If a bulb has a current of 20mA and voltage of 5V, and the current cost of electricity is £3 for a kW/hour. How much money would you spend to power the bulb for 8 hours? Are these good estimates for the current, voltage and cost of electricity?

Well we know that the power of the bulb is given by P = IV. Therefore the power = (20 x 10^-3 Coulomb per second) * (5 Joules per Coulomb) = 100 x 10^-3 Joules per second = 0.1 W.

So the bulb uses 0.1 Joules of energy per second or 0.1 Watts. 

To power the bulb for one hour you need 0.1 W  = 0.0001kW

Therefore 0.0001 * 300 pence = 0.03 pence per hour. 

So for 8 hours it would cost 0.24 pence.