Energy
Energy is the capacity to do work, and in physics it is tracked as it transfers between stores, such as kinetic, gravitational potential, elastic potential, chemical and thermal energy stores. Useful energy transfers always waste some energy, usually as heat, which is why no device is 100% efficient. This KS3 topic covers energy transfers, efficiency, power and renewable/non-renewable resources.
Before you start
Make sure you're comfortable with these topics first:
Method
- Identify the main energy store(s) involved in a situation, such as kinetic for a moving object, gravitational potential for a raised object, or chemical for a fuel or food.
- Use KE = 1/2 x m x v^2 for kinetic energy and GPE = m x g x h for gravitational potential energy, remembering v is squared in the kinetic energy equation.
- Use efficiency = (useful output energy / total input energy) x 100 to calculate how much of the energy supplied to a device is usefully transferred, rather than wasted, usually as heat.
- Use power = energy transferred / time (P = E / t) to calculate power in watts, remembering 1 watt equals 1 joule per second.
- Use the specific heat capacity equation E = m x c x change in T to calculate the energy needed to change the temperature of a substance.
- Classify energy resources as renewable (e.g. solar, wind, biomass) or non-renewable (fossil fuels, nuclear fuel), and weigh up reliability, cost and environmental impact when comparing them.
Worked example
An electric drill is supplied with 80 J of electrical energy. Only 60 J is usefully transferred to the kinetic energy store of the drill bit; the rest is wasted, mostly as heat and sound. Calculate the efficiency of the drill.
- Write down the equation: efficiency = (useful output energy / total input energy) x 100.
- Substitute the values: efficiency = (60 / 80) x 100.
- Calculate: 60 / 80 = 0.75, so efficiency = 75.
- Final answer: 75%.
Practice questions
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Q1Name the main energy store of a stretched bow, just before an arrow is released.Show answer
Answer: Elastic potential energy store
Q2State whether solar power is a renewable or non-renewable energy resource.Show answer
Answer: Renewable
Q3A toaster transfers 600 J of energy in 20 s. Use P = E / t. Calculate the power of the toaster.Show answer
Answer: 30 W (600 divided by 20)
Q4A book of mass 1.2 kg is lifted to a height of 2 m above a table. Use GPE = m x g x h with g = 10 N/kg. Calculate the gravitational potential energy store of the book.Show answer
Answer: 24 J (1.2 x 10 x 2)
Q5A skateboarder of mass 50 kg is moving at 6 m/s. Use KE = 1/2 x m x v^2. Calculate the kinetic energy store of the skateboarder.Show answer
Answer: 900 J (0.5 x 50 x 36)
Q63 kg of water is heated so its temperature rises by 15 degrees C. The specific heat capacity of water is 4200 J/(kg degrees C). Use E = m x c x change in T. Calculate the energy needed.Show answer
Answer: 189000 J (3 x 4200 x 15)
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
A washing machine motor transfers 900 J of energy in 45 s. Use P = E / t. Calculate the power of the motor.
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A laptop charger has a total power input of 18 W. Of this, 15 W is usefully transferred to the laptop battery's chemical energy store; the rest is wasted, mostly heating the charger casing. (a) Calculate the wasted power output of the charger. (1) (b) Use efficiency = (useful output energy / total input energy) x 100. Calculate the efficiency of the charger. (2) (c) Suggest one change to the charger's design that would reduce the amount of wasted energy. (1)
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An energy company is deciding where to invest its next round of funding: a new solar farm or a new gas-fired power station. Evaluate the case for each option as a way of generating the UK's electricity, weighing up how reliable each is, what each costs, and the environmental impact of each, before reaching a justified conclusion.
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Free printable worksheet
Want more practice on paper? Download the energy worksheet pack - 14 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.
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