13. In a closed and well-insulated room, the door of an ordinary running refrigerator
is opened and left open for some time. From the time the door is opened, I measure
the temperature at some point in the room far from the refrigerator. After some time I
find that the temperature increases. This is mainly due to:
a. The second law of thermodynamics
b. Anomalous expansion of water
c. The greenhouse effect
d. None of the above
Introduction
In a refrigerator door open closed room temperature increase scenario, many expect the room to cool down. However, physics proves the opposite. This classic thermodynamics puzzle highlights the principles of heat pumps and energy efficiency limits—concepts essential for exams like CSIR NET Life Sciences (interdisciplinary physics).
Option Analysis
- a. The second law of thermodynamics: Refrigerators transfer heat from a cold interior to a warmer exterior using electrical work. Due to inefficiencies, the expelled heat exceeds that extracted. This extra heat from the compressor and motor raises the room temperature, directly illustrating the second law.
- b. Anomalous expansion of water: Water expands below 4°C due to molecular structure changes. However, no such phenomenon affects air or the refrigerator’s mechanism here.
- c. The greenhouse effect: This involves atmospheric gases trapping infrared radiation, unrelated to the refrigerator’s thermodynamic behavior in a sealed room.
- d. None of the above: Incorrect—option (a) accurately explains the observed effect.
Refrigerator Heat Pump Principle
Refrigerators function as reverse heat engines. They absorb heat from the cold interior (\(Q_c\)) and reject it to the warmer surroundings (\(Q_h\)) by performing electrical work (\(W\)). The energy balance follows the relation:
Qh = Qc + W
This means the room receives more heat than what the refrigerator removes. When the door remains open, the interior warms to match the room temperature, causing the compressor to run continuously, increasing the total heat output.
Second Law of Thermodynamics
The second law states that heat cannot spontaneously flow from a colder to a hotter region without external work. Real systems are inherently inefficient, producing waste heat and increasing entropy. The Coefficient of Performance (COP) obeys the Carnot limit:
COP < Tc / (Th − Tc)
Since no real system achieves perfect efficiency, \(Q_h > Q_c\), ensuring the room’s net heat content rises over time.
Common Misconceptions
- Cold air spilling out may cause a brief cooling effect, but warm compressor output soon dominates.
- In a closed or insulated room, expelled heat accumulates, steadily increasing temperature.
- There is no role for anomalous water behavior or greenhouse gas effects in this process.
