WJEC Physics P1 Topic 3

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  • Created by: MrTom
  • Created on: 11-01-16 18:46

Electrical energy

Electricity is useful to us as it can be transferred around the country through the National Grid.

Electrical energy is transferred to other forms of energy in devices, e.g. a t.v. converts electrical energy into useful light and sound energy, as well as heat energy.

Energy is measured in joules, J.

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Power/Energy Equation

Power (watts) = energy transfer (J)

                                 time (s)

or: energy transfer = power x time

These equations are just re-arranged versions of each other

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Watts and Joules

For the exam you will need to remember that:

1 Watt = 1 Joule per second or 1 j/s


A more powerful kettle transfers more electrical energy into heat energy every second, so the water should boil quicker. This is because the greater the power of a device the faster it transfers energy from one form to another

eg a 2 watt kettle transfers 2 j/s whereas a 5 watt kettle  will transfer 5 j/s (More energy is transferred because there is more to start with)

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Cost of Electricity

Firstly use the prior equation to calculate the number of units

Cost = number of units x cost per unit

The cost per unit will be displayed in the question and will usually be around 15p (pence)

Using the example before, let's calculate the cost:

Cost = 0.5 x 15p = 7.5 pence (no need to round or convert to £'s)

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Micro generation is the small scale generation of electricity usually using wind turbines or solar panels at home or in schools or small businesses.

Generating your own electricity isn’t only a good way to save money on your bills - it also reduces your carbon footprint. The less electricity you use from the National Grid the less electricity has to be produced in the first place.

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Payback Time

Since wind turbines and solar panels are expensive to install and produce little electricity it is important to consider the cost-effectiveness of it (Will it end up costing you more?) You can do this easily by using the equation below:

Payback Time (Years) = Cost of fitting / (Yearly) Saving

Payback time is an estimate and subject to change if...

1) The cost of electricity changes

2) The weather is unpredictable - it could be a VERY sunny year

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