Sec 3 Physics Energy, Work & Power notes

The syllabus describes energy with stores — places where energy sits — and pathways — the ways it moves. Stores: kinetic (moving things), gravitational potential (raised things), chemical (food, fuel, batteries), elastic (stretched springs), nuclear (nuclei), and internal (the warmth of particles). Pathways: mechanically (a force acting over a distance), electrically (a current doing work), by heating (a temperature difference), and by waves (light and sound carrying energy from a source).

A good energy story names stores and pathways in order: "a battery (chemical store) drives a current (electrical pathway) that spins a fan (kinetic store) and warms the motor (internal store)". Exam questions award marks for that grammar — the store emptied, the store filled, and the pathway in between.

Kinetic energy is the energy of motion: Ek = ½mv². The square matters: a car at 30 m/s carries four times the kinetic energy it had at 15 m/s, not twice — one reason braking distances grow so steeply with speed. Gravitational potential energy near the Earth is Ep = mgh, where h is measured from whatever level you choose as zero; only changes in height matter, so the choice cancels in every real calculation.

When an object falls, the potential store empties into the kinetic store: mgh = ½mv², so v = √(2gh) — remarkably, the mass cancels, which is why (ignoring air) every object falls with the same speed change. At 45 m: v = √(2 × 10 × 45) = √900 = 30 m/s.

The principle of conservation of energy: energy cannot be created or destroyed, only transferred between stores. It is the ultimate checking tool — every exam energy chain must add up, and any "missing" energy has gone somewhere specific (usually warming things up). Work done is the mechanical receipt: work done = force × distance moved in the direction of the force, in joules. One joule is one newton pushing through one metre.

The phrase in the direction of the force carries marks. Carrying a heavy bag horizontally along a corridor does no work against gravity: the force you exert is upward and the motion is sideways. But your muscles still tire — they do internal work stretching and contracting, transferring energy to thermal stores; the physics statement is about the net force on the bag, not about how sweaty you get.

Power is the rate of energy transfer: P = E ÷ t, measured in watts (1 W = 1 J/s). A 60 W lamp transfers 60 J every second; a 2.0 kW kettle transfers 2000 J each second. Two machines can do the same work at different prices in time — the more powerful one is simply faster.

Efficiency = useful energy output ÷ total energy input, quoted as a fraction or a percentage and never exceeding 1 (100%). The complement — input minus useful output — is the wasted energy, usually transferred by heating to thermal stores of the machine and its surroundings. Efficiency has no units; quoting a percentage is a bookkeeping convention, not a unit.

Non-renewable resources draw down stores that will not be replaced: fossil fuels (coal, oil, natural gas — ancient chemical stores) and nuclear fuel (uranium, a nuclear store). Renewable resources tap ongoing flows: biofuel, wind, tides, hydropower, geothermal heat and solar radiation. Every generating technology is judged on four axes the syllabus names: efficiency of the energy transfer, cost to build and run, reliability (does it work on a calm, cloudy day?), and environmental impact.

Two system-level points earn marks. First, renewable electricity is often intermittent, so real grids pair it with storage (pumped hydro, batteries) or with reliable backup. Second, comparing sources fairly means counting the whole chain — a gas turbine is ~40–60% efficient at converting its fuel’s chemical store to electricity, while a typical large solar panel converts maybe 20% of the incident light but pays nothing for fuel.