Energy transfers in a system

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.

Students should be able to describe with examples where there are energy transfers in a closed system, that there is no net change to the total energy.

Students should be able to describe, with examples, how in all system changes energy is dissipated, so that it is stored in less useful ways. This energy is often described as being ‘wasted’.

---

Students should be able to explain ways of reducing unwanted energy transfers, for example through lubrication and the use of thermal insulation.

The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material.

Students should be able to describe how the rate of cooling of a building is affected by the thickness and thermal conductivity of its walls.

Students do not need to know the definition of thermal conductivity.

Efficiency

The energy efficiency for any energy transfer can be calculated using the equation:

\(\text{efficiency} = \dfrac{\text{useful output energy transfer}}{\text{total input energy transfer}}\)

Efficiency may also be calculated using the equation:

\(\text{efficiency} = \dfrac{\text{useful power output}}{\text{total power input}}\)

---

Students should be able to describe ways to increase the efficiency of an intended energy transfer.