Energy Transfers in a System

Six flasks of hot water, all the same, wrapped in different things. Press Go and watch them cool for ten minutes. Then switch to the layers comparison and see what happens when you keep adding bubble wrap — the answer is not what most students expect.

Useful transfers and wasted ones

Energy is never created or destroyed — but that doesn't mean it all ends up somewhere useful. In any real system, some energy is transferred to stores we didn't want:

• A light bulb gives out light (useful) and warms the room (wasted).
• A car engine turns the wheels (useful) and heats the engine and the air (wasted).
• A flask of hot water keeps the drink warm (useful) and warms the room (wasted).

Wasted energy is dissipated to the thermal store of the surroundings. It is spread out so thinly that we can't get it back — but it is not destroyed.

Reducing unwanted energy transfers

• Lubrication. Oil between moving parts reduces friction, so less energy is dissipated to the thermal store.
• Thermal insulation. Wrapping something in a material with a low thermal conductivity slows the rate at which energy escapes.
• Streamlining. A smoother shape reduces air resistance, so less energy is wasted.

Thermal conductivity

This is the key idea of the whole spec point, and it has a precise meaning.

Metals have a high thermal conductivity — that's why a metal spoon in a hot drink gets hot quickly. Materials that trap air, like wool, polystyrene and bubble wrap, have a low thermal conductivity, which is what makes them good insulators.

Working out a rate of change of temperature

You don't need a fancy equation. It is just the temperature change divided by the time taken:

Keeping a building warm

Exactly the same physics applies to houses. The rate at which a building cools depends on:

• the thermal conductivity of the walls — lower is better;
• the thickness of the walls — thicker is better;
• the temperature difference between inside and outside — a bigger difference means a faster rate of energy transfer.

So cavity wall insulation, loft insulation and double glazing all work the same way: they add a layer of low thermal conductivity material (usually one that traps air), which gives the house a lower rate of change of temperature.

Whatever the investigation, the conclusion follows the same shape. Fill in the gaps:

The investigation demonstrated end to end, including how to describe the results using the keywords.