How is energy stored, transferred, and why can't any transfer be 100% efficient?
Energy cannot be created or destroyed — only stored differently and transferred between stores. When a ball rolls to a stop, its kinetic energy store doesn't disappear; it transfers to thermal energy stores in the ball and the floor through work done by friction. This is the first law of thermodynamics, and it underpins every calculation in this topic. The skill is identifying which stores are involved at the start and end of a change, then tracking what moved between them.
Efficiency captures the idea that every real energy transfer wastes some energy — usually to thermal stores due to friction or resistance. A more efficient device doesn't "create" energy, it just dissipates less of it uselessly. The National Grid minimises wasted energy in transmission by using very high voltages (and therefore very low currents), since power lost to heating = I²R.
A 2 kg ball is dropped from 5 m above the ground. Calculate its speed at the point of impact (assume no air resistance). If the impact transfers 80 J to thermal stores, what percentage of the original energy was usefully transferred to kinetic energy just before impact?
Key equations
Kinetic energy: Ek = ½mv² recall
Gravitational PE: Ep = mgh recall
Power: P = E/t = W/t recall
Efficiency: useful output energy / total input energy (also as power ratio) recall
Elastic PE: Ee = ½ke² equation sheet
Thermal energy change: ΔE = mcΔθ equation sheet
What students must understand
Energy stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, electrostatic, magnetic
A system is an object or group of objects; changes in a closed system have no net change in total energy
Tracing energy stores before and after changes (projectile, braking vehicle, kettle, spring)
Kinetic energy: Ek = ½mv² — recall and apply
Gravitational PE: Ep = mgh — recall and apply
Elastic PE: Ee = ½ke² (up to limit of proportionality) — equation sheet
Power is the rate of energy transfer: P = E/t (watts = joules per second)
Specific heat capacity: energy needed to raise 1 kg by 1 °C; ΔE = mcΔθ
Energy is conserved but dissipated (wasted to thermal stores); cannot be destroyed
Efficiency = useful output / total input; expressed as decimal or percentage
Lubrication and insulation reduce unwanted energy transfers
Thermal conductivity: rate of conduction increases with conductivity, decreases with wall thickness
Renewable vs non-renewable energy resources; environmental impacts
HT Methods to increase efficiency of energy transfers
Linking questions
How does electrical work connect to energy stores? → P2 Electricity
How is elastic PE related to Hooke's Law? → P5 Forces
How does specific heat capacity connect to changes of state? → P3 Particle Model