Question

Suppose a woman does 500 J of work, and 9500 J of heat transfer occurs into the environment in the process. (a) What is the decrease in her internal energy, assuming no change in temperature or consumption of food? (That is, there is no other energy transfer.) (b) What is her efficiency?

a) −10,000 J, 4.8%
b) −9,000 J, 5.6%
c) −8,500 J, 6.3%
d) −7,500 J, 7.1%

Answer 1

The decrease in internal energy is 9000 J and the efficiency is 5%.

Step-by-step explanation:

(a) Decrease in internal energy

The decrease in internal energy can be calculated using the first law of thermodynamics:

ΔE = Q - W

Where ΔE is the change in internal energy, Q is the heat transfer, and W is the work done. In this case, the woman does 500 J of work and 9500 J of heat transfer occurs into the environment:

ΔE = 9500 J - 500 J = 9000 J

Therefore, the decrease in her internal energy is 9000 J.

(b) Efficiency

The efficiency can be calculated using the following formula:

Efficiency = (Useful work / Total energy input) x 100%

In this case, the useful work is 500 J and the total energy input is the sum of work done and heat transfer, which is 9500 J + 500 J = 10,000 J:

Efficiency = (500 J / 10,000 J) x 100% = 5%

Therefore, her efficiency is 5%.