Unit conversion
A quantity written using one unit may be more easily used or understood if it is converted into a different unit. The size of the measurement will be the same.
Converting unit prefixes
Converting a measurement expressed with a prefix back to standard form is necessary to perform calculations.
Example
Calculate the electrical power when a 50 mA current passes through a 230 V light bulb.
The standard unit for electrical power is watts (W). Electrical power is calculated by multiplying current by voltage. This calculation will come out in watts if the current is in amperes and the voltage in volts.
The current is 50 mA = 50 × 0.001 A = 0.05 A
power = current × voltage = 0.05 x 230
power = 11.5 W
Question
What is the power when a 12 mV LED has a current of 0.02 A pass through it?
The voltage is 12 mV = 12 × 0.001 V = 0.012 V
power = current × voltage = 0.02 x 0.012 = 2.4 × 10-4 W
power = 0.24 mW
Converting different units
In some cases a measurement may be given in a non-SI unit and needs to be converted into the SI equivalent to be useful. A common example of this is when a time is stated in hours but, for a calculation, it has to be given in seconds.
Example
Calculate the electrical energy transferred by a 35 W light bulb when it is on for four hours.
The standard unit for energy is joules (J). Energy is calculated by multiplying power by time. This calculation will come out in joules if the power is in watts and the time in seconds.
The time is 4 hours.
Each hour is 60 minutes long, and each minute is 60 seconds long.
1 hour = 60 × 60 = 3,600 s
energy = power × time
energy = 35 × (4 × 3,600) = 35 × 14,400 = 504,000 J
energy = 504 kJ
