Energy runs through the whole Combined Physics syllabus: mechanics, thermal physics, waves and electricity all describe the same quantity being shifted from one place to another. Energy is a scalar quantity measured in joules (J), and it is conserved. It is never created, used up or destroyed, only moved between stores.
At O-Level, marks go to answers written in the stores-and-transfers language. The older “forms of energy” phrasing, such as “chemical energy turns into light energy”, is no longer accepted. The topic also covers calculations with kinetic and gravitational potential energy, work done, power and efficiency, and how to compare energy resources in extended answers.
What these notes cover
- •Energy stores and transfers
- •The six energy stores
- •The four transfer pathways
- •Writing correct energy statements
- •Kinetic energy
- •Gravitational potential energy
- •The principle of conservation of energy
- •Work done, power and efficiency
- •Renewable and non-renewable energy resources
- •Structuring a 4–5 mark “discuss” answer
- •Common mistakes and exam technique
- •Summary
Key definitions
- Principle of conservation of energy
- Energy cannot be created or destroyed, only transferred from one store to another. The total amount of energy stays constant.
- Energy store
- Where energy is held. The six stores are kinetic, gravitational potential, chemical potential, elastic potential, nuclear and internal.
- Transfer pathway
- How energy gets from one store to another: mechanically, electrically, by heating or by waves. A pathway is not a store, so energy is never “in” a pathway.
- Internal store
- Energy held because of the random kinetic energy and potential energy of the particles inside an object. It replaces the old “heat energy” and “thermal energy” labels.
- Renewable resource
- A resource that nature replenishes as fast as, or faster than, we use it, so it will not run out. A non-renewable resource is used up far faster than nature can replace it.
Key formulas
Kinetic energy
Eₖ = ½mv²
m is mass in kg, v is speed in m/s, and Eₖ is in joules (J). Speed is squared, so doubling v makes Eₖ four times larger.
Gravitational potential energy
Eₚ = mgh
m is mass in kg, g is gravitational field strength (take g = 10 N/kg) and h is vertical height in m above a chosen reference level. It is valid near the Earth’s surface.
Fall from rest (no air resistance)
mgh = ½mv², so v = √(2gh)
Loss in GPE = gain in KE. Mass cancels, so the impact speed does not depend on mass.
Work done
W = Fd
F is force in N and d is the distance moved in the direction of the force, in m. W is in joules, where 1 J = 1 N × 1 m.
Power
P = E ÷ t (or P = W ÷ t)
E or W is the energy transferred in J and t is the time in s. P is in watts, where 1 W = 1 J/s.
Efficiency
efficiency = (useful energy output ÷ total energy input) × 100%
This also works with power. Efficiency has no unit and is always less than 100% for real devices.
Worked example
A ball of mass 0.50 kg is dropped from rest from a height of 5.0 m above the ground. Take g = 10 N/kg. (a) Assuming air resistance is negligible, calculate the gravitational potential energy lost by the ball, and hence find its speed just before it hits the ground. (b) In a real experiment, the speed just before impact is measured to be 9.0 m/s. Calculate the energy transferred to the internal store by air resistance, and describe this transfer.
- 1(a) The gravitational potential energy lost is Eₚ = mgh = 0.50 × 10 × 5.0 = 25 J.
- 2By conservation of energy, all of this is transferred from the gravitational potential store to the kinetic store, so ½mv² = 25 J.
- 3Rearranging, v² = (2 × 25) ÷ 0.50 = 100 m²/s², so v = 10 m/s (2 s.f.).
- 4(b) The actual kinetic energy just before impact is Eₖ = ½ × 0.50 × 9.0² = 20.25 J.
- 5The energy transferred away by air resistance is 25 − 20.25 = 4.75 J ≈ 4.8 J (2 s.f.).
- 6This energy is transferred by heating, due to friction between the ball and the air, to the internal store of the ball and the surrounding air. The total energy is still 25 J.
Watch out: Square the speed before multiplying by ½m. Do not say the energy is “lost”: it is transferred by heating to the internal store of the surroundings.
Common mistakes
Saying energy is “lost” or “used up”
Wrong: Some energy is lost due to friction.
Right: Some energy is transferred by heating to the internal store of the surroundings (due to friction or air resistance), so less energy is transferred to the useful store.
Forgetting to square the speed
Wrong: For m = 2 kg and v = 6 m/s: Eₖ = ½ × 2 × 6 = 6 J.
Right: Eₖ = ½ × 2 × 6² = 36 J.
Using slope length instead of vertical height
Wrong: Using the distance along the slope (d = 6.5 m) in Eₚ = mgh.
Right: Use the vertical rise (h = 2.4 m). The slope length is only used for the work done by a force acting along the slope, W = Fd.
Questions students ask
What are the six energy stores in O-Level Combined Physics?
The six stores are kinetic, gravitational potential, chemical potential, elastic potential, nuclear and internal. Energy moves between stores along four pathways: mechanically, electrically, by heating or by waves. “Electrical energy”, “light energy” and “sound energy” are not stores.
Does a heavier object hit the ground faster if dropped from the same height?
No, if air resistance is ignored. Loss in GPE = gain in KE gives mgh = ½mv², and the mass cancels to give v = √(2gh). A 1 kg rock and a 100 kg boulder dropped from the same height land at the same speed.
Can efficiency be more than 100%?
No. Every real device transfers some energy by heating to the surroundings, so the useful output is always less than the total input. If your working gives more than 100%, you have swapped the input and output.
Is nuclear energy renewable or non-renewable?
Nuclear fuel is non-renewable because uranium is finite. Calling it renewable “because it lasts a long time” loses marks. Nuclear power releases no CO₂ during operation, but it leaves radioactive waste that must be stored securely for thousands of years.
In the full PDF
- •Energy-flow diagram for a torch and a Sankey diagram for a filament lamp
- •Graph of Eₚ, Eₖ and total energy against height for a falling object
- •Further worked examples: describing transfers (torch, kettle, falling durian), work done, power and efficiency
- •Full comparison table of renewable resources, plus model “discuss” answers on solar power and on wind versus gas
- •The remaining misconceptions and the eight-step mark-winning checklist for energy calculations
- •End-of-topic summary of the five key ideas