Physics · Ch 8 — Heat and Thermodynamics
Refrigerator
Refrigerator
A refrigerator is essentially a Carnot engine run in reverse (Figure 8.49): instead of extracting heat from a hot reservoir to do work, it uses external work (supplied by a compressor) to pump heat OUT of a cold interior (at ) and dump a larger amount of heat INTO the warmer surrounding air (at ); by the first law, . Its efficiency is measured by the coefficient of performance (COP), , which using becomes , and for an ideal (Carnot) refrigerator, since , reduces to A larger COP means a more efficient refrigerator (typical real refrigerators achieve COP around 5-6, Example 8.27); the smaller the gap between the cooling chamber and room temperature, the higher the COP. Pumping heat "uphill" from cold to hot requires external work and does …
What this figure shows. A two-part figure. Part (a) is a schematic block diagram: a 'Refrigerator' block sits between a labelled 'Cold reservoir (inside of refrigerator)' box, from which heat QL is drawn, and a 'Hot reservoir (room temperature of kitchen)' box, into which heat QH is rejected, with an arrow labelled W showing external work being supplied into the refrigerator block by a compressor. Part (b) is a more physically realistic cutaway of an actual refrigerator, showing the compressor compressing the refrigerant to high pressure liquid, which flows through condenser coils where it releases heat QH to the outside kitchen air, before the low-pressure refrigerant returns to absorb heat from inside the refrigerator's cold interior -- illustratin …
Green House Effect (Case Study)
The atmosphere warms Earth's surface by about 33°C between its coldest top layer (-19°C) and the ground (+14°C), an effect called the greenhouse effect, caused by gases -- mainly carbon dioxide and water vapour, plus smaller amounts of methane, nitrous oxide, ozone and others -- that readily absorb the infrared radiation Earth re-emits after absorbing visible sunlight from the Sun (CO₂ and water vapour absorb infrared strongly because they have far more vibrational degrees of freedom than nitrogen or oxygen, the atmosphere's main constituents). Since the 1900s, human activity -- overwhelmingly the burning of fossil fuels, driven heavily by the growth in automobile use -- has roughly doubled the CO₂ fraction of the atmosphere, raising Earth's average temperature by about 1°C, a phenomenon called global warming, with serious consequences for polar ice and ocean chemistry. Among human-made greenhouse gases, CO₂ accounts for about 55% and CFCs (chlorofluorocarbons, historically used as refrigerator coolants) for about …