Efficiency Flashcards

All 5 cards in this deck

What is the equation for efficiency in terms of energy?

efficiency $= \dfrac{\text{useful output energy transfer}}{\text{total input energy transfer}}$

What is the equation for efficiency in terms of power?

efficiency $= \dfrac{\text{useful power output}}{\text{total power input}}$

What are the steps to calculate the efficiency of a device?
e.g. a lamp is supplied 200 J and transfers 50 J usefully as light

1. Identify the useful output (50 J) and total input (200 J)
2. Divide: $\dfrac{50}{200} = 0.25$
3. Multiply by 100 for a percentage: 25%

What are the steps to find the useful power output from an efficiency?
e.g. efficiency 0.75, total power input 0.24 W

1. Write $0.75 = \dfrac{\text{useful power output}}{0.24}$
2. Rearrange: useful power output $= 0.75 \times 0.24$
3. $= 0.18$ W

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

False. The useful output can never be more than the total input, because energy cannot be created.