Efficiency Flashcards

All 7 cards in this deck

What is the equation for efficiency in terms of energy?

$\text{efficiency} = \dfrac{\text{useful energy transferred by the device}}{\text{total energy supplied to the device}}$

What is the equation for efficiency in terms of power?

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

What are the steps to calculate the efficiency of a device as a percentage?
e.g. a motor is supplied with $200\ \text{J}$ and usefully transfers $50\ \text{J}$

1. Write the equation: efficiency = useful energy transferred ÷ total energy supplied
2. Substitute: $50 \div 200 = 0.25$
3. Multiply by 100: efficiency = $25\%$

What are the steps to find the total energy supplied to a device from its efficiency?
e.g. a device of efficiency $0.5$ usefully transfers $20\ \text{J}$

1. Substitute into the equation: $0.5 = \dfrac{20}{\text{total}}$
2. Rearrange: total = useful energy ÷ efficiency
3. Evaluate: $20 \div 0.5 = 40\ \text{J}$

What does an efficiency of $0.4$ tell you about a device?

$40\%$ of the energy supplied to it is transferred usefully; the other $60\%$ is wasted.

True or false? A device can have an efficiency greater than $1$.

False. Useful energy transferred can never be more than the total energy supplied.

Why is the efficiency of a real engine always less than $100\%$?

The useful output energy is less than the input energy, because some energy is dissipated to less useful stores (heating the surroundings).