Efficiency & Power Flashcards

All 6 cards in this deck

What is the equation for efficiency in terms of energy?

efficiency = useful energy transferred by the device ÷ total energy supplied to the device

What is the equation for efficiency in terms of power?

efficiency = useful power output ÷ total power input

What are the steps to find the useful energy output of a device from its efficiency?

e.g. efficiency 0.70 with 6500 kJ supplied

1. efficiency = useful output ÷ total input
2. Rearrange: useful output = efficiency × input = $0.70 \times 6500$
3. Useful output = 4550 kJ

What are the steps to find the energy wasted by a device from its efficiency?

e.g. 200 J supplied, efficiency 0.25

1. Useful energy = $0.25 \times 200 = 50$ J
2. Wasted = total supplied − useful = $200 - 50$
3. Wasted energy = 150 J

What are the steps to write an efficiency given as a decimal as a percentage?

e.g. efficiency 0.35

1. Multiply by 100: $0.35 \times 100 = 35$
2. Add the percent sign: efficiency = 35%

True or false? A device can have an efficiency greater than 1 (100%).

False. The useful output can never be more than the total energy supplied.