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As shown in Figure 5, the refrigerator contains 1 an electrically-powered compressor that does work on Freon gas, and 2 a series of coils that allow heat to be released outside on the back of the refrigerator or absorbed from inside the refrigerator as Freon passes through these coils.
This is a schematic diagram of the major functional components of a refrigerator. The major features include a compressor containing Freon CCl 2 F 2 gas, an external heat-exchange coil on the outside back of the refrigerator in which the Freon passes and condenses, an expansion valve, and a heat-exchange coil inside the insulated compartment of the refrigerator blue in which the Freon is vaporized, absorbing heat from inside the refrigerator and thus lowering its temperature.
Figure 6 below traces the phase transitions of Freon and their associated heat-exchange events that occur during the refrigeration cycle. The steps of the refrigeration cycle are described below the figure. The numbers in the figure correspond to the numbered steps below. This diagram shows the major steps in the refrigeration cycle. For a description of each step indicated by the green numbers , see the numbered steps below. In this figure, blue dots represent Freon gas, and solid blue areas represent liquid Freon.
Small arrows indicate the direction of heat flow into or out of the refrigerator coils. Please click on the pink button below to view a QuickTime movie showing an animation of the refrigeration cycle shown in the figure above and described below.
Click the blue button below to download QuickTime 4. Outside of the refrigerator, the electrically-run compressor does work on the Freon gas, increasing the pressure of the gas. As the pressure of the gas increases, so does its temperature as predicted by the ideal-gas law. Next, this high-pressure, high-temperature gas enters the coil on the outside of the refrigerator.
Heat q flows from the high-temperature gas to the lower-temperature air of the room surrounding the coil. This heat loss causes the high-pressure gas to condense to liquid, as motion of the Freon molecules decreases and intermolecular attractions are formed.
Hence, the work done on the gas by the compressor causing an exothermic phase transition in the gas is converted to heat given off in the air in the room behind the refrigerator. If you have ever felt the coils on the back of the refrigerator, you have experienced the heat given off during the condensation of Freon. Next, the liquid Freon in the external coil passes through an expansion valve into a coil inside the insulated compartment of the refrigerator. Now, the liquid is at a low pressure as a result of the expansion and is lower in temperature cooler than the surrounding air i.
Since heat is transferred from areas of greater temperature to areas of lower temperature, heat is absorbed from inside the refrigerator by the liquid Freon, causing the temperature inside the refrigerator to be reduced. The absorbed heat begins to break the intermolecular attractions of the liquid Freon, allowing the endothermic vaporization process to occur.
When all of the Freon changes to gas, the cycle can start over. The cycle described above does not run continuously, but rather is controlled by a thermostat. When the temperature inside the refrigerator rises above the set temperature, the thermostat starts the compressor.
Once the refrigerator has been cooled below the set temperature, the compressor is turned off. This control mechanism allows the refrigerator to conserve electricity by only running as much as is necessary to keep the refrigerator at the desired temperature. Refrigerators are essentially heat engines working in reverse. Whereas a heat engine converts heat to work, reverse heat engines convert work to heat.
Heat from the environment is used to vaporize the refrigerant, which is then condensed to a liquid in coils within a house to provide heat. The energy changes that occur during phase changes can be quantified by using a heating or cooling curve. As the temperature of the ice increases, the water molecules in the ice crystal absorb more and more energy and vibrate more vigorously. At the melting point, they have enough kinetic energy to overcome attractive forces and move with respect to one another.
Once all the ice has been converted to liquid water, the temperature of the water again begins to increase. Now, however, the temperature increases more slowly than before because the specific heat capacity of water is greater than that of ice.
At this point, the temperature again begins to rise, but at a faster rate than seen in the other phases because the heat capacity of steam is less than that of ice or water. Thus the temperature of a system does not change during a phase change. Many cooks think that food will cook faster if the heat is turned up higher so that the water boils more rapidly.
Instead, the pot of water will boil to dryness sooner, but the temperature of the water does not depend on how vigorously it boils. A superheated liquid is a sample of a liquid at the temperature and pressure at which it should be a gas. Superheated liquids are not stable; the liquid will eventually boil, sometimes violently.
When a test tube containing water is heated over a Bunsen burner, for example, one portion of the liquid can easily become too hot. Superheating is the reason a liquid heated in a smooth cup in a microwave oven may not boil until the cup is moved, when the motion of the cup allows bubbles to form. At this temperature, the steam begins to condense to liquid water.
No further temperature change occurs until all the steam is converted to the liquid; then the temperature again decreases as the water is cooled. This region corresponds to an unstable form of the liquid, a supercooled liquid. If the liquid is allowed to stand, if cooling is continued, or if a small crystal of the solid phase is added a seed crystal , the supercooled liquid will convert to a solid, sometimes quite suddenly. As the water freezes, the temperature increases slightly due to the heat evolved during the freezing process and then holds constant at the melting point as the rest of the water freezes.
Subsequently, the temperature of the ice decreases again as more heat is removed from the system. For example, supercooling of water droplets in clouds can prevent the clouds from releasing precipitation over regions that are persistently arid as a result. Clouds consist of tiny droplets of water, which in principle should be dense enough to fall as rain. In fact, however, the droplets must aggregate to reach a certain size before they can fall to the ground. A phase change is a physical process in which a substance goes from one phase to another.
Usually the change occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point of the substance.
The melting point is the temperature at which the substance goes from a solid to a liquid or from a liquid to a solid. The boiling point is the temperature at which a substance goes from a liquid to a gas or from a gas to a liquid. The nature of the phase change depends on the direction of the heat transfer. Heat going into a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat from a substance changes a gas to a liquid or a liquid to a solid. Two key points are worth emphasizing.
Take water H 2 O as an example. However, if heat is added, some of the solid H 2 O will melt and turn into liquid H 2 O. If heat is removed, the opposite happens: some of the liquid H 2 O turns into solid H 2 O.
Water is a good substance to use as an example because many people are already familiar with it. Other substances have melting points and boiling points as well. Again, consider H 2 O as an example. Only after all of the solid has melted into liquid does the addition of heat change the temperature of the substance.
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