Energy
Energy is a physical quantity that describes how able a system is to make something happen, such as moving, heating or lighting an object, and it is measured in joules. GCSE physics tracks it as it transfers between stores such as kinetic, gravitational potential and thermal energy, with the total always conserved.
Before you start
No specific prerequisites - this is a good place to start.
Method
- Identify which energy stores are involved before and after the change, such as kinetic, gravitational potential, elastic potential, chemical or thermal.
- Choose the correct equation for each store you need, for example kinetic energy = 0.5 x mass x speed^2, or gravitational potential energy = mass x gravitational field strength x height.
- Substitute the numbers from the question into the equation, keeping units consistent (kg, m, m/s, J).
- Work through the calculation carefully, squaring speeds or extensions before multiplying by the other terms.
- Apply conservation of energy: total energy in equals total energy usefully transferred plus energy wasted (dissipated).
- For efficiency or power questions, divide the useful output by the total input, or divide energy transferred by time.
Worked example
A ball of mass 0.80 kg is dropped from a height of 5.0 m. Assuming air resistance is negligible and g = 9.8 N/kg, calculate the kinetic energy of the ball just before it hits the ground.
- As the ball falls, its gravitational potential energy store transfers completely to its kinetic energy store, since air resistance is negligible.
- Calculate the loss in gravitational potential energy: GPE = mass x g x height = 0.80 x 9.8 x 5.0.
- 0.80 x 9.8 = 7.84, then 7.84 x 5.0 = 39.2 J.
- Since all of this energy transfers to the kinetic store, the final answer is: kinetic energy just before impact = 39.2 J.
Practice questions
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Q1Name the energy store associated with a moving car.Show answer
Answer: Kinetic (energy store)
Q2A spring is compressed. Which energy store increases?Show answer
Answer: Elastic potential (energy store)
Q3Calculate the kinetic energy of a 2.0 kg trolley moving at 4.0 m/s.Show answer
Answer: 16 J (0.5 x 2.0 x 4.0^2 = 0.5 x 2.0 x 16)
Q4A 5.0 kg object is lifted 3.0 m. Calculate its gain in gravitational potential energy (g = 9.8 N/kg).Show answer
Answer: 147 J (5.0 x 9.8 x 3.0)
Q5A motor is supplied with 800 J of energy and wastes 240 J as heat. Calculate its efficiency as a percentage.Show answer
Answer: 70% (560/800 x 100, since 800 - 240 = 560 J useful)
Q6A kettle transfers 360000 J of energy in 150 s. Calculate its power in watts.Show answer
Answer: 2400 W (360000/150)
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A car of mass 900 kg is travelling at 20 m/s. Calculate the kinetic energy of the car.
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A student carries out the required practical to determine the specific heat capacity of a metal block. A 1.0 kg block is heated by an electric heater that transfers 9000 J of energy to it, and the temperature of the block rises by 20 degrees C. (a) Calculate the specific heat capacity of the metal. Use: specific heat capacity = energy transferred / (mass x temperature change) (3 marks) (b) State one way the student could reduce energy losses to the surroundings during the experiment. (1 mark)
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Compare the use of solar panels and a coal-fired power station for generating electricity, including advantages and disadvantages of each.
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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 worksheet pack - 10 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 1 of GCSE Physics Workbook, the whole course as one free printable PDF.
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