Work, energy and power

Energy stores and transfers, conservation of energy, kinetic and gravitational potential energy, work, power, efficiency and energy resources for G3 Pure and O-Level Physics.

SEC G3 Physics 2027
8 lessons

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Before you begin

Energy is the one quantity that is never created or destroyed, only moved from store to store. The lessons start with energy stores and the four ways energy is transferred, then calculate kinetic and gravitational potential energy and use conservation of energy to find speeds and heights, with and without dissipation. Work, power and efficiency follow, and the topic ends by comparing the resources used to generate electricity.

Be comfortable with: forces, for force and weight, and kinematics, for speed and motion graphs.

Learning goals

Recognise kinetic, potential, nuclear and internal energy stores
Describe mechanical energy transfer by a force acting over a distance
Describe electrical energy transfer by an electric current
Describe energy transfer by heating due to a temperature difference
Describe energy transfer by electromagnetic and mechanical waves
Recall and apply Ek = ½mv² in new situations
Recall and apply Ep = mgh near the Earth's surface in new situations
State and apply the principle of conservation of energy
Recall and apply work done = force × distance moved in the force direction
Recall and apply power = energy transfer / time taken
Calculate efficiency as useful energy output / total energy input
Evaluate prescribed electricity-generation resources by efficiency, cost, reliability and environmental impact

Syllabus statements covered

show an understanding that there are energy stores, e.g. kinetic, potential (gravitational, chemical, elastic), nuclear and internal, and that energy can be transferred from one store to another: (i) Mechanically (by a force acting over a distance) (ii) Electrically (by an electric current) (iii) By heating (due to a temperature difference) (iv) By propagation of waves (both electromagnetic and mechanical)
recall and apply the relationships for kinetic energy (Ek = ½mv²) and gravitational potential energy near the Earth's surface (Ep = mgh) to new situations or to solve related problems
state the principle of the conservation of energy and apply the principle to new situations or to solve related problems
recall and apply the relationship work done = force × distance moved in the direction of the force to new situations or to solve related problems
recall and apply the relationship power = energy transfer / time taken to new situations or to solve related problems
calculate the efficiency of an energy transfer using the formula efficiency = useful energy output / total energy input
discuss the use of non-renewable energy resources such as fossil fuel and nuclear fuel, and renewable energy resources such as biofuel, wind, tides, hydropower, geothermal reservoirs and solar to generate electricity in terms of efficiency of energy transfer, cost, reliability and their environmental impact

Lessons

Work through them in order.

Syllabus lessons

Practise and check

Topic reference

Quick reference

Quantity	Equation	Unit
kinetic energy	Eₖ = (1/2)mv²	joule (J)
change in GPE	Δ Eₚ = mgΔ h	joule (J)
work done	W = Fd	joule (J), 1 J = 1 N m
power	P = E/t = W/t	watt (W), 1 W = 1 J s⁻¹
efficiency	useful output ÷ total input × 100%	no unit

In Δ Eₚ = mgΔ h, Δ h is the vertical height change. In W = Fd, d is the distance moved in the direction of the force. Near Earth’s surface, g ≈ 10 N kg⁻¹, which is the same as 10 m s⁻²; use the value a question gives.

Core knowledge to remember

Energy is held in stores: kinetic, gravitational potential, elastic, chemical, nuclear and internal.
Energy is transferred mechanically (by a force moving through a distance), electrically (by a current), by heating (because of a temperature difference) and by waves.
Principle of conservation of energy: energy cannot be created or destroyed. It can only be transferred from one store to another, so the total energy of an isolated system stays constant.
Friction, air resistance and collisions dissipate energy into the internal stores of the surroundings. The energy still exists but is no longer useful.
Work done is the force multiplied by the distance moved in the direction of the force.
Power is the rate of doing work, or the rate of energy transfer.
Efficiency is useful energy output divided by total energy input. Total input = useful output + energy dissipated.
Non-renewable resources (fossil fuels, nuclear fuel) will run out; renewable resources (solar, wind, tides, hydropower, geothermal, biofuel) are replaced naturally. Compare resources by efficiency, cost, reliability and environmental impact.

Common mistakes

Using the distance along a slope instead of the vertical height change in mgΔ h.
Halving the kinetic energy when the speed halves. Kinetic energy depends on v², so it falls to a quarter.
Using a distance that is not in the direction of the force in W = Fd.
Dividing by minutes instead of seconds, or using kilowatts instead of watts, in P = E/t.
Giving the dissipated share as the efficiency, or quoting an efficiency above 100%.
Calling heat or light an energy store, or saying energy is “lost” without saying where it goes.
Treating a resource with variable output, such as wind, as one with low efficiency.