The units you must use

Before any calculation, learn the units 4PH1 expects you to recognise and use throughout Section 4:

• kilogram (kg) — mass
• joule (J) — energy and work done
• metre (m) — distance and height
• metre/second (m/s) — speed
• metre/second² (m/s²) — acceleration
• newton (N) — force
• second (s) — time
• watt (W) — power

Exam habit: always put quantities into these base units first. A mass given in grams must become kilograms, a time in minutes must become seconds — otherwise the joules and watts come out wrong.

Pick the unit

Stores, and the ways energy moves

Energy sits in stores and moves between them by transfers. 4PH1 lists eight stores:

• Mechanically — gravity accelerates a falling object and fills its kinetic store.
• Electrically — a current through a lamp transfers energy that is given out as light and heat.
• By heating — a flame transfers energy to a pan.
• By radiation — sound waves carry energy through the air; an object emits electromagnetic radiation.

Watch out: "heat", "electrical" and "light" are not stores — they describe the transfer. The store is where energy ends up (e.g. thermal, kinetic).

Name the transfer

Tap the way energy is mainly transferred in each case.

Energy is conserved

The principle of conservation of energy: energy cannot be created or destroyed, only transferred from one store to another. Total energy before = total energy after.

Misconception: energy is never "used up". When a device seems to "lose" energy, that energy has been transferred to the surroundings (usually heating them) — it still exists, it is just spread out and no longer useful.

Tracking the energy

Efficiency & Sankey diagrams

In every device, only some of the input energy ends up doing the useful job — the rest is wasted (usually heating the surroundings). Efficiency compares the two:

A Sankey diagram shows this with arrows whose width is proportional to the energy. The wider the useful arrow, the more efficient the device.

Misconception: no real device is 100% efficient — there is always some wasted transfer. Efficiency has no units; write it as a ratio (0.6) or a percentage (60%), never in joules.

Your turn — efficiency

Conduction, convection & radiation

Thermal energy moves from hotter to cooler regions in three ways:

• Conduction — heated particles vibrate more and pass energy to neighbours. In metals, free electrons carry energy quickly, so metals are good conductors; non-metals are thermal insulators.
• Convection — in fluids (liquids and gases) the warmed part expands, becomes less dense and rises; cooler, denser fluid sinks to take its place, setting up a current (4.7).
• Radiation — infrared radiation needs no medium. Dull black surfaces are the best emitters and absorbers; shiny white surfaces are the best reflectors. Higher temperature and larger surface area mean more is emitted (4.8).

Practical (4.9): investigate thermal energy transfer by conduction, convection and radiation — e.g. compare how fast water cools in shiny vs dull, black vs white containers (insulation reduces unwanted transfer, 4.10).

Which transfer is it?

Work done = energy transferred

Whenever a force moves an object in the direction of the force, work is done and energy is transferred between stores:

The work done is equal to the energy transferred (4.12) — both are measured in joules.

Your turn — work done

Gravitational potential energy

Lift an object up and you fill its gravitational store:

Your turn — gravitational P.E.

Kinetic energy

Every moving object has energy in its kinetic store. It depends on mass and, much more strongly, on speed:

Because speed is squared, doubling the speed gives four times the kinetic energy.

Conservation of energy links GPE, KE and work (4.15): when an object falls, GPE is transferred to KE, so GPE lost = KE gained (ignoring air resistance).

Your turn — kinetic energy

Power is the rate of transfer

Power is the rate at which energy is transferred, or the rate at which work is done (4.16):

One watt is one joule per second (1 W = 1 J/s). A more powerful device transfers the same energy in less time.

Your turn — power

Generating electricity

The big divide is renewable (replenished as fast as it is used — won't run out) vs non-renewable (finite — will eventually run out):

How they make electricity (4.18P): wind (KE of moving air), water/HEP (KE of moving water or GPE of stored water), geothermal (thermal energy from the ground), solar heating (energy from the Sun) and solar cells (light converted directly to electricity), fossil fuels (chemical store released by burning) and nuclear power (energy from atomic nuclei). Except for solar cells, the energy turns a turbine → drives a generator → electricity.

Trade-offs (4.19P): fossil fuels and nuclear are reliable with a large output, but fossil fuels release CO₂ (climate change) and nuclear makes radioactive waste. Renewables won't run out and produce little or no CO₂ as they generate, but many (wind, solar, tidal) are intermittent / less reliable and have high set-up costs.

Renewable or non-renewable?

Tap a resource, then tap the box it belongs in.

Choosing a resource

The equations to know

You've covered the whole of 4PH1 Section 4 — units, stores & transfers, conservation, efficiency & Sankey diagrams, conduction/convection/radiation, work, GPE, KE, power, and (Paper 2 only) energy resources & electricity generation. Press Finish to see your score.