Electrical Power Systems Technology

Power System Fundamentals 23

Sample Problem: Work

Work is done whenever a force (F) is moved a distance (d), or:

W = F × d, where

W = work in joules

F = force in newtons

d = distance the force moves in meters

Given: An object with a mass of 22Kg is moved 55 meters.

Find: The amount of work done when the object is moved.

Solution: The force of gravity acting on the object is equal to 9.8 (a

constant that applies to objects on earth) multiplied by the mass of the ob-

ject, or:

F = 9.8 × 22 Kg = 215.6 newtons

W = F × d

= 215.6 × 55

W = 11,858 joules

A third important term is "power." Power is the rate at which work is

done. It concerns not only the work that is performed but the amount of

time in which the work is done. For instance, electrical power is the rate at

which work is done as electrical current flows through a wire. Mechanical

power is the rate at which work is done as an object is moved against op-

position over a certain distance. Power is either the rate of production of

energy or the rate of use of energy. The watt is the unit of measurement of

power.

Sample Problem: Power

Power is the time rate of doing work, which is expressed as:

W

P = ——, where

t

P = power in watts

W = work done in joules

t = time taken to do the work in seconds

Given: An electric motor is used to move an object along a convey-