OCR - P2: Sustainable energy

P2.1 How much energy do we use?
Background to the topic What you should be able to do:

Energy is considered as being stored in a limited number of ways: chemical, nuclear, kinetic, gravitational, elastic, thermal, electrostatic and electromagnetic and can be transferred from one to another by processes called working and heating.

Electricity is a convenient way to transfer energy from source to the consumer because it is easily transmitted over distances and can be used to do work in many ways, including heating and driving motors which make things move or to lift weights.

When energy is used to do work some energy is usually wasted in doing things other than the intended outcome, it is dissipated into the surroundings, ultimately into inaccessible thermal stores.

The power of an appliance or device is a measure of the amount of energy it transfers each second, i.e. the rate at which it transfers energy.

1. Describe how energy in chemical stores in batteries, or in fuels at the power station, is transferred by an electric current, doing work on domestic devices, such as motors or heaters

2. Explain, with reference to examples, the relationship between the power ratings for domestic electrical appliances, the time for which they are in use and the changes in stored energy when they are in use

3. Recall and apply the following equation in the context of energy transfers by electrical appliances:
energy transferred (J, kWh) = power (W, kW) × time (s, h)

4. Describe, with examples, where there are energy transfers in a system, that there is no net change to the total energy of a closed system (qualitative only)

5. Describe, with examples, system changes, where energy is dissipated, so that it is stored in less useful ways

6. Explain ways of reducing unwanted energy transfer e.g. through lubrication, thermal insulation

7. Describe the effects, on the rate of cooling of a building, of thickness and thermal conductivity of its walls (qualitative only)

Sankey diagrams are used to show all the energy transfers in a system, including energy dissipated to the surroundings; the data can be used to calculate the efficiency of energy transfers.

8. Recall and apply the equation:
efficiency = useful energy transferred ÷ total energy transferred
to calculate energy efficiency for any energy transfer, and describe ways to increase efficiency

9. Interpret and construct Sankey diagrams to show understanding that energy is conserved