Transferring Electrical EnergyOxford AQA IGCSE Physics: Revision notes

Electrical appliances are designed to transfer energy from the electrical supply to other, useful forms.

• A kettle transfers electrical energy to thermal energy (heating the water)
• A lamp transfers electrical energy to light energy (and some wasted thermal energy)
• A motor transfers electrical energy to kinetic energy
• A loudspeaker transfers electrical energy to sound energy

Every appliance is chosen and designed for the specific energy transfer it needs to make.

Power is the rate at which energy is transferred, and can be calculated in two ways:

P = E ÷ t

where P is power (W), E is energy transferred (J), and t is time (s).

P = I × V

where I is current (A) and V is potential difference (V). This second equation is useful for determining the correct size of fuse needed for an appliance — the fuse rating must be just above the normal operating current.

Energy transferred can also be calculated from potential difference and charge:

E = V × Q

where E is energy transferred (J), V is potential difference (V), and Q is charge (C). This equation reflects the definition of potential difference as the energy transferred per unit charge.

Energy used by mains electricity is measured in kilowatt-hours (kWh) rather than joules, since household energy use involves very large numbers of joules.

E (kWh) = P (kW) × t (h)

To find the cost of electricity used, multiply the energy used (in kWh) by the cost per kilowatt-hour charged by the electricity supplier.

cost = energy used (kWh) × cost per kWh