Energy transfers and dissipation
The big idea behind P1.5 is that energy is conserved overall, but it is not always useful. As energy moves around a system, some of it always ends up in places we don't want — usually as a thermal store of the surroundings — and once it's there it is hard to recover. This is dissipation.
The conservation principle
Energy cannot be created or destroyed. It can only be transferred between stores.
A closed system (one that exchanges no energy with its surroundings) keeps the same total energy. The only thing that changes is which stores hold it.
Useful and wasted transfers
In every real device, some of the supplied energy ends up in the intended store (useful) and some ends up elsewhere (wasted).
| Device | Useful transfer | Common wasted transfers |
|---|---|---|
| Electric kettle | Electrical → thermal (water) | Thermal store of casing, sound from boiling |
| Filament lamp | Electrical → light radiation | Thermal store of bulb (most of the input!) |
| Petrol car engine | Chemical → kinetic of car | Thermal store of engine, exhaust, brakes |
| Speaker | Electrical → sound radiation | Thermal store of coil |
| Wind turbine | Kinetic of wind → electrical | Sound + thermal of bearings (small) |
Dissipation
Once thermal energy spreads out into the surroundings, it becomes too dilute to recover usefully. This is what we mean by dissipation. Friction is the most common cause:
- Sliding a book across a table — kinetic store empties; the surfaces' thermal stores fill.
- A bouncing ball — small heat in the ball and floor each bounce; ball reaches lower height each time.
- Bicycle brakes — kinetic of cyclist → thermal of brake pads (you can feel them get hot).
Reducing wasted transfers
You need to know practical ways to reduce dissipation in three contexts:
Mechanical (friction):
- Lubrication — oil/grease between moving parts.
- Smooth, polished surfaces in bearings.
- Streamlining shapes to cut air resistance.
Thermal (heat loss from a building):
- Loft insulation (fibreglass) — air pockets reduce conduction and convection.
- Cavity wall insulation (foam between two brick layers).
- Double glazing — air gap is a poor conductor.
- Draught excluders to stop convection currents through gaps.
- Reflective foil behind radiators — reflects IR back into the room.
Electrical:
- Thicker wires (lower resistance — less $I^2 R$ heating).
- Higher voltage transmission in the National Grid (lower current for same power).
✦Worked example— Example calculation
A motor receives 200 J of electrical energy. 150 J ends up in the kinetic store of a load; 50 J in the thermal store of the motor windings.
- Total energy is conserved (200 J in, 200 J out).
- Useful = 150 J. Wasted = 50 J. Efficiency = 75%.
Why entropy isn't on the spec — but is hinted at
Once energy is dissipated to the surroundings as a thermal store at low temperature, it cannot do useful work. This is the everyday version of the second law of thermodynamics. GCSE doesn't ask for the formal statement, but examiners do reward students who say "the energy is too spread out / too dilute to be useful".
⚠Common mistakes
- Saying energy "is lost". It is dissipated, not destroyed.
- Calling sound a "store". Sound is a radiation pathway that ends in a thermal store of the surroundings.
- Forgetting that insulation reduces but does not eliminate heat loss.
- Confusing conduction (through a solid by particle vibration), convection (in fluids by bulk movement) and radiation (electromagnetic, no medium needed).
➜Try this— Quick check
A pupil writes: "Friction destroys energy, and that's why moving things slow down." Rewrite this sentence using the correct GCSE language.
"Friction transfers energy from the kinetic store of the moving object to the thermal store of the surfaces, which is then dissipated to the surroundings. Energy is conserved overall."
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