Question

The efficiency of a motor is 15%. The student calculated the useful output energy transfer is 1.20J. Calculate the total input energy transfer.

Answer 1

Answer:

$8.0\; \rm J$.

Explanation:

The efficiency of a machine is the ratio between the useful output and the energy input:

$\begin{aligned}\text{efficiency} &= \frac{\text{useful output}}{\text{energy input}} \times 100\%\end{aligned}$

Rearrange this equation to find energy input in terms of efficiency and useful output:

$\displaystyle \text{energy input} = \frac{\text{useful output}}{\text{efficiency} / (100\%)}$.

Substitute in the values: $\text{useful output} = 1.2\; \rm J$ and $\text{efficiency} = 15\%$. Evaluate to find the value of $\text{energy input}$:

$\begin{aligned} \text{energy input} &= \frac{\text{useful output}}{\text{efficiency} / (100\%)} \\ &= \frac{1.20\; \rm J}{15\% / (100\%)} \\ &= 8.0\; \rm J\end{aligned}$.

(Rounded to two significant figures as in the value of efficiency.)