

OCR  P2: Sustainable energy
P2.1 How much energy do we use? 
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 
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 

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