Topic Menu

Cyber-chess a chess instruction site

Electrical Power

Power is the rate of doing work.

P = E/t

It is measured in watts (W) - a Joule per second.

1 W = 1 J/s

We tend to think of electrical devices 'having a lot of energy' when they are in reality we should say they are 'powerful' because they can 'do work' - perform a task (transfer energy from one form to another) - very quickly.

The higher the wattage of an electrical system the faster it can 'do work'. (Also, the more costly it is to run - see kWh).

For example a 100W bulb is brighter than a 40W one because it transfers the electrical energy to light energy at a faster rate - therefore more photons hit your eye per second and you think of it as brighter.

The equation for electrical power is:

P = I V

We know that

V = IR

so

If we substitute for V into the power equation we get:

P = I2R

Therefore if we consider a wire of resistance R, the energy conversion that takes place due to current flow depends on the square of the current. If you double the current you increase the energy transfer into heat energy by a factor of four - triple it and the increase is a factor of nine.

The current 'heats the wire' it passes through it - the higher the current the more of the electrical energy is transferred to heat energy and is dissipated in the wire.

This is the reason why we transfer electrical energy at high voltage and low current - to minimise the heat energy transferred to the surroundings during transmission - minimising the 'wasting' of electrical energy in transit between the power stations and the consumers. We are able to do this using transformers.

Now I = V/R so if we substitute for that we get:

P = V2/R

Let's calculate the energy transferred by electricity

We know that

I = Q/t

therefore:

P = QV/t

Now,

P = E/t

So,

E = QV

(energy transfered by the electricity = charge x potential difference)