15.1 The First Law of Thermodynamics

A cylinder is divided in half by a movable disk in the middle. Each half is filled with an equal number of gas molecules, but one half is at a higher temperature than the other. Which choice best describes what happens next?

1. Nothing.
2. The high temperature side expands, compressing the low temperature side.
3. Heat moves from hot to cold, so the low temperature side will gradually increase in temperatu…

A system has 300 J of work done on it, and has a heat transfer of -320 J. Compared to prior to these processes, the internal energy is:

1. 20 J less
2. 20 J more
3. 620 J more
4. 620 J less

A potato cannon has the fuel combusted, generating a lot of heat and pressure, which launch a potato. The combustion process _____ the internal energy, while launching the potato _____ the internal energy of the potato cannon.

1. increases, increases
2. increases, decreases
3. decreases, increases
4. decreases, decreases

What is the change in internal energy of a car if you put 12.0 gal of gasoline into its tank? The energy content of gasoline is $1 .\text{ } 3 \times \text{10 }^{8} \text{J}/\text{gal }$. All other factors, such as the car’s temperature, are constant.

A system does $1 . \text{80} \times \text{10}^{8} \text{ J}$ of work while $7 . \text{50} \times \text{10}^{8} \text{ J}$ of heat transfer occurs to the environment. What is the change in internal energy of the system assuming no other changes (such as in temperature or by the addition of fuel)?

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) What is the average metabolic rate in watts of a man who metabolizes 10,500 kJ of food energy in one day? (b) What is the maximum amount of work in joules he can do without breaking down fat, assuming a maximum efficiency of 20.0%? (c) Compare his work output with the daily output of a 187-W (0.250-horsepower) motor.

(a) A woman climbing the Washington Monument metabolizes $6 .\text{ } \text{00 } \times \text{10 }^{2} \text{kJ }$ of food energy. If her efficiency is 18.0%, how much heat transfer occurs to the environment to keep her temperature constant? (b) Discuss the amount of heat transfer found in (a). Is it consistent with the fact that you quickly warm up when exercising?