Energy Stores, Transfers and Calculations
Energy is not created or destroyed; it is transferred between stores such as kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic and nuclear, along pathways that are mechanical, electrical, by heating or by radiation. Three calculations carry most of the marks: kinetic energy = 0.5 x mass x speed squared, gravitational potential energy = mass x gravitational field strength x height, and energy = mass x specific heat capacity x temperature change.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Describe a transfer as store to store, naming the pathway: for a falling ball, the gravitational potential store empties and the kinetic store fills, transferred mechanically by the force of gravity.
- Use the conservation principle to link two stores: for a falling object with no air resistance, the gravitational potential energy lost equals the kinetic energy gained, which lets you calculate a speed without knowing the time.
- Recall and use kinetic energy = 0.5 x mass x speed squared, remembering that doubling the speed multiplies the kinetic energy by four, which is the physics behind stopping distances.
- Recall and use gravitational potential energy = mass x gravitational field strength x height, with g = 9.8 N/kg unless the question states otherwise, and with height measured vertically.
- For temperature change, use energy = mass x specific heat capacity x temperature change, converting the mass to kilograms and using the change in temperature rather than the final temperature.
- Finish an explanation by saying where the wasted energy goes: it is dissipated to the surroundings by heating, warming the surroundings slightly, and becomes spread out so it is no longer useful.
Worked example
An immersion heater transfers energy to 2.0 kg of water, raising its temperature from 20 degrees Celsius to 50 degrees Celsius. The specific heat capacity of water is 4200 J/kg per degree Celsius. Calculate the energy transferred, and explain why the heater must supply more than this.
- Find the temperature change: 50 - 20 = 30 degrees Celsius.
- Write the equation: energy = mass x specific heat capacity x temperature change.
- Substitute: 2.0 x 4200 x 30.
- Calculate step by step: 2.0 x 4200 = 8400, and 8400 x 30 = 252 000 J.
- State the answer: 252 000 J, which is 252 kJ.
- Explain the excess: some energy is transferred to the container and dissipated to the surroundings by heating, so the heater must supply more than 252 kJ for the water alone to gain that amount.
Practice questions
Try each question, then tap to reveal the answer.
Q1Name four energy stores.Show answer
Answer: Any four of: kinetic, gravitational potential, elastic potential, thermal, chemical, magnetic, electrostatic, nuclear.
Q2Calculate the kinetic energy of a 0.5 kg ball moving at 6 m/s.Show answer
Answer: 0.5 x 0.5 x 6 squared = 0.5 x 0.5 x 36 = 9 J.
Q3By what factor does kinetic energy increase if speed is doubled?Show answer
Answer: By a factor of four, because kinetic energy is proportional to speed squared.
Q4State the equation for gravitational potential energy.Show answer
Answer: Gravitational potential energy = mass x gravitational field strength x height.
Q5Define specific heat capacity.Show answer
Answer: The energy needed to raise the temperature of one kilogram of a substance by one degree Celsius.
Q6A 3 kg object is raised 2 m. Calculate the gravitational potential energy gained. (g = 9.8 N/kg)Show answer
Answer: 3 x 9.8 x 2 = 58.8 J.
Q7What happens to the energy described as 'wasted' in a transfer?Show answer
Answer: It is dissipated to the surroundings, usually by heating, becoming spread out and no longer useful. It is not destroyed.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A ball of mass 0.20 kg is dropped from a height of 1.8 m. Assuming no air resistance, calculate its speed just before it hits the ground. (g = 9.8 N/kg)
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Explain why the water in a swimming pool warms up much more slowly on a sunny day than the concrete around it, even though both receive energy at a similar rate per square metre.
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See real GCSE Science past-paper questions, with official mark schemes →
Free printable worksheet
Want more practice on paper? Download the energy stores, transfers and calculations worksheet pack - 13 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 16 of GCSE Physics Workbook, the whole course as one free printable PDF.
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