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
