Electric power 

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.

1 ampere =
1 coulomb / 1 second

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.

1 volt =
1 joule / 1 Coulomb

We can multiply a current of 1 ampere by a voltage of 1 volt to see what we end up with.

current  × voltage =
1 coulomb / 1 second
1 joule / 1 coulomb

1 coulomb gets cancelled since it is on top and at the bottom.

current × voltage =
1 joule / 1 second

The above also means 1 joule per second.

In the lesson about power, we saw that 1 joule of energy per second  = 1 watt and watt is a unit of power.

Therefore, power = current × voltage =
1 joule / 1 second

We can generalize the formula. Keep in mind that joule represents energy and 1 second represents time.

Electric power definition #2

Electric power =
energy / time

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.

10 / 2
= 5, then 10 = 5 × 2

If electric power =
energy / time
  then, energy = electric power × time

energy = electric power × time is the formula to use to calculate the cost of electric energy.

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