Section 3 - Technical Basics

3.2 Power

Power is a measure of the electrical energy used by a device. It can be used by the device in any number of ways, but most  often electrical energy is converted to heat. A bulb filament is a resistance, when current flows through it, it gets hot, so hot that it glows white  Notice that a higher power bulb glows brighter than a lower power one. This is because it is converting more electrical energy to heat (and light) than the lower power one.

Note that this conversion of electrical energy into heat occurs whenever electric current flows through a resistance. All wires have some resistance, which is why they can get hot if we try to pass too much current through them!

POWER and has the symbol P. It is measured in Watts, abbreviated W.

We can calculate power as follows;

Power (P) in WATTS = Voltage (in Volts) multiplied by the Current (in Amps) or
P = I x V, as before the "x" for multiply is normally omitted so we get

P = I V

We have a triangle "aide memoir" for this too which we use in the same way as the one we used for Ohm's law previously by covering the term we want to calculate.

We can see that both voltage and current are related to power. In the case of mains light bulbs, since the voltage is the same, a higher power bulb must be drawing more current than the lower powered one. Let's have a look at using the triangle to calculate a few examples.

1. What is the current drawn by a 100W light bulb assuming a 230V mains supply?

We want to calculate the current (I), so we cover I on the triangle showing that
I = P / V = 100W / 230V = 0.44 A

2. A bulb is connected to a 12V battery and the current drawn is measured as 0.5A. What is the power consumption of the bulb?

We want to calculate the power (P), so we cover P on the triangle showing that
P = I x V = 0.5A x 12V = 6W

3. A motor consumes 1495W at a current of 6.5A. What is the supply voltage?

We want to calculate the voltage (V), sop we cover V on the triangle showing that
V = P / I = 1495W / 6.5A = 230V

That wasn't any harder than the Ohm's law questions earlier was it? The maths never gets any more complicated than this at this level. Indeed, it only gets easier from now on!