Work, Energy, and Power
ENERGY
- Energy is the ability or the capacity to do work.
- The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another.
Sources of Energy
- There are natural and artificial sources of energy. The natural sources include:
- The Sun – gives radiant energy which warms up the earth. Plants manufacture starch using the energy from the sun.
- Food – When food is broken down in the body chemical energy is released which makes us grow and carry out our activities.
- Wood
- Natural gas
- Wind – can be used to run engines and generate electricity.
- Water – can be used to drive hydroelectric generators for production of electricity
- Artificial sources of energy are:
- Electrical generators – produce electricity
- Batteries
- Petroleum products – E.g. Kerosene, petrol, diesel oil.
Forms of Energy
Forms | Uses |
Heat Energy | Ironing clothes, cooking, drying clothes, etc. |
Light Energy | Photosynthesis, Seeing, etc. |
Sound Energy | Communication, to kill cancerous cells in the body, etc. |
Electrical Energy | To power electrical equipment, etc. |
Chemical Energy | To do work, etc. |
Nuclear Energy | To produce electricity, etc. |
Mechanical Energy | To create motion, etc. |
- Mechanical Energy is divided into:
- Potential Energy – The type of energy due to its position. This energy increases as the height increases, and decreases as it moves towards the ground.
P.E. = mgh;
where m – mass of the body in kg
g – acceleration due to gravity in m/s²
h – distance of the body from the groundExample
- A mass of 100 kg is released from a height of 200 m. With what energy does the body strike the ground? (Take g = 10 m/s²).
P.E. = mgh
= 100 × 10 × 200
= 200000 J
- A mass of 100 kg is released from a height of 200 m. With what energy does the body strike the ground? (Take g = 10 m/s²).
- Potential Energy – The type of energy due to its position. This energy increases as the height increases, and decreases as it moves towards the ground.
-
- Kinetic Energy – This type of energy is possessed by a moving body. E.g. A moving car, a fired bullet, falling objects etc.
K.E. = Work done
= F × speed
= ½mv²Example
- A bullet of mass 40 g is moving with a speed of 2.16 km/h. Calculate its kinetic energy
mass = 40 g = 40/1000 = 0.04 kg
velocity = (2.16 × 1000 m)/(60 × 60 s)= 2160 m/3600 s= 0.6 m/s
- A bullet of mass 40 g is moving with a speed of 2.16 km/h. Calculate its kinetic energy
- Kinetic Energy – This type of energy is possessed by a moving body. E.g. A moving car, a fired bullet, falling objects etc.
Transformation of Energy
- Energy can be transformed from one form to another. For instance:
- Chemical energy is transformed to light energy in candles and matches.
- Electrical energy is converted to heat energy in electrical irons.
- Chemical energy is converted to mechanical energy when petrol is burnt in a combustion engine
- Light energy is converted to heat energy when clothes are dried by sunlight
Renewable and Non-renewable Energy
- Renewable energy is energy that can be restored to its original form after being used.
- Non-renewable energy is energy that can not be restored to its original form after being used.
- Examples of renewable energy include energy from:
- Sun
- Wind
- Water
- Examples of non-renewable energy include energy from:
- Natural gas
- Gas
- Diesel oil
- Petrol
- Kerosene
- Wood
- Battery
WORK
- Work is the product of force and distance moved, i.e. work is said to be done when a force moves a body through a distance.
- It is measured in Joules (J)
- Work done = Force × Distance
- Work done = F × h = mgh or ½mv²
-
- where W – Work done in Joules (J)
- h – Distance moved in meters (m)
- g – Acceleration due to gravity in m/s²
- v – Velocity or Speed in m/s
-
Examples
- What work is done when a mass of 5 kg is raised through a vertical height of 2.5 m. (Take g = 10 m/s).
- W = mgh
- = 5 × 10 × 2.5
- = 125 J
- Calculate the work done if a box is pulled by a boy with a force of 150 N through a distance of 50 m.
- W = F × h
- = 150 × 50
- = 7500 J
- A body is pulled along a horizontal plane by a force of 10 N applied to the plane
- Calculate the work done in moving the body through a distance of 20 m.
- W = F × h
- = 10 × 20
- = 200 J
- Calculate the work done on the same body for the same distance but applied in a direction making an angle of 60° to the horizontal.
- W = Fhcosθ
- = 10 × 20 × cos 60°
- = 10 × 20 × 0.5
- = 100 J
- Calculate the work done in moving the body through a distance of 20 m.
POWER
- Power is defined as the rate of doing work with time and it is measures in Watts (W).
- Power = Work done ÷ Time
= (F × v)/t
= (½mv²)/t
Examples
- What is the power of a girl that has done work of 50 J in 10 seconds?
- Power = Work done/time
- = 50/10
- = 5 W
- An engine raised 100 kg of water through a height of 60 m in 20 seconds. What is the power of the engine? (Take g = 10 m/s²).
- Power = mgv/t
- = (100 × 10 × 60)/20
- = 60000/20
- = 3000 W