8.4. Electric power

We have always treated electricity as a form of energy, when electrical appliances are powered they will  transform this electrical energy consumed, the period of time they are connected to the power source.

• E is the energy transferred in kilowatt-hours, kWh

• P is the power in kilowatts, kW

• T is the time in hours, h.

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.

For example, 1,000 W = 1,000 ÷ 1,000 = 1 kW.

Exercise.

The rate at which the work is being done in an electrical circuit is called an electric power. In other words, the electric power is defined as the rate of the transferred of energy 

Electrical power measures the energy that is consumed by a load device or delivered by a power source over a certain period. It is measured in watts (W) or kilowatts (kW). 

In general, the power consumed by household appliances is always indicated on them.

Exercise.

Joule effect

As electrons move across electrical conductors, some of their kinetic energy is transformed into heat due to the friction. This is known as the Joule effect.

Sometimes, we want this effect (for example, with electric heaters, ovens, toasters, etc.). Other times, we do not want this heat but it is unavoidable. In these cases, we consider heat as an energy loss. An example of this is electrical wires, which can overheat and cause a fire.

Joule’s law formula is: 

Q = I2 . R . t

where:

• Q is the amount of heat, in Joules (J)

• I is the electric current that flows through the wire, in amperes (A)

• R is the value of the electrical resistance of the wire, in ohms (R)

• t is the amount of time that current passes through the wire, in seconds (s).

Joule’s law can be established as the amount of heat (Q) generated in a wire with resistance (R), when a current (I) passes through it for a period of time (t).