a relationship between the power of the device and the thickness of the cable?

When wiring a house some devices need more power than others so different cables must be used. To protect the cables from overload a circuit breaker is installed, this will switch off if the current becomes too big for the cables. Have a look in the switch cupboard.

  • What is the current value for the largest circuit breaker?
  • What sort of devices are connected to this circuit?
  • What sort of device is the lowest current circuit used for?
  • Estimate the total current that can be supplied.
  • Calculate the maximum power that could be delivered.
  • Electrical energy is measured in kWh, how many Joules is equivalent to 1 kWh?


The total amount of energy that can be convert from chemical to electrical during the life of a battery can be calculated from the Ah (Amp hour) rating. A 1 Ah rating means the battery can deliver 1 A for 1 hour. A typical 1.5 V AA has a rating of about 2 Ah.

  • Calculate the total energy that can be delivered by an AA battery (in Joules).
  • Find out the rating of different makes of battery. Do the expensive ones contain more energy?

Exercises on page 228 - 229

Power and potential dividers multiple choice


Energy 6

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