Energy stores and systems: bringing water to a boil in an electric kettle.

The idea

In an electric kettle, energy is transferred from the chemical store of the power station fuel via the national grid. Electrical work is done on the kettle's heating element by the current, transferring energy to the thermal store of the element. Energy is then transferred by heating to the thermal store of the water. As the water heats, its temperature rises until it reaches boiling point, undergoing a change of state from liquid to gas. During boiling, energy is transferred to the water's thermal store without a change in temperature; this energy is the latent heat of vaporisation. Some energy is also dissipated by heating to the surroundings, reducing efficiency.

What earns the marks

• Electrical work is done on the kettle's heating element by the current, increasing its thermal store.
• Energy is transferred by heating from the element to the thermal energy store of the water.
• The temperature of the water rises until it reaches 100 °C at standard atmospheric pressure.
• At boiling point, further energy transfer causes a change of state from liquid to gas without a change in temperature.
• Energy is also transferred by heating to the surroundings, so the kettle is not 100% efficient.
• The total energy transferred from the electrical supply equals the increase in thermal energy of the water plus energy transferred to the surroundings.

Common mistakes and how to put them right

• Stating that electrical work transfers energy directly to the water.

  Put it right: Electrical work is done on the heating element, which then transfers energy to the water by heating.

• Saying the water's temperature continues to rise while it is boiling.

  Put it right: During a change of state, energy is transferred without a change in temperature.

• Assuming all electrical energy is transferred to the water's thermal energy store.

  Put it right: Some energy is transferred to the surroundings by heating, so efficiency is less than 100%.

• Confusing the energy transfer with the temperature change.

  Put it right: Energy transfer can cause a temperature change or a change of state, but not both at the same time.

Exam tips

• Clearly distinguish the pathways: electrical work to the element, then heating to the water.
• Use the correct terms 'thermal energy store' and 'change of state' rather than vague words like 'heat' or 'boiling'.
• If asked about efficiency, calculate useful energy output divided by total energy input, and explain where the rest goes.