An important aspect of any electrical or electronic circuit is the power associated with it. It is found that when a current flows through a resistor, electrical energy is converted into heat. This fact is used by electrical heaters which consist of a resistor through which current flows. Light bulbs use the same principle, heating the element up so that it glows white hot and produces light. At other times much smaller resistors and very much smaller currents are used. Here the amount of heat generated may be very small. However if some current flows then some heat is generated. In this instance the heat generated represents the amount of electrical power being dissipated.
The unit of power is the watt which is often denoted by the symbol W. A typical light bulb may consume 60 watts of power. A domestic electric heater may consume a thousand watts or a kilowatt (kW). Even higher power levels are measured in Megawatts (MW) or millions of watts.
At the other end of the scale powers below a watt are commonly encountered. A milliwatt (mW) is a thousandth of a watt, and a microwatt is a millionth of a watt.
The amount of power dissipated in a circuit can be easily determined. It is simply the product of the potential difference or voltage across the particular element, multiplied by the current flowing through it. In other words an electrical fire running from a 250 volt supply, and consuming 4 amps of current will dissipate 250 x 4 = 1000 watts or 1 kilowatt. In other words.
W = V x I
W = power in watts
V = potential in volts
I = current in amps
In some instances the actual resistance of the circuit element may be known. By using Ohm's Law ( V = I x R) it is possible to calculate the power if either the voltage or current is known. For example the mains voltage may be known to be 250 volts and the element resistance may be known to be 62.5 Ohms.
By performing some simple algebra it is possible to discover the very useful formulae:
W = V^2 / R
W = I^2 x R
Using these formulae it is simple to work out the power dissipated in the 62.5 ohm resistor when a voltage of 250 volts is placed across it