GCSE Physics Energy stores revision

Energy moves between stores along four kinds of pathway. Mechanically: a force acts through a distance (lifting a suitcase, pushing a trolley). Electrically: charge is driven around a circuit by a p.d. (a kettle heating up). By heating: energy flows from hot to cold because of a temperature difference (a mug warming your hands). By radiation: waves carry energy with no matter (sunlight warming skin, a lamp lighting a room). GCSE answers earn marks for naming both the store and the pathway: “chemical energy in the battery is transferred electrically to the thermal store of the lamp”.

A system is the object or group of objects you choose to study; everything else is the surroundings. The principle of conservation of energy: energy cannot be created or destroyed — it can only be moved between stores. In a closed system nothing enters or leaves, so the total energy is constant no matter what happens inside. The four exam-favourite systems:

Energy that spreads out to the surroundings is dissipated — a physical way of saying “wasted”. A moving car warms the air by friction; a hot drink warms the room. The energy still exists (conservation), but it is now in thermal stores of the surroundings, spread too thinly to be useful. Machines are never 100% efficient precisely because some energy always dissipates.

Efficiency = useful energy output ÷ total energy input. The same ratio works with power in place of energy. An efficiency can be written as a ratio (0.75) or a percentage (75%) — but never above 1 or 100%, because you cannot get more useful energy out than the energy you put in. Machines are made more efficient by lubricating moving parts (less friction heating), streamlining (less drag), using wires with lower resistance, and capturing waste heat (e.g. heat exchangers in power stations warming homes).

Power = energy transferred ÷ time, measured in watts: 1 W = 1 J/s. Energy companies bill in kilowatt-hours: 1 kWh is the energy a 1 kW appliance uses in 1 hour (3 600 000 J). Sankey-style arrows and Sankey diagrams simply picture this ratio — a fat useful arrow and a thin wasted one.

Unwanted transfers are reduced in two arenas. In machines: lubrication (oil, grease) cuts friction between surfaces, so less kinetic energy becomes thermal energy of the parts. In buildings: thermal insulation slows conduction from the warm inside to the cold outside. The rate of cooling depends on the walls’ thickness and their thermal conductivity: thicker walls give a longer path, lower-conductivity materials pass less heat per second. Practical countermeasures, each with its physics:

• Cavity walls: two brick layers with an air (or foam-filled) gap — air is a poor conductor, so the house loses less heat per second.
• Loft insulation: thick fibreglass or foam traps air; the low-conductivity layer slows conduction through the roof.
• Double glazing: two panes with dry air or argon between — conduction through the gas is much slower than through glass.
• Draught excluders: stop warm air escaping by convection rather than conduction.
• Aluminium foil behind radiators: reflects infrared radiation back into the room, cutting transfer by radiation.

The specific heat capacity (c) of a substance is the energy needed to raise the temperature of 1 kg of it by 1 °C. Water’s enormous c (4 200 J/kg °C) is why it is used as a coolant and why coastal towns have mild winters. The working equation: change in thermal energy ΔE = m c Δθ. In symbols: m is the mass (kg), c the specific heat capacity (J/kg °C), Δθ the temperature change (°C), and ΔE the energy supplied (J).

1. 1Measure the block

   Weigh the metal block (e.g. 1.0 kg of aluminium) and record its starting temperature.

2. 2Insert the heater

   Push an electric immersion heater into the block’s drilled hole; add a drop of water or oil for good thermal contact.

3. 3Supply energy and time

   Connect the heater to a joulemeter (or a voltmeter and ammeter and use E = I V t). Switch on for a fixed time, e.g. 5 minutes, stirring nothing — the block is solid.

4. 4Record the rise

   Watch the thermometer and record the highest steady temperature; Δθ is the rise from the start.

5. 5Calculate

   c = energy supplied ÷ (mass × temperature rise). Compare with the accepted value and explain any gap.

Variables: independent — the substance (or the mass of block); dependent — the temperature rise for a fixed energy input (or the energy needed for a fixed rise); controls — heater power, heating time, insulation around the block, starting temperature. Risks: the heater and block become hot enough to burn skin — let them cool before handling; the heater leads must be kept away from water; use a heatproof mat. Sources of error: some energy escapes from the block to the air and the thermometer, so the measured c comes out higher than the accepted value; a joulemeter reading that starts part-way through adds error — zero it first; insulation (a layer of bubble wrap or foam around the block) improves the result.

Three habits earn method marks: convert before substituting (cm → m, kJ → J, minutes → seconds), write the equation first and then substitute, and quote the unit with every final answer. Exam papers expect 2–3 significant figures unless told otherwise.