Power (Also seen in GCSE Physics 1)

Power is a measure of how quickly something can transfer energy. Power is linked to energy by the equation:

Power is measured in Watts, W

Energy is measured in Joules, J

Time is measured in seconds, s

New Equations

If we look at the equations from the QVIRt lesson we can derive some new equations for energy and power.

Energy

can be rearranged into and we know that so combining these equations we get a new one to calculate the energy in an electric circuit:

<---------------------- so (1)

Power

If we look at the top equation, to work out power we divide energy by time:

which cancels out to become (2)

If we substituteinto the last equation we get another equation for power:

<---------------------- so (3)

We can also rearrange into and substitute this into to get our last equation for power:

<---------------------- so (4)

Energy again

Two more equations for energy can be derived from the equation at the top and equations 3 and 4

Energy = Power x time

Equation 3 becomes (5)

Equation 4 becomes (6)

Fuses (Also seen in GCSE Physics 2)

Electrical devices connected to the Mains supply by a three-pin plug have a fuse as part of their circuit. This is a thin piece of wire that melts if the current through it exceeds its maximum tolerance. The common fuses used are 3A, 5A and 13A. A 100W light bulb connected to the UK Mains would have a 240V potential difference across it. Using we can see that the current would be 0.42A so a 2A fuse would be the best to use.

Applications

The starter motor of a motor car needs to transfer a lot of energy very quickly, meaning its needs a high power. Millions of Joules are required in seconds; since the voltage of the battery is unchanging we need current in the region of 160A which is enormous.

The power lines that are held by pylons and form part of the National Grid are very thick and carry electricity that has a very high voltage. Increasing the voltage lowers the current so if we look at the equation we can see that this lowers the energy transferred to the surroundings.