2.1 - How Appliances Transfer Energy

Appliances transfer energy electrically

• A moving charge transfers energy because it does work against the resistance of a circuit.
• Electrical appliances transfer energy electrically from a power source to their components, and from there to other stores:
  • A battery stores energy in its chemical store.
  • The mains supply delivers energy from a power station.

Examples of energy transfer

• Kettle - Transfers energy electrically from the mains supply to the thermal energy store of its heating element.
• Handheld fan - Transfers energy electrically from the battery's chemical store to the kinetic energy store of the fan's motor.

Not all energy is useful

• No appliance transfers all of its energy to useful stores.
• Some energy is always dissipated to the thermal energy stores of the wires and components, and then to the surroundings.

Power ratings of appliances

• An appliance's power rating is its maximum operating power: the maximum energy it transfers each second when in use.
• The lower the power rating, the less energy transferred in a given time, so the cheaper it is to run.
• The higher the power rating, the faster the energy transfer.
• For example, a 1,200 W heater transfers twice as much energy each second as a 600 W heater.

A higher power rating isn't always better

• A higher power rating does not always mean more useful energy is transferred.
• An inefficient appliance with a higher power rating can transfer the same or less useful energy than a lower-power, more efficient one, because more of its energy is wasted to thermal stores.

The kilowatt-hour as a unit of energy

• The kilowatt-hour (kWh) is a unit of energy used by electricity companies to measure domestic energy supply.
• A kilowatt-hour is the energy transferred by a 1 kW device running for one hour.
• 1 kWh = 3,600,000 J (3.6 MJ).

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Formula for energy transferred in kilowatt-hours

• To convert power from watts to kilowatts, divide by 1,000.
• To convert time from minutes to hours, divide by 60.

Worked example - Energy in kilowatt-hours

A 1,200 W microwave oven is used for 15 minutes. Calculate the energy transferred in kilowatt-hours.

The cost of domestic electricity

• Electricity bills are based on the number of kilowatt-hours a household uses.
• Electricity companies charge a set price per kilowatt-hour.

Formula for cost

The price is usually given in pence per kilowatt-hour, so the cost comes out in pence.

Worked example - Cost of using an appliance

An electricity company charges 17.4p per kWh. A 1,400 W electric lawnmower is used for 24 minutes. Calculate the cost of using it, to the nearest penny.