Question

Consider a household refrigerator that has just been filled up with roomtemperature food. Define a control volume (mass) and examine its boundaries for rates of work and heat transfer, including sign.

a. Immediately after the food is placed in the refrigerator

b. After a long ** of time has elapsed and the food is cold

Answer 1

I. C.V. Food.

(a) short term.: -Q from warm food to cold refrigerator air. Food cools.

(b) Long term: -Q goes to zero after food has reached refrigerator T.

II. C.V. refrigerator space, not food, not refrigerator system

(a) short term: +Q from the warm food, +Q from heat leak from room into cold space. -Q (sum of both) to refrigeration system. If not equal the refrigerator space initially warms slightly and then cools down to preset T.

(b) long term:small -Q heat leak balanced by -Q to refrigeration system.

Note: For refrigeration system CV any Q in from refrigerator space plus electrical W input to operate system, sum of which is Q rejected to the room.