Energy changes in systems — AQA GCSE Physics
Test yourself on Energy changes in systems with AQA GCSE practice questions.
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Energy changes in systems explained
Heat a substance and its particles gain kinetic energy, so its internal store rises and its temperature climbs.
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How much energy that takes depends on three things: how much substance there is, how many degrees you want, and what the substance is made of. Those three appear in ΔE = m c Δθ. Water has a specific heat capacity of 4200 J/kg °C, so it takes 4200 J to warm one kilogram by one degree; the same energy would raise a kilogram of copper, at 385 J/kg °C, by about 11 °C. The equation works both ways, so a cooling object releases the same energy that heating it would need, which is why a high specific heat capacity material makes a good store and a good coolant.
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 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, c, is the energy needed to raise the temperature of one kilogram of a substance by one degree Celsius, measured in J/kg °C. In the equation ΔE = m c Δθ, mass is in kilograms, Δθ is the temperature change in degrees Celsius and ΔE comes out in joules. To measure c for water, heat a known mass in an insulated container with an immersed electric heater, record the energy from a joulemeter, or from E = P t, and measure the temperature rise with a thermometer. Then c = ΔE ÷ (m Δθ). The value always comes out too high because some energy heats the container and escapes to the surroundings rather than the water. Insulating the container, fitting a lid and starting the water below room temperature all reduce that error.
Your focus
- Calculate the energy needed to raise a named mass of water through a stated temperature rise.
- Rearrange ΔE = m c Δθ to find the specific heat capacity from experimental readings.
- Explain why a material with a high specific heat capacity is chosen for a storage heater or a coolant.
Show all 8 objectives
- Predict which of two substances reaches the higher temperature when both receive the same energy.
- State the unit of specific heat capacity and explain what a value of 4200 J/kg °C tells you about water.
- Describe a method that uses a joulemeter, balance and thermometer to determine the specific heat capacity of water.
- Calculate a value of c from readings of energy supplied, mass and temperature rise.
- Explain why energy transferred to the container and the surroundings makes the experimental value too high.
Energy changes in systems exam tips
Marking Points
- one mark for selecting ΔE = m c Δθ and identifying the three quantities from the question
- one mark for using the temperature change rather than the final temperature
- one mark for a correct substitution with mass in kilograms
- one mark for the energy in joules, or for the correct rearrangement when c or Δθ is the unknown
- one mark for measuring the mass of the substance with a balance and recording the starting temperature
- one mark for measuring the energy supplied using a joulemeter or from power multiplied by time
- one mark for measuring the temperature rise after a stated time with a thermometer
- one mark for substituting into c = ΔE ÷ (m Δθ) and giving the unit J/kg °C
- one mark for insulating the container and identifying energy transfer to the surroundings as a source of error
Examiner Tips
- 💡Write Δθ as final minus initial in its own line before substituting, so the sign and size are both clear.
- 💡A temperature change of 1 °C and of 1 K are the same size, so no conversion is needed.
- 💡If the answer comes out in the millions, check whether the mass should have been in kilograms.
- 💡In a six-mark method, give the measuring instrument for every quantity and the equation you will use at the end.
- 💡Name insulation and a lid as controls, then say they reduce energy transfer to the surroundings.
- 💡State the direction of any error: heating the container makes the measured specific heat capacity too high.
Common Mistakes
- putting the final temperature into the equation instead of the difference between start and finish
- using grams for mass with a specific heat capacity quoted per kilogram
- assuming a material that heats quickly must be storing more energy, when it has a low specific heat capacity
- using ΔE = m c Δθ for the part of a process where the substance is changing state, when that part needs the specific latent heat equation E = m L instead
- recording only the final temperature, leaving no way to work out Δθ
- quoting the unit as J/kg or as J/°C rather than J/kg °C
- assuming all the electrical energy reaches the substance, so ignoring the container and the surroundings
- starting the timer before the heater is switched on, which inflates the energy supplied