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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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.
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)
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.
Free printable worksheet
Want more practice on paper? Download the energy worksheet pack - 14 pages of exam-style questions with a full mark scheme. No sign-up, no email wall - just the PDF, free for personal and classroom use.
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