Energy stores and systems: heating

The idea

Heating is one way a system can be changed: energy is transferred from a hotter region to a cooler region, raising the temperature of the cooler material or changing its state. The energy transferred for a temperature change depends on mass, specific heat capacity and temperature change, using ΔE = m × c × Δθ. For example, heating 0.50 kg of water (c = 4200 J/kg°C) through 20 °C transfers ΔE = 0.50 × 4200 × 20 = 42 000 J. During a change of state, temperature stays constant while energy breaks or forms bonds. Here, use the specific latent heat equation E = m × L. Energy is conserved: energy gained by the cooler object equals energy lost by the hotter one, ignoring dissipation.

What earns the marks

• Recognise heating as a pathway that transfers energy from a hotter object to a cooler one, not as a substance that flows.
• Use ΔE = m × c × Δθ for a temperature change, identifying mass, specific heat capacity and temperature change correctly.
• Use the specific latent heat equation E = m × L for a change of state, where temperature remains constant during melting, boiling or condensing.
• Convert units before substituting: grams to kilograms and, where needed, minutes to seconds for power calculations.
• Interpret a heating graph: sloping sections show temperature change, flat sections show a change of state.
• Apply conservation of energy so that energy lost by the hotter object equals energy gained by the cooler object in an insulated system.

Common mistakes and how to put them right

• Using the final temperature instead of the temperature change.

  Put it right: Δθ is final temperature minus initial temperature.

• Assuming temperature rises during melting or boiling.

  Put it right: During a change of state the energy breaks bonds, so temperature stays constant.

• Confusing specific heat capacity with specific latent heat.

  Put it right: Specific heat capacity applies to temperature change, specific latent heat to change of state.

• Forgetting that the unit of specific heat capacity is J/kg°C, so mass must be in kilograms.

Exam tips

• Write down m, c, L and Δθ separately before substituting, so each value is clearly identified.
• For change-of-state questions, check whether the temperature is constant; if it is, use E = m × L rather than specific heat capacity.
• Read heating graphs carefully: label the flat sections as changes of state and the sloping sections as temperature changes.
• Give the energy unit as J or kJ and keep the same unit throughout a multi-step calculation.