Energy
Energy stores
Energy cannot be created or destroyed, only transferred between stores (conservation of energy). The main energy stores:
- Kinetic (KE): energy due to motion. KE = ½mv²
- Gravitational potential (GPE): energy due to height. GPE = mgh (g = 9.8 J/kg·m on Earth)
- Elastic/strain potential: energy stored in stretched/compressed springs.
- Chemical: energy stored in chemical bonds (food, fuel, batteries).
- Thermal (internal): kinetic + potential energy of all particles in an object.
- Nuclear: energy in atomic nuclei.
- Electrostatic/magnetic: energy in fields.
Energy transfers
Energy is transferred by:
- Mechanical work (force × distance)
- Electrical work (charge × voltage)
- Heating (conduction, convection, radiation)
- Radiation/waves (light, sound)
Work done
W = Fd cos θ — but for CCEA GCSE: W = Fd when force and displacement are in the same direction. W in joules (J), F in newtons (N), d in metres (m). One joule = one newton-metre.
Energy transferred = work done when there are no other losses.
Kinetic energy
KE = ½mv² (J), where m = mass (kg), v = speed (m/s).
Doubling speed → quadruples KE. This is why road safety is so speed-sensitive.
Gravitational potential energy
GPE = mgh (J), where h = change in height (m), g = 9.8 N/kg.
When an object falls freely from height h: GPE lost = KE gained (energy conservation, ignoring air resistance). ½mv² = mgh → v = √(2gh)
Power
Power = energy transferred / time: P = E / t (W = J/s) Also: P = Fv (power = force × velocity, for constant force and speed — a useful CCEA Higher formula).
Efficiency
Efficiency = useful output energy / total input energy (× 100 for %)
Or: Efficiency = useful output power / total input power
Efficiency is always ≤ 1 (or ≤ 100%). Energy not usefully transferred is usually wasted as thermal energy (heat).
Ways to improve efficiency: reduce friction (lubrication), insulation, using LED bulbs over filament bulbs, regenerative braking.
Sankey diagrams
CCEA may provide a Sankey diagram showing energy input and outputs. The width of each arrow is proportional to energy. Useful output is typically shown going in the intended direction; wasted energy branches off.
⚠Common mistakes
- Using height above ground instead of change in height for GPE — always use Δh.
- Not squaring velocity in KE — ½mv² not ½mv.
- Expressing efficiency > 100% — if you get this, check your values are in the same units.
- Confusing energy and power — energy in J, power in W (= J/s). "A 2 kW heater" gives 2000 J per second.
AI-generated · claude-opus-4-7 · v3-ccea-physics