Energy and particles · GCSE Physics
Energy
The flagship opening of GCSE Physics: energy stores and pathways, conservation, efficiency, power, specific heat capacity and energy resources.
Energy is stored, transferred or dissipated — it is not used up. Write the store at the start, the pathway in the middle, and the wasted thermal store at the end. Efficiency is useful divided by total.
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. Transfers happen mechanically, electrically, by heating or by radiation.
- 2
Energy is conserved: the total in a closed system stays the same. In real devices some energy is dissipated to thermal stores in the surroundings, so the useful output is less than the input.
- 3
Efficiency = useful energy output / total energy input (or useful power / total power). It has no unit and is often given as a percentage. It cannot exceed 1, or 100%.
- 4
Power is the rate of energy transfer: P = E / t, in watts. One watt is one joule per second. Electrical work also uses P = VI.
- 5
Change in gravitational potential energy: ΔE = mgh. Kinetic energy: E = ½mv². Elastic energy: E = ½ke². Work done: W = Fs when the force is along the displacement.
- 6
Specific heat capacity is the energy needed to raise 1 kg of a substance by 1 °C: ΔE = mcΔθ. Water has a high value, so it stores a lot of energy for a small temperature change — useful in heating systems, a nuisance when you want water to boil quickly.
- 7
Thermal conductivity describes how quickly energy transfers through a material by heating. Insulation, trapped air, cavity walls and loft lagging reduce the rate of dissipation.
- 8
Energy resources are renewable (solar, wind, hydro, geothermal, tides, biofuels) or non-renewable (coal, oil, gas, nuclear fuel). Compare reliability, output, start-up time, environmental impact and whether they can meet base load.
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 is done against friction and the thermal stores of the brakes and air rise. 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 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.
Go deeper
Efficiency and power are the two numbers examiners want
Efficiency is a ratio. Keep the units the same on top and bottom — both joules, or both watts. A 60% efficient kettle still conserves energy: 40% has gone to heating the kitchen. Power tells you how fast. A 2 kW kettle transfers 2000 J every second. Time in seconds, energy in joules, power in watts. If a question gives kilowatt-hours, you are on electricity bills: energy (kWh) = power (kW) × time (h). That unit is for cost, not for most mechanics questions. Specific heat capacity practicals: measure mass, temperature change and electrical energy in (or use P × t), then rearrange ΔE = mcΔθ to find c. Heat loss to the air makes experimental c larger than the data-book value.
Go deeper
Resources are a comparison, not a slogan
Renewable does not automatically mean always available. Wind and solar have low running emissions but are unreliable; they need storage or a back-up. Hydro and nuclear can supply a steady base load; nuclear is non-renewable because uranium will run out, but it does not produce carbon dioxide in generation. Coal is dispatchable and energy-dense, and it produces greenhouse gases and particulates. Biofuels are renewable if plants are replanted, but they still release carbon dioxide when burned and they compete with land for food. A six-mark evaluation names two resources, compares reliability, environmental impact and suitability for the job (a small island versus a city), and finishes with a reasoned choice. “Renewables are better” is not a conclusion.
See the idea in action
A 1500 W kettle heats 0.50 kg of water from 20 °C to 100 °C. c for water = 4200 J/kg°C. Useful energy = mcΔθ = 0.50 × 4200 × 80 = 168 000 J. Time at 1500 W if all energy stayed in the water: t = E/P = 168 000 / 1500 = 112 s. In the kitchen it takes 140 s, so total energy from the mains = 1500 × 140 = 210 000 J. Efficiency = 168 000 / 210 000 = 0.80, or 80%. The rest is dissipated heating the kettle and the air. Energy is conserved; it is not 80% “destroyed”.
Exam technique
Turn knowledge into marks
Start every answer with the store or the equation in standard form. Substitute with units, then calculate. For efficiency, write useful ÷ total. For resources, compare at least two factors (reliability, environment, output) before you judge. Never write that energy is used up.
Common mistakes
Do not give these marks away
- 01
Saying energy is used up, or mixing up stores (thermal) with pathways (heating).
- 02
Calculating efficiency as total ÷ useful, or quoting a value over 100%.
- 03
Using mass in grams in ΔE = mcΔθ, or leaving time in minutes in P = E/t.
A lamp transfers 200 J electrically. 40 J is usefully transferred as light. What is its efficiency?
A0.20
B0.40
C0.80
D5.0
Show the answer
0.20. Efficiency = useful output / total input = 40 / 200 = 0.20, or 20%. The other 160 J is dissipated, usually as thermal energy.
Quick questions
If this is the bit you searched
What is the difference between energy and power?
Energy is the quantity transferred, measured in joules. Power is how fast that transfer happens, in watts: P = E/t.
How do you calculate efficiency?
Useful energy (or power) output divided by total energy (or power) input. Multiply by 100 for a percentage. It cannot be greater than 1.
What does specific heat capacity mean?
The energy required to raise the temperature of 1 kg of a substance by 1 °C. In symbols, ΔE = mcΔθ.