Question

please answer all 10 questions!

Question 1

If the refrigerator's efficiency is 61.7% and Q(cold) is 935 W, what is Q(hot) in kW?

Question 2

Find the net radiative energy exchange (W) when: the area of radiative surface is 68 m^2, surface emissivity is 0.49, the cold temp is 433 K, and the hot temp. is 534 K.

Question 3

Consider a device where the minimum input is 18, the actual input is 62, and the ideal output is 49. What is it's second law efficiency? (write as fraction, not percent)

Question 4

Find the surface area (m^2) when: the rate of heat transfer by convection is 107 W, h is 4 W/Km^2, surface temp. is 470 K, and the surrounding temp. is 287 K.

Question 5

Suppose that the amount of useful energy coming out of a device is 27, the total energy put in is 72, and the ideal output is 47. What is the first law efficiency of this device? (write as a fraction)

Question 6

A house gains 304 MJ of heat during the day (8 hours). If the room temperature is reasonably maintained by a 7-kW air conditioner what is the COP of the unit?

(express result with one decimal and no units)

Question 7

What is the rate of heat transfer by conduction (in W), when the area of conducting surface is 46 m^2, T1 is 299 K, T2 is 409 K, the thermal conductivity is 3 W/m.K, and the slab has a thickness of 84 dm?

Question 8

I like to drink my 215-g tea at exactly 75 oC. After 2 minutes and 30 seconds, the microwave gives me tea at 98 oC. Too hot to me. How much tap water (19 oC) should I add to my microwaved tea to be able to have it just at the right temperature?

(express result with no decimals followed by g units)

Question 9

A heat pump is used to maintain a house at 22°C on a wintry night, when the outside air temperature is -3°C. The house is losing heat at a rate of 2,391 kJ/min. What is the pump power in kW?

(express result with two decimals and include the corresponding units)

Question 10

If Q(cold) is 36 W, and Q(hot) is 73 W, what is the refrigerator's efficiency?(give as a fraction)

Answer 1