Temperature changes in a system and specific heat capacity — AQA GCSE Physics
Test yourself on Temperature changes in a system and specific heat capacity with AQA GCSE practice questions.
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Temperature changes in a system and specific heat capacity explained
Heating a system raises its internal energy, and how far the temperature climbs is set by three things.
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More energy in gives a bigger rise. More mass gives a smaller rise, because the same energy is shared between more particles. The material matters through its specific heat capacity: 4200 J will raise 1 kg of water by 1 °C but will raise 1 kg of copper (c = 385 J/kg °C) by about 11 °C. Rearranging ΔE = m c Δθ makes this explicit, Δθ = ΔE ÷ (m c), so temperature rise is proportional to energy in and inversely proportional to both mass and specific heat capacity. This is why a reactor wall is built from a material with a very high specific heat capacity: a huge energy transfer then produces only a small temperature rise, keeping the wall below its melting point.
The following equation applies: change in thermal energy = mass × speci f ic heat capacity × temperature change ∆E = m c ∆ θ change in thermal energy, ∆E, in joules, J mass, m, in kilograms, kg specific heat capacity, c, in joules per kilogram per degree Celsius, J/kg °C temperature change, ∆θ, in degrees Celsius, °C.
ΔE = m c Δθ links the energy transferred to a temperature change. Mass must be in kilograms, so 250 g becomes 0.25 kg before you substitute. Δθ is a difference, not a final reading: water taken from 20 °C to 80 °C has Δθ = 60 °C. So 0.5 kg of water heated through 60 °C needs ΔE = 0.5 × 4200 × 60 = 126 000 J. In the required practical the energy in comes from an electric heater, ΔE = P × t or a joulemeter reading, the block is insulated, and c is found from c = ΔE ÷ (m Δθ). Because some energy always escapes to the surroundings, the measured ΔE is larger than the energy that actually reached the block, so the experimental value of c comes out too high.
The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.
Specific heat capacity is an energy price per kilogram per degree. The word specific means per kilogram, and the unit J/kg °C carries the whole definition: joules, per kilogram, per degree Celsius. Water is unusually expensive at 4200 J/kg °C, while aluminium is about 900 and copper about 385. A high value has two consequences worth remembering: the substance needs a lot of energy to warm up, and it also releases a lot as it cools. That is why water is used in heating systems and engine coolants, and why the walls of a fusion reactor are chosen for a very high specific heat capacity, so that an enormous energy transfer produces only a small temperature rise and the walls stay below their melting point.
Your focus
- Predict which of two equal masses of different materials reaches the higher temperature when both are given 10 kJ, using their specific heat capacities.
- Explain why a reactor wall or a storage heater is built from a material with a high specific heat capacity, in terms of the temperature rise produced by a large energy transfer.
- Compare the temperature rise of 1 kg and 2 kg of water supplied with the same energy, and justify the difference.
Show all 9 objectives
- Calculate the energy needed to heat a stated mass of water or metal through a stated temperature change, converting the mass to kilograms first.
- Rearrange ΔE = m c Δθ to find the specific heat capacity or the temperature change when the other quantities are given.
- Explain how energy transferred to the surroundings during the heating practical shifts the experimental value of c away from the true value, and in which direction.
- Describe how to determine the specific heat capacity of water or a metal block, naming every quantity measured and the equation used.
- Explain why water, at 4200 J/kg °C, is chosen for central heating systems and engine coolant rather than a metal.
- Justify the choice of a material with a very high specific heat capacity for a component that must absorb a large energy transfer without melting.
Temperature changes in a system and specific heat capacity exam tips
Marking Points
- one mark for stating that a larger energy input produces a larger temperature rise when mass and material are unchanged
- one mark for stating that a larger mass of the same substance gives a smaller temperature rise for the same energy input
- one mark for naming the property of the material as its specific heat capacity
- one mark for linking a high specific heat capacity to a small temperature increase for a large energy transfer, for example a reactor wall that does not reach its melting point
- one mark for correct substitution into ΔE = m c Δθ, with mass converted to kilograms
- one mark for using the temperature change rather than the starting or final temperature
- one mark for the calculated answer given in joules, or in kJ with the conversion shown
- one mark for a correct rearrangement, such as c = ΔE ÷ (m Δθ) when the specific heat capacity is the unknown
- one mark for identifying energy transferred from the sample to the surroundings as the main source of error in the practical
- one mark for the energy needed to raise the temperature of one kilogram of the substance
- one mark for specifying a rise of one degree Celsius
- one mark for stating that a large energy transfer causes only a small temperature increase when the specific heat capacity is high
- one mark for the consequence in context, for example that the reactor walls do not reach their melting point
Examiner Tips
- 💡When a question says which factors affect the temperature rise, name all three: energy transferred, mass, and the material's specific heat capacity.
- 💡If a question asks why a material was chosen, answer in terms of the size of the temperature change it produces, not just in terms of the value of c.
- 💡Hold the other factors constant in your answer; examiners look for comparisons of the same mass or the same energy input.
- 💡Write the equation, then a line of substituted numbers, then the answer; substitution earns a mark even if the arithmetic slips.
- 💡Convert grams to kilograms as the very first step and cross out the original figure so you cannot use it by mistake.
- 💡If the answer to a practical question is a value of c, say whether your value is too high or too low and name the energy transfer that caused it.
- 💡Learn the definition as three separate conditions: energy, one kilogram, one degree Celsius; each is worth checking before you move on.
- 💡Quote the unit J/kg °C when asked for it; it reads the definition back to you if you forget the wording.
- 💡In an application question, always finish with the effect on temperature, not just the value of c.
Common Mistakes
- claiming a larger mass reaches a higher temperature for the same energy supplied
- using the words temperature and thermal energy as though they mean the same thing
- assuming two equal masses given the same energy always end at the same temperature, whatever the material
- treating specific heat capacity as a measure of how hot a substance can get rather than how much energy each degree costs
- substituting a mass in grams and leaving the answer a thousand times too large
- using the final temperature, for example 80, instead of the change of 60
- rearranging to c = ΔE × m × Δθ instead of dividing
- quoting the energy in J/kg °C, which is the unit of c and not of ΔE
- forgetting that a heater supplies ΔE = P × t, and reading the power as though it were the energy
- omitting the one kilogram and defining it as the energy to raise the temperature of the substance by one degree
- writing the energy needed to raise the temperature to 1 °C rather than by 1 °C
- giving the definition of specific latent heat, which involves no temperature change at all
- assuming a high specific heat capacity means the material cannot store much energy