Energy, Work and Power

Work Done

Work done = force × distance (in the direction of force)

W = Fd (joules, J)

Work done is equivalent to energy transferred. When a force acts through a distance, energy is transferred from one store to another.

Example: A 500 N force moves a box 3 m → W = 500 × 3 = 1500 J of energy transferred (from chemical store in muscles to KE/thermal stores).

If force is not parallel to displacement: W = Fd cos θ (Higher tier).

Power

Power = energy transferred ÷ time (or work done ÷ time)

P = E/t = W/t (watts, W = J/s)

Example: A 2 kW kettle uses 2000 J every second.

Alternative form: P = Fv (force × velocity — useful for moving vehicles).

Unit conversions: 1 kW = 1000 W; 1 MW = 10⁶ W.

Energy Efficiency in Everyday Life

Key principle: devices transfer input energy into useful and wasted forms.

Reducing energy waste:

• Insulation (loft, cavity wall, double glazing) — reduces thermal energy loss from homes.
• LED bulbs — much more efficient than incandescent bulbs (95% light vs 5% light).
• Electric vehicles — more efficient than petrol/diesel engines (~20% efficiency).

Power Stations

All power stations generate electricity by spinning a generator (except solar PV and fuel cells).

Fossil fuels: burn fuel → heat water → steam → turbine → generator. High carbon footprint. Non-renewable.

Nuclear: fission heats water → steam → turbine → generator. Low CO₂ but produces radioactive waste.

Renewable: wind turbines (KE → electrical), hydroelectric (GPE → KE → electrical), solar thermal (heat → steam → turbine), tidal.

The National Grid

Electricity is transmitted across the country at very high voltage (400,000 V) and low current to minimise energy loss in cables (P_loss = I²R — smaller I means far less power loss for the same resistance).

• Step-up transformer (at power station): increases voltage (e.g. 25,000 V → 400,000 V), decreases current.
• Step-down transformer (at substation): decreases voltage for safe use in homes (230 V in UK) and industry.

Why high voltage? Power = VI, so for the same power, higher V → lower I → less I²R heating in cables → less energy wasted. Extremely efficient transmission.

Transformers (Higher Tier)

Ideal transformer equation: V₁/V₂ = N₁/N₂ = I₂/I₁

Where V = voltage, N = number of turns, I = current. An ideal transformer has 100% efficiency: V₁I₁ = V₂I₂.

Step-up: more turns on secondary (N₂ > N₁) → higher voltage, lower current. Step-down: fewer turns on secondary (N₂ < N₁) → lower voltage, higher current.