Electric power 

The electric power, expressed in watts, is the rate at which electric energy is converted into another form of energy such as light or heat.

Electric power

1 watt is the consumption of 1 joule of energy every second.

1000 watts = 1000 joules every second

  • A 40-watt light bulb burns 40 joules of energy every second (40 J/s)
  • An iron could use 1500 joules of energy every second (1500 J/s)
  • An electric car, charged at home, could use about 7200 joules of energy every second (7200 J/s)

40 J/s, 1500 J/s, and 7200 J/s are all examples of electric powers.

Another way to express the electric power

The electric power is also the product of current and voltage.

Electric power = current × voltage

1 watt = 1 ampere × 1 volt

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


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

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

Expanding the definition for electric power

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.

If
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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