Efficiency

• What can energy be transferred into?

  Another form of energy
• Why are most devices not 100% efficient?

  Because energy is lost in the process
• What is efficiency in terms of energy output?

  • Proportion of input energy converted to useful output
  • Useful energy output is considered light energy
  • Waste energy is considered heat energy
• How is efficiency calculated in terms of power?

  Efficiency = useful power output / total power input
• How is efficiency calculated in terms of energy?

  Efficiency = useful energy output / total energy input
• How do you calculate the efficiency of two lamps with given inputs and outputs?

  1. Calculate efficiency for each lamp:

  • Incandescent: \( \frac{45 \text{ J}}{300 \text{ J}} = 0.15 \)
  • LED: \( \frac{225 \text{ J}}{300 \text{ J}} = 0.75 \)

  1. Compare efficiencies to find how many times more efficient one is.
• How many times more efficient is the LED lamp compared to the incandescent lamp?

  5 times more efficient
• How do you convert efficiency figures into percentage terms?

  Multiply by 100
• What is a common mistake when calculating efficiency?

  Getting the division the wrong way around
• What happens if you calculate efficiency incorrectly?

  You may get a value greater than one
• What does an efficiency rating greater than 100% imply?

  More energy output than input, which is impossible
• What is the conservation of energy principle?

  • Energy can only be transferred
  • Energy cannot be created or destroyed
• How do you calculate useful power output from efficiency and total power input?

  Useful power output = efficiency × total power input
• What is the efficiency of the microwave mentioned?

  70% efficient
• What is the total power input of the microwave?

  800 watts
• What is the useful power output of the microwave?

  560 watts
• What is the exception to the rule of devices not being 100% efficient?

  • Devices designed to produce heat
  • Example: Electric heaters
• What is a common form of waste energy produced by devices?

  Thermal energy
• What counts as useful output for devices that produce heat?

  Thermal energy produced by the device