efficency

• 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 joules of thermal energy, and 12,000 joules of thermal energy passes into the water. The efficiency is 0.8 or 80%.
  • An electric current delivers 60 watts of power to a light bulb, and 3 watts of electrical energy are transferred to light energy. The efficiency is 0.05 or 5%.
• Efficiency values greater than 1 or 100% are not possible
• Incandescent bulbs vs LED bulbs

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

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

  2. Place the heating element inside the water, as in an electric kettle, to reduce heating the pan itself
• Thermal conductivity


  How well a material conducts heat
• Plastic has lower thermal conductivity than metal, so less thermal energy is lost through the walls and lid of an electric kettle compared to a pan