Efficiency

• Efficiency


  The fraction of the energy we put into an appliance that is transferred to useful forms of energy
• Calculating efficiency


  1. Useful output energy transfer / Total input energy transfer

  2. Useful power output / Total power input
• Efficiency equations are not given in the exam, so you need to learn them
• Calculating efficiency


  • An electric hob transfers 15,000 J of thermal energy, and 12,000 J passes into the water. Efficiency = 12,000 / 15,000 = 0.8 or 80%
• Efficiency greater than 1 or 100% is not possible
• Calculating efficiency


  • An electric current delivers 60 W to a light bulb, and 3 W is transferred to light energy. Efficiency = 3 / 60 = 0.05 or 5%
• Incandescent bulbs vs LED bulbs

  • Incandescent bulbs are less efficient, with most energy transferred to thermal energy
  • LED bulbs are more efficient, with a greater percentage of energy transferred to light
• Increasing efficiency of heating water

  1. Use a pan with a wider base and lid to reduce unwanted thermal energy transfer

  2. Place the heating element inside the water, as in an electric kettle, to avoid heating the pan itself

  3. Use materials with lower thermal conductivity, like plastic, to reduce heat loss through the walls and lid
• Thermal conductivity will be covered in a later video