Energy transfers in everyday appliances: Work is done when charge flows in a circuit.

The idea

When charge flows through a component, energy is transferred from the source to that component. The battery or power supply does work on the charge, raising its electrical potential energy. As the charge passes through a lamp, heater or motor, that energy is transferred to other stores, such as the thermal store of the surroundings by heating and light, or to a kinetic store by a motor. The size of the energy transfer depends on both the potential difference across the component and the amount of charge that passes. For example, a 12 V motor driving a current for 60 s transfers more energy than the same motor running for 10 s, because more charge has flowed. Energy transferred, charge and potential difference are linked by E = QV, where E is energy in joules, Q is charge in coulombs and V is potential difference in volts.

What earns the marks

• State that charge must flow through a component for electrical work to be done.
• Identify the energy transfer pathway: source does work on charge, then the component transfers energy to other stores, such as a kinetic store for a motor.
• Use E = QV correctly, substituting charge in coulombs and potential difference in volts.
• Explain that a larger charge flow or a larger potential difference increases the energy transferred.
• Recognise everyday examples such as a lamp transferring energy by light and heating, or a motor transferring energy to a kinetic store.

Common mistakes and how to put them right

• Thinking work is done only when a current is switched on, rather than whenever charge flows.

  Put it right: Work is done continuously while charge moves through a potential difference.

• Confusing charge with current.

  Put it right: Charge Q = It is measured in coulombs, while current I is the rate of flow of charge in amperes.

• Using E = QV with V in millivolts or Q in milliampere-hours without converting.

  Put it right: Convert to volts and coulombs before calculating.

• Stating that a battery raises a charge's potential difference.

  Put it right: A battery raises the charge's electrical potential energy; potential difference is the work done per unit charge.

Exam tips

• Write the equation E = QV, then rearrange only after substituting known values to reduce arithmetic slips.
• Check units: joules for energy, coulombs for charge and volts for potential difference.
• Link the calculation to a named appliance so the energy transfer is described accurately, such as a motor transferring energy to a kinetic store.