The electric power is the product of current and voltage or current × voltage
The electric power is expressed in watts.
1 watt = 1 ampere × 1 volt
Say for instance an iron is rated at 1200 watts. If your home outlet is 120 volts, the iron will draw a current of 10 amperes.
Why? It is because 1200 = 10 × 120
At this point, you may not quite see why we are multiplying the current and the voltage to get the power. A little math will clarify this!
Recall that in the lesson about electric current, we saw that 1 ampere = 1 coulomb of charge per second.
The word per means divided by. Therefore, we can rewrite 1 coulomb of charge per second as shown below.
Recall also that in the lesson about electric potential, we saw that 1 volt = 1 joule of energy per 1 coulomb of charge.
Therefore, we can rewrite 1 joule of energy per 1 coulomb of charge as shown below.
We can multiply a current of 1 ampere by a voltage of 1 volt to see what we end up with.
1 coulomb gets cancelled since it is on top and at the bottom.
In the lesson about power, we saw that 1 joule of energy per second = 1 watt and watt is a unit of power.
From the formula, you can see that the electric power is the rate at
which energy is transferred. This second definition of electric power
can also help us to calculate the cost of electric energy and we will
show you how in a different lesson.