efficiency

• What does efficiency tell us about energy transfer?

  It tells us what fraction of the energy put into an appliance is transferred to useful forms of energy
• What are the equations used to calculate efficiency?

  • Efficiency = Useful output energy transfer / Total input energy transfer
  • Efficiency = Useful power output / Total power input
• If an electric hob transfers 15,000 J of thermal energy and 12,000 J passes into the water, what is the useful output energy transfer?

  12,000 J of thermal energy
• What is the total input energy transfer if an electric hob transfers 15,000 J of thermal energy?

  15,000 J of thermal energy
• How do you calculate the efficiency of the energy transfer in the example given?

  Efficiency = $\frac{12,000 \text{ J}}{15,000 \text{ J}} =$$0.8$
• What is the maximum possible efficiency value?

  1 or 100%
• How can efficiency be expressed besides as a decimal?

  As a percentage by multiplying the decimal value by 100
• If an electric current delivers 60 W to a light bulb and 3 W is transferred to light energy, what is the efficiency of the bulb?

  Efficiency = $\frac{3 \text{ W}}{60 \text{ W}} =$$0.05 \text{ or } 5\%$
• Why are LED bulbs more efficient than incandescent bulbs?

  • More energy is transferred to light
  • Less energy is wasted as thermal energy
• What happens to thermal energy when using a pan to heat water?

  Much thermal energy is wasted to the air around the hob and the surface of the water
• What are two ways to increase the efficiency of energy transfer when heating water?

  • Use a pan with a wider base
  • Use a lid to reduce heat loss
• Why is metal a poor choice for a kettle's walls compared to plastic?

  Metal is a good conductor of heat, allowing more thermal energy to escape
• What is thermal conductivity?

  It is a measure of how well a material conducts heat
• How can you reduce thermal energy loss when heating water?

  By placing the heating element inside the water
• What makes a kettle more efficient than a pan for heating water?

  • Less thermal energy escapes through the walls and lid
  • Heating element is submerged in water
• What should you be able to describe after watching the video?

  Ways to increase the efficiency of energy transfer
• Where can you find more questions on calculating efficiency?

  In the vision workbook linked in the video