Thermodynamics Homework 1 Solutions

1. An 8 kW electrical coil in an electric water heater runs for 3 hours to raise the water

temperature to the desired level. Determine the amount of electric energy used in both

kWh and kJ.

2. People are most comfortable when the air temperature they are sitting in is between about

65°F and 75°F. Express these temperature limits in °C. Convert the size of this temperature

range (10°F) to K, °C, and R. Is there any difference in the size of this range as measured in

relative or absolute units?

3. What is the difference between gage pressure and absolute pressure?

Gage pressure is the pressure above atmospheric pressure. Gage pressure can be negative

(in a vacuum). Absolute pressure is the actual pressure that we use when specifying

thermodynamic states. A perfect vacuum has an absolute pressure of zero. Absolute

pressure can never be less than zero.

4. A person has a total footprint area of 400 cm2. He wants to walk on snow, but his boots

keep sinking in. Unbeknownst to him, the snow can only withstand a pressure of 0.5 kPa.

He weighs 70 kg, and he decides to wear snowshoes. What is the minimum snowshoe area

that he must wear to support himself? Consider the fact that he’ll have his whole weight on

one snowshoe for periods of time while walking.

5. 1-95

6. 1-114