P5 Energy

Energy stores and transfers

Energy is stored in different stores:

• Kinetic (moving objects)
• Gravitational potential (objects above ground)
• Elastic potential (stretched/compressed springs)
• Chemical (fuels, food, batteries)
• Thermal (hot objects)
• Nuclear (unstable nuclei)
• Electrostatic / Magnetic

Energy is transferred between stores by:

• Mechanically (forces doing work)
• Electrically (charge flow)
• By radiation (light, sound, EM waves)
• By heating (conduction, convection, radiation)

Energy is conserved — total energy in a closed system is constant.

Work done

When a force moves an object through a distance:

W = Fd cos θ

For force and displacement in the same direction: W = Fd.

Units: joules (J) or newton-metres (N m). 1 J = 1 N m.

Kinetic energy

Eₖ = ½mv²

Doubling mass doubles KE. Doubling speed quadruples KE.

Gravitational potential energy

Eₚ = mgh

where g = 10 N/kg, h = height above reference level (m).

Elastic potential energy

Eₑ = ½ke²

where k = spring constant (N/m), e = extension (m). [Same as P2]

Power

P = W / t = E / t

Units: watts (W). 1 W = 1 J/s.

Also P = Fv (force × velocity) for a constant velocity situation.

Efficiency

Efficiency = useful energy output / total energy input (× 100 for %)

or

Efficiency = useful power output / total power input

Energy is never destroyed; "wasted" energy is dissipated as thermal energy (heat) and/or sound.

A Sankey diagram shows energy transfers to scale: arrow width represents energy quantity.

PAG P5.1: Investigate energy transfers (e.g. bouncing ball, model car on ramp). Measure KE and GPE, compare to identify losses.

Energy resources

Non-renewable (fossil fuels + nuclear)

• Coal, oil, natural gas → burn to heat water → steam → turbine → generator → electricity.
• Reliable (can be used on demand — controllable).
• Non-renewable — finite reserves.
• Release CO₂ → climate change; other pollutants (SO₂ → acid rain).
• Nuclear: no CO₂ from fission; but radioactive waste is problematic; high start-up cost.

Renewable

• Solar: PV panels convert light → electricity; or solar thermal. No CO₂ in operation; intermittent.
• Wind: turbines; no CO₂; intermittent; noise/visual impact.
• Hydroelectric: most reliable renewable; dam environmental impact; high efficiency.
• Tidal/wave: reliable (tidal predictable); limited suitable locations.
• Geothermal: heat from Earth's core; limited to volcanic regions.
• Biomass: burning plant/animal waste; considered carbon-neutral (CO₂ absorbed during growth).

The National Grid

Electricity is transmitted at high voltage (400 kV) to reduce current → reduce I²R losses. Step-up transformers at power stations; step-down transformers at substations before homes.