Energy and particles · GCSE Physics
Energy stores and transfers
Teacher-written GCSE Physics revision on energy stores and transfers: name kinetic, gravitational and thermal stores, then follow mechanical, electrical, heating and radiation pathways with conservation.
Energy is stored, transferred or dissipated — it is not used up. Write the start store, the pathway and the end store before you reach for a number.
The important bits
What you need to know
- 1
Name stores, not a vague “energy”: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic and electrostatic.
- 2
Transfers happen along four pathways: mechanically (forces doing work), electrically (charge flowing), by heating, or by radiation (light, infrared, sound).
- 3
Conservation of energy: in a closed system the total energy stays the same. Units of energy are joules (J). One kilojoule is 1000 J.
- 4
A falling object loses gravitational potential store and gains kinetic store. ΔE_p = mgh and E_k = ½mv² if dissipative forces are ignored.
- 5
A stretched spring stores elastic potential energy E_e = ½ke², with k in N/m and extension e in metres, not centimetres.
- 6
Dissipated energy still exists. Friction and air resistance transfer energy to thermal stores in the surroundings, so it is no longer useful for that job.
- 7
Sankey diagrams show conservation: the width of the incoming arrow equals the sum of the outgoing arrows, useful and wasted.
- 8
Work done is a mechanical transfer: W = Fs when the force is along the displacement. W is in joules, F in newtons, s in metres.
Quotations worth analysing
Short evidence. Real method.
“Energy cannot be created or destroyed.”
The mark scheme wants conservation, not “energy is used up”. Name where the joules go: another store, or dissipated heating the surroundings.
“Write the store at the start, the pathway in the middle, and the wasted thermal store at the end.”
A battery-powered lamp is chemical store → electrical pathway → light and thermal stores. Missing any of the three parts loses the explain mark.
“The total energy of a closed system is constant.”
Closed means you have accounted for every store. If the kinetic store falls by 200 J, 200 J must appear somewhere else, usually thermal.
Go deeper
Stores and pathways stop the “energy is used up” sentence
A falling apple loses gravitational potential store and gains kinetic store. A braking car loses kinetic store; work against friction raises thermal stores in the brakes and the air. A battery-powered lamp transfers chemically stored energy electrically, then by light and heating. If you cannot name the start store, the pathway and the end store, you are not yet answering in GCSE Physics language. Conservation means the total is constant, not that every joule is useful. Dissipated energy still exists; it is just spread out so it is no longer useful for that job. Sankey diagrams show this: the incoming arrow equals the sum of the outgoing arrows, useful and wasted. If those widths do not add up, the diagram is wrong.
Go deeper
Choose the equation after you have named the stores
Gravitational potential energy changes when height changes: ΔE = mgh, with m in kilograms, g in N/kg and h in metres. Kinetic energy depends on speed squared: E = ½mv², so doubling speed quadruples kinetic energy. Elastic energy is ½ke² only while Hooke’s law holds. Work W = Fs is the mechanical pathway that links a force to a change of store. Students grab ½mv² because they see a moving object, then miss that the question asked for the energy needed to lift it. Read the story first. If height is the thing that changes, start with mgh. If speed is the thing that changes, start with ½mv². Then substitute with units on the same line as the numbers.
See the idea in action
A 0.40 kg trolley is lifted through 1.5 m. g = 9.8 N/kg. Change in gravitational potential store ΔE = mgh = 0.40 × 9.8 × 1.5 = 5.88 J. It is then released from rest and reaches 2.4 m/s at the bottom if drag is ignored: E_k = ½mv² = 0.5 × 0.40 × (2.4)² = 1.152 J. Those two numbers do not match, so some energy has been dissipated to thermal stores. The missing 4.73 J has not been destroyed; it has heated the rails and the air.
Exam technique
Turn knowledge into marks
Start every answer with the store or the equation in standard form. Substitute with units, then calculate. Never write that energy is used up. If a Sankey diagram is given, check that the arrows add up.
Common mistakes
Do not give these marks away
- 01
Saying energy is used up, or mixing up stores (thermal) with pathways (heating).
- 02
Using mass in grams in ΔE = mgh or E = ½mv², or leaving height in centimetres.
- 03
Grabbing kinetic energy when the question is a lift, or gravitational energy when only speed changes.
A 2.0 kg bag is lifted 1.5 m at a steady speed. g = 10 N/kg. How much energy is transferred to the gravitational potential store?
A3.0 J
B13.3 J
C30 J
D300 J
Show the answer
30 J. ΔE = mgh = 2.0 × 10 × 1.5 = 30 J. Steady speed means kinetic store is not changing; the work done goes to the gravitational store.
Quick questions
If this is the bit you searched
What is the difference between a store and a pathway?
A store is where energy is held (kinetic, gravitational, chemical, thermal). A pathway is how it is transferred: mechanically, electrically, by heating or by radiation.
Does conservation mean devices are 100% efficient?
No. The total energy is still conserved, but some is dissipated to thermal stores in the surroundings, so it is not useful for the intended job.
When do I use ΔE = mgh and when do I use E = ½mv²?
Use mgh when height in a gravitational field changes. Use ½mv² when speed changes. Many questions need both, linked by conservation if heating is ignored.
What does a Sankey diagram show?
The width of each arrow is proportional to energy or power. The input width equals the useful output plus the wasted output, which is conservation drawn as a picture.