Students session


Energy is defined as the ability to do work.

Animals, including humans, obtain energy from food. When we eat food, energy is stored in our bodies for immediate and future usage.

The SI unit of energy is joules (J).


  • Light energy
  • Chemical energy
  • Electrical energy
    Solar energy
  • Heat energy
  • electric iron etc.
  • Nuclear energy
  • Sound energy
  • Mechanical energy
  •  Potential energy
  • Kinetic energy

Potential energy (P.E.)

Potential energy (P.E.): This is the energy a body has as a result of its position or
state. Examples of potential energy include:
i. A cat lying on a mat
ii. A piece of cloth hanging on line
iii. A girl sitting on a chair
iv. Mango hanging on a tree
The formula for potential energy is given as
P.E. = m x g x h
where m = mass, g = acceleration due to gravity and h = height of the body
A mango fruit of mass 0.06 kg hangs 4.0 m above the ground. Calculate its potential
energy. [g = 10 ms-2] Solution
P.E. = mgh
m = 0.06 kg, h = 4 m, g = 10 ms-2
P.E. = 0.06 x 10 x 4
P.E. = 2.4 J

Kinetic energy (K.E.)

This is the energy possessed by a body due to its motion. Examples of K.E. include:
i. A boy riding a bicycle
ii. A fan whirling around
iii. A car speeding
Kinetic energy is expressed as:
K.E. = ½ x m x v²
where m = mass and v = velocity Example
A particle of mass 20 g moves with a velocity of 3 m/s. What is its kinetic energy?
K.E. = ½ mv²
m = 20 g = 20
1000 = 0.02 kg, v = 3 ms-1
K.E. = ½ x 0.02 x 32
K.E. = 0.09 J

Energy transformation

Energy transformation is the process of changing energy from one form to another.

The law of conservation of energy

The law of conservation of energy states that energy cannot be made or lost, but can be changed from one form to another.

How to conserve energy

1. Switch off all electrical appliances that
are not in use.
2. Do not leave doors of fridges and
freezers open.
3. Do not put hot food in fridges and
4. Turn off and remove mobile phone
chargers from the power outlets after
charging mobile phones
5. Iron clothes in bulk.
6. Close all doors and windows when
using air-conditioners.
7. Use energy saving lamps.
8. Open doors and windows to allow in
fresh air instead of electric fans and air conditioners.
9. Turn off light during the (bright and
sunny) day.
10. Have your electric metre earthed.

Efficiency of energy

Efficiency of energy is the ratio of energy output to the energy input. Efficiency is expressed in percentage.
In other words, it is the ratio of energy put into a work to the energy obtained from it.

Efficiency = 𝑒𝑛𝑒𝑟𝑔𝑦 𝑜𝑢𝑡𝑝𝑢𝑡 / energy input x 100%

Efficiency of energy is never 100%. This is because parts of the energy put into work are used to overcome friction, inertia and gravity. This makes the energy output always less than the energy input.

Sample question:

A machine was supplied with 90 J of energy. It could do 75 J of work. Calculate its efficiency.
Energy output = 75 J, energy input = 90 J
Efficiency = energy output / energy input x 100% = 75 / 90 x 100 = 83%

Students study aid purpose only


Leave a Reply

Your email address will not be published.

Back to top button