Conservation of Energy & Energy Transfer

A very important and useful fact about energy is that it can change from one form to another. When a machine does work the energy is not used up rather it is transferred – converted from one form to another. The amount of energy stays the same or is said to be conserved.

This fact is known as the Principle of Conservation of Energy.

Principle of Conservation of Energy:
Energy can neither be created nor destroyed, but can be changed from one form to another.

 

The principle of conservation of energy can be explained by considering the energy changes in a simple pendulum.



Example of Energy Transfer

A cement work uses a conveyor to transport 25kg cement bags from the ground floor to the pallet sorting machine situated on the first floor. The conveyor uses an electric motor with a voltage of 220V and current of 3 A. The time taken for the cement bag to reach the first floor is 5 s.

Calculate the work done in lifting one bag of cement from the ground floor to the first floor and the electrical energy transferred by the motor in 5 seconds?

Solution:

Work done in lifting one bag

Work done = energy transferred
Cement is lifted to a height of 6m so it will gain gravitational potential energy.

GPE = mass x acceleration due to gravity x height
GPE = 25 x 9.8 x 6 = 1470 Joules

Tip: If we look closely at the gravitational potential energy equation it is the same as that for Work done = force x distance

Force = mass x acceleration, here the acceleration is due to gravity therefore,
Force = m x g, substituting this value in the work done equation gives:

Work done = m x g x distance, here the distance is height thus
Work done = m x g x h

Electrical energy transferred in 4 seconds

Electrical Energy = V x I x t
Electrical Energy = 220 x 3 x 5 = 3300 Joules

Not all the electrical energy is transferred to gravitational potential energy but is loss as heat via friction to the moving parts of the motor and conveyor and also as noise energy as the moving parts contact each other.