AQA GCSE Physics: Conservation and Dissipation of Energy — Revision Notes

Condensed for the final weeks. For the full explanation, use the Conservation and Dissipation of Energy study guide.

Stores and pathways

Energy stores: kinetic, gravitational potential, elastic potential, chemical, thermal, magnetic, electrostatic, nuclear.

Transfer pathways: mechanically (by a force), electrically (by a current), by heating, and by radiation (light and sound).

Energy is transferred between stores; it is not “used up” or “created”. Marks are lost by writing “heat energy” or “light energy” as stores — those are transfer pathways, not stores. Say the thermal store increases, not heat energy is made.

Key equations

efficiency = useful output / total input     (x100 for a percentage)

The energy-store equations (kinetic, gravitational potential, elastic potential, thermal energy and power) belong to sub-topic 4.1.1 Energy changes in a system – see the Energy Changes in a System revision notes for those. This sub-topic, 4.1.2, uses them (for example, kinetic energy in the worked examples below) but does not introduce them.

Note that Eₖ depends on v², so doubling the speed quadruples the kinetic energy. That is why braking distance rises so sharply with speed, and it is the standard applied question.

Worked example. A 1200 kg car at 15 m/s has Eₖ = 0.5 × 1200 × 15² = 135,000 J. At 30 m/s, Eₖ = 0.5 × 1200 × 30² = 540,000 J — quadruple, not double, confirming the v² relationship, and the reason four times as much work must be done by the brakes to stop the car.

GPE-to-KE worked example. A 0.30 kg ball dropped from 2.5 m (g = 9.8 N/kg) loses Ep = mgh = 0.30 × 9.8 × 2.5 = 7.35 J of gravitational potential energy. If this converts entirely to kinetic energy (no air resistance), ½mv² = 7.35 gives v = √(2 × 7.35 ÷ 0.30) = 7.0 m/s. In reality the ball is slower, because some energy is transferred to the thermal store of the surrounding air by air resistance, and some to sound.

Conservation of energy

Energy cannot be created or destroyed, only transferred, stored or dissipated.

A closed system is one where no energy enters or leaves, so the total energy is constant, even though it moves between stores.

Dissipation — energy transferred to less useful stores, usually the thermal store of the surroundings, where it becomes spread out and difficult to use again. Friction and air resistance are the usual culprits.

“Wasted” energy is not destroyed. It is still there, just dissipated to the surroundings in a form too dilute to be useful. That distinction is examined regularly.

Efficiency

efficiency = useful output energy / total input energy

No device can be 100% efficient (except an electric heater, where all the output is the intended thermal energy), because some energy is always dissipated by friction, sound or unwanted heating.

Efficiency can never exceed 1 (100%), since useful output energy can never exceed total input energy. A calculated efficiency above 1 signals an arithmetic or measurement error, not a genuinely efficient device — the input and output values should be re-checked rather than the impossible figure reported.

Reducing unwanted transfers:

Method	How it works
Lubrication	Reduces friction between moving surfaces
Streamlining	Reduces air resistance
Thermal insulation	Reduces the rate of energy transfer by conduction

Insulating a house — the two factors that matter:

1. Thickness of the walls — thicker walls transfer energy more slowly.
2. Thermal conductivity of the material — lower conductivity means a slower rate of transfer.

Cavity wall insulation traps air, which has low thermal conductivity, and prevents convection currents in the cavity. Double glazing works the same way. Note the answer is about the rate of transfer, not about “stopping” heat.

Required Practical: investigating insulators. Identical containers of hot water are wrapped in different materials (or thicknesses of the same material), and the temperature of each is measured at regular time intervals as it cools. The material giving the smallest temperature drop over time is the most effective insulator — comparing several materials or thicknesses this way is what the required practical tests.

Power

Power is the rate of energy transfer, measured in watts (1 W = 1 J/s).

A more powerful device transfers the same energy in less time, or more energy in the same time. Comparing two devices means comparing energy per second, not total energy.

Sub-topic 4.1.3 National and Global Energy Resources (renewable and non-renewable resources, and evaluating them for reliability, environmental impact and cost) is covered in its own National and Global Energy Resources study guide, the next sub-topic after this one.

Exam traps

• Calling heat or light an energy store.
• Saying energy is “used up” or “lost”.
• Forgetting that Eₖ depends on v squared.
• Explaining insulation without mentioning the rate of transfer.
• Explaining a transfer without identifying which stores gain and which lose energy.
• Using cm or g in equations that need m and kg.

Self-test

1. Name four energy stores and the four transfer pathways.
2. State the principle of conservation of energy.
3. What actually happens to “wasted” energy?
4. What two properties of a wall determine its rate of energy transfer?
5. If a car doubles its speed, what happens to its kinetic energy, and why does that matter?

Answers: 1. Stores include kinetic, gravitational potential, elastic potential, chemical, thermal, magnetic, electrostatic and nuclear; pathways are mechanical, electrical, heating and radiation. 2. Energy cannot be created or destroyed, only transferred between stores, stored, or dissipated. 3. It is dissipated to the thermal store of the surroundings, becoming spread out and too dilute to be useful — it is not destroyed. 4. Its thickness and its thermal conductivity. 5. Kinetic energy quadruples, because Eₖ depends on v²; this is why braking distance increases so steeply with speed.