Concept of Work, Energy and Power – JSS2 Basic Science Lesson Note (Second Term)

Work is done when a force moves a body through a distance in the direction of the force.

• Energy: the ability to do work.
• Power: the rate of doing work.
• Units: work and energy are measured in joules (J), and power in watts (W), where 1 watt is 1 joule per second.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain the meaning of work, energy and power;
2. state the formulae for work, force and power, and their units;
3. explain why the sun is the primary source of energy;
4. list examples of secondary sources of energy;
5. list the different forms of energy.

What are work, energy and power?

Work, energy and power are three linked quantities: energy is needed to do work, and power tells how fast the work is done.

• Energy change: when work is done, energy is changed from one form to another.
• Muscles: the energy stored in our muscles is made available when we do work.
• Stored to moving: stored energy (potential energy) changes to kinetic energy as work is being done.

What is work?

Work is done whenever a force is applied to make a body move.

• Example: you lift a bucket of water from the ground to your head. You have done work by lifting it.
• No movement, no work: if you then stand still for hours holding the bucket, you do no more work.
• Needs: a force and movement in the direction of the force.

What is energy?

Energy is the ability to do work by exerting a force to move an object through a distance.

• Anything that can do work has energy: a boy running a 100-metre race has energy because he can move over a distance.
• Unit: work and energy are measured in the same unit, the joule (J).

What is power?

Power is the rate at which work is done, that is, the work done in one second.

• Example: two boys of the same weight climb the same flight of stairs.
• Greater power: the boy who reaches the top first has more power, because he did the same work in a shorter time.
• Formula: power = work done (or energy used) ÷ time taken.
• Unit: if work is in joules and time is in seconds, power is in watts (W).

What are the units of power?

How are work, force and energy related?

Work is the product of force and the distance moved in the direction of the force.

• Work and energy: the work done is equal to the energy put in.
• Energy input: for any work to be done, there must be energy, since energy is the capacity to do work.
• Joule: the unit of work and energy is named after the scientist James Prescott Joule, who carried out early studies on energy.

What is the formula for work?

Work done = force (F) × distance (d) moved in the direction of the force.

• Unit of force: newton (N).
• Unit of distance: metre (m).
• Unit of work: newton metre, which is the joule (J).
• Worked example: a force of 50 N moves a box 4 m. Work = 50 × 4 = 200 J.

What is force?

Force is that which changes a body’s state of rest or of uniform motion in a straight line.

• Formula: force = mass × acceleration, that is, F = m × a.
• Meaning of letters: F is force, m is mass and a is acceleration.
• Unit: newton (N), with mass in kilograms and acceleration in metres per second squared.
• Worked example: a mass of 5 kg accelerates at 2 m/s². Force = 5 × 2 = 10 N.
• Link to work: since F = m × a, work = mass × acceleration × distance.

How do we calculate power?

1. Write the formula: power = work done ÷ time taken.
2. Put in the values, with work in joules and time in seconds.
3. Divide and write the answer in watts.
4. Example: work of 200 J done in 10 s gives power = 200 ÷ 10 = 20 W.

What are the quantities, formulae and units?

Why is the sun the primary source of energy?

The sun is the primary source of energy because almost all the energy we use comes from it, directly or indirectly.

• Supply: the sun gives out energy every day and human use does not use it up.
• Light and heat: it sends solar energy to the Earth as light and heat, which can change into other forms.
• Food: green plants use solar energy to make food by photosynthesis.
• Passing it on: man and animals get this stored energy when they eat yam, rice, cassava, beans and grasses.
• Other uses: solar energy can be changed to chemical and electrical energy for heating, boiling water and lighting homes and streets.
• Secondary sources: other sources of energy depend on the sun and are called secondary sources.

What are examples of secondary sources of energy?

A secondary source of energy is one that gets its energy from another source, such as the sun.

1. Food: energy stored by plants and passed on to animals and man.
2. Petrol: a fuel formed from the remains of ancient living things.
3. Wood: energy stored by trees and released when burnt.
4. Water: moving water can turn turbines to make electricity.
5. Chemicals: chemical energy is released in reactions.
6. Magnet: it has magnetic energy.
7. Battery: it stores chemical energy and gives electrical energy.

What are the forms of energy?

There are ten common forms of energy: mechanical, kinetic, potential, chemical, atomic, sound, electrical, light, heat and magnetic.

Energy can be changed from one form to another.

Evaluation

1. Define work, energy and power.
2. Write the formula for power and state its unit.
3. State the unit of force and the formula for finding it.
4. Why is the sun called the primary source of energy?
5. Convert 1 kW and 1 MW to watts.

Assignment

1. List five forms of energy, then explain how the energy in the yam you eat came originally from the sun.