Power

Power

Like work and energy, the word "power" is used in a special way in physics. Power may be defined as the energy converted per unit time.

The word "converted" is to be understood in a rather general sense. It can simply mean light energy flowing from the sun to the earth. It can mean chemical energy in a piece of burning wood changing to heat. It can mean the electrical energy in a light bulb changing to light.

The definition of power can be written as a formula: P = E/t, where P is power in watts (W), E is energy in joules (J) and t is time seconds (s). If the energy is converted to work, the formula may also be written as P = W/t, where W is the work done (J).

There is another useful formula. Suppose the work done refers to a force F pushing a body over a distance d. Then W = Fd, and the power is P = W/t = Fd/t. But d/t is just the velocity v. So power can also be written as P = Fv.

Can we still use P = Fv is velocity is changing? Yes. In that case, the F and v would be the instantaneous force and velocity, i.e. at an instance in time.

We should mention that efficiency can also be defined in terms of power.

The efficiency of a practical device is defined as the fraction of energy input that is useful. Since power is just the energy in a certain time, we can also replace the word "energy," by "power." So we can have another definition of efficiency: the fraction of power input that is useful.

The Physics Notes