Efficiency

• What does energy efficiency measure?

  The proportion of energy which is wasted
• Why is it important to reduce the rate at which energy is wasted?

  To save energy, especially thermal energy
• What is the definition of efficiency in the context of energy devices?

  • Efficiency is how good a device is at transferring energy input to useful energy output.
  • It is the fraction of the energy supplied to a device which is transferred in a useful form.
• How can efficiency be calculated?

  Efficiency = \(\frac{useful \ energy \ transferred}{total \ energy \ supplied}\)
• What is the formula to calculate the percentage of efficiency?


  Percentage of efficiency = efficiency \(\times 100\)
• If a device supplies 140 J of energy and transfers 108 J usefully, what is its efficiency?

  Efficiency = \(\frac{108}{140} = 0.77\)
• What is the percentage efficiency of a device that has an efficiency of 0.77?

  Percentage efficiency = \(0.77 \times 100 = 77\%\)
• What happens to energy that is dissipated?

  • Dissipated energy spreads out and transfers into less useful forms.
  • It often raises the temperature of the surroundings and is referred to as 'wasted' energy.
• What is the maximum possible efficiency of a device?

  Efficiency cannot be greater than 1 or 100%
• What does breaking the law of conservation of energy imply about efficiency?

  It implies that more energy cannot be transferred than is supplied
• What are some reasons devices waste energy?

  • Friction between moving parts
  • Electrical resistance
  • Unwanted sound energy
• How can mechanical devices be made more efficient?

  • By using lubrication to reduce friction between moving parts
• How can the efficiency of thermal energy transfer systems be improved?

  • By reducing the wasteful dissipation of thermal energy to the surroundings
  • Using insulation to minimize heat loss