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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
FormsUses
Heat EnergyIroning clothes, cooking, drying clothes, etc.
Light EnergyPhotosynthesis, Seeing, etc.
Sound EnergyCommunication, to kill cancerous cells in the body, etc.
Electrical EnergyTo power electrical equipment, etc.
Chemical EnergyTo do work, etc.
Nuclear EnergyTo produce electricity, etc.
Mechanical EnergyTo 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 ground

      Example
      • 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
    • 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
      = ½mv²

      Example
      • 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
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
  1. 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
  2. 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
  3. A body is pulled along a horizontal plane by a force of 10 N applied to the plane
    1. Calculate the work done in moving the body through a distance of 20 m.
      • W = F × h
      • = 10 × 20
      • = 200 J
    2. 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
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
  1. 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
  2. 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
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