The COPs are defined differently for heat pumps and air conditioners. Explain why.

A supermarket is trying to decide how many cash registers to keep open. Suppose an average of 18 customers arrive each hour, and the average checkout time for a customer is 4 minutes. Interarrival times and service times arc exponential, and the system may be modeled as an M/M/s/GD/∞/∞ queuing system. It costs $20 per hour to operate a cash register, and a cost of 25¢ is assessed for each minute the customer spends in the cash register area. How many registers should the store open?

Explain why it is important to consider NPSH when designing and operating a pumping system.

At steady state, a heat pump provides $30,000\ \mathrm{Btu} / \mathrm{h}$ to keep a dwelling at $68^{\circ} \mathrm{F}$ on a day when the outside temperature is $35^{\circ} \mathrm{F}$. The power input to the heat pump is $5\ \mathrm{hp}$. If electricity costs 8 cents per $\mathrm{kW} \cdot \mathrm{h}$, compare the actual operating cost with the minimum theoretical operating cost for each day of operation.

Can you warm a kitchen in winter by leaving the oven door open? Can you cool the kitchen on a hot summer day by leaving the refrigerator door open? Explain.

Explain why the temperature of a gas increases when it is adiabatically compressed.

Can the temperature of a system remain constant even though heat flows into or out it? If so, give examples.

Choose the letter that best answers the question or completes the statement. Which of the following is a FALSE statement about a heat pump? a. It requires no work. b. It moves heat from a cold to a hot area. c. It uses a refrigerant. d. Air conditioners are heat pumps.

A heat pump is a heat engine run in reverse. In winter it pumps heat from the cold air outside into the warmer air inside the building, maintaining the building at a comfortable temperature. In summer it pumps heat from the cooler air inside the building to the warmer air outside, acting as an air conditioner. $(a)$ If the outside temperature in winter is $-5.0^{\circ} \mathrm{C}$ and the inside temperature is $17.0^{\circ} \mathrm{C}$, how many joules of heat will the heat pump deliver to the inside for each joule of electrical energy used to run the unit, assuming an ideal Carnot cycle? $(b)$ Suppose you have the option of using electrical resistance heating rather than a heat pump. How much electrical energy would you need in order to deliver the same amount of heat to the inside of the house as in part $(a)$? Consider a Carnot heat pump delivering heat to the inside of a house to maintain it at $68^{\circ} \mathrm{F}$. Show that the heat pump delivers less heat for each joule of electrical energy used to operate the unit as the outside temperature decreases. Notice that this behavior is opposite to the dependence of the efficiency of a Carnot heat engine on the difference in the reservoir temperatures. Explain why this is so.

Can mechanical energy ever be transformed completely into heat or internal energy? Can the reverse happen? In each case, if your answer is no, explain why not; if yes, give one or two examples.

Can mechanical energy ever be transformed completely into heat or internal energy? Can the reverse happen? In each case, if your answer is no, explain why not; if yes, give one or two examples.

How do ground-source (geothermal) heat pumps (GSHPS) use geothermal energy?

An "emergency blanket" is a thin shiny (metal coated) plastic foil. Explain how it can help to keep an immobile person warm.

Which of the following conditions would MOST likely cause flushed skin?

• A: Low blood pressure

• B: Blood loss

• C: Hypothermia

• D: Exposure to heat