Force, Work, Power and Energy (With Electrical Units) | Engineering

In this article we will discuss about force, work, power and energy with their electrical units.

Force:

A force is that physical cause which when acting on a body changes or tends to change the state of rest or of uniform motion of the body in a straight line.

The MKS or SI unit of force is the newton which is defined as the force which when applied to a mass of one kg, gives it an acceleration of 1 m/s².

Hence the force F required to give a mass m an acceleration a is:

F (newton) = in (kg) × a (m/s²).

Work is said to be done by or against a force, when its point of application moves in or opposite to the direction of the force and is measured by the product of the force and the displacement of the point of application in the direction of force.

I.e. Work done, W = Force [F] × distance [d]

The SI or MKS unit of work is the joule, which is defined as the work done when a force of one newton acts through a distance of one metre in the direction of the force.

Hence if a force F acts through distance d in its own direction:

W = F [newtons] × d [metres] = Fd joules

Power is defined as the rate of doing work or the amount of work done in unit time.

The MKS or SI unit of power is the joule/second or watt. In practice, the watt is often found to be inconveniently small and so a bigger unit, the kilowatt is frequently used.

1 kilowatt = 1,000 watts

The bigger unit of power, most commonly used in engineering practice (not at all in SI system) is horse power defined as below:

Metric Horse Power:

It is the practical unit of power in MKS system (not in SI system) which according to ISI specifications is equal to 75 kgf-m of work done per second.

Energy is defined as the capacity of doing work. Its units are same as those of work, mentioned above. If a body having mass m, in kg, is moving with velocity v, in metres/second.

Kinetic energy = 1/2 mv² joules

If a body having mass m, in kg, is lifted vertically through height h, in metres, and if g is the gravitational acceleration, in metres/second² in that region, potential energy acquired by the body.

= Work done in lifting the body = mgh joules = 9.81 mh joules

As already stated, in SI system the unit of energy of all forms is joule. Bigger unit of energy is mega joules (MJ) where 1 MJ = 10⁶ J.

The thermal units of energy, calorie (gm-calorie) and kilo-calorie (kilogram-calorie), are defined below:

Calorie:

It is the amount of heat required to raise the temperature of one gram of water through 1 °C.

1 calorie = 4.18 J = 4.2 J

Kilo-Calorie:

It is the amount of heat required to raise the temperature of 1 kg of water through 1 °C.

1 k. calorie = 1,000 calories

= 4,180 joules = 4,200 J

Electrical Units of Work, Power and Energy:

The unit of work done and of energy expended is joule. It is equal to the energy expended in passing 1 coulomb of charge through a resistance of 1 ohm i.e., the energy expended in passing one ampere current for 1 second through a resistance of one ohm is taken as one joule. It may also be expressed as 1 watt- second i.e., one watt of power consumed for one second.

i.e., 1 joule = 1 watt-second

The unit of energy, joule or watt-second is too small for practical purposes, so a bigger unit Mega joule (MJ) or kilowatt-hour (kWh) is used in electrical engineering.

1 kWh = 1,000 watt-hours = 1,000 × 3,600 watt-seconds or joules = 3.6 MJ

The kWh, also called the Board of Trade (BOT) unit, is the energy absorbed by supplying a load of 1 kW or 1,000 watts for the period of one hour. This is legal unit on which charges for electrical energy are made, and, therefore, it is called the Board of Trade (BOT) unit.

Watt:

It is defined as the power expended when there is an unvarying current of one ampere between two points having a potential difference of one volt. As already stated the bigger unit of power is kW or Mega-watt.

1 kW = 1,000 watts

1 MW = 1,000 kW = 1 × 10⁶ watts