Reducing Unwanted Energy Transfers Flashcards

All 6 cards in this deck

How does lubrication reduce unwanted energy transfer in a machine?

Oil between the moving surfaces reduces friction, so less energy is dissipated to the thermal store of the surroundings.

How do the thickness and the thermal conductivity of a building's walls affect its rate of cooling?

Thicker walls cool the building more slowly; walls of lower thermal conductivity also cool it more slowly.

Hot water cans are wrapped in two different materials. How do you tell which material is the better insulator?

The better insulator is around the can whose water temperature falls the least in the same time (it cools more slowly).

Why do insulating materials such as loft insulation and cavity wall insulation contain trapped air?

Trapped air is a poor thermal conductor and cannot form convection currents, so energy is transferred out much more slowly.

True or false? Insulating a hot water tank stops energy being transferred to the surroundings completely.

False. Insulation only reduces the rate of energy transfer; the water still cools, just more slowly.

What does the thermal conductivity of a material describe?

How readily energy is conducted through the material: the higher it is, the faster energy passes through. e.g. copper is high, polystyrene foam is low.