Chapter 5 Principles of Refrigeration 83 Copyright by Goodheart-Willcox Co., Inc. Conduction Conduction is heat transfer by direct contact between two objects. For instance, if you pick up a cold soft drink can, you immediately feel the heat leaving your hand for the can metal. Another example is the heating of a cooking pot by direct contact with the burner, Figure 5-2. In the refrig- eration system, the most common example of conduction is the transfer of heat between the air and the metal of the evaporator. Radiation Radiation occurs when heat travels from an object by infrared rays. Th ese rays are not visible and do not contain heat in themselves. However, when the infrared rays strike an object, they cause its molecules to vibrate, heating it. Th e most obvious example of radiation is how the sun heats an object through millions of miles of space, Figure 5-3. Radiation is a factor in automotive air conditioning as sunlight has an eff ect on system effi ciency. Change of State Everything in the universe is in one of three states, or physical conditions. Th e three states are solid, liquid, and gas. All matter can switch between solid, liquid, and gas if conditions are right. Th e switching of the physical condition of a substance is called change of state. Th e usual way to cause a change of state is by adding or removing heat. Changing from a solid to a liquid or from a liquid to a gas is called changing to a higher state, and requires the addition of heat. Changing from a gas to a liquid or from a liquid to a solid is called changing to a lower state, and requires the removal of heat. See Figure 5-4. Pressure also aff ects the change of state. Th e tempera- ture at which a substance will move to a higher or lower state will increase if its pressure is increased, or decrease if its pressure is decreased. Any kind of matter can change its state from gas to liquid to solid if the temperature is low enough. Matter can also switch from solid to liquid to gas if the temperature is high enough. No matter how much heat is removed from a solid, it remains a solid. A gas, no matter how much heat is added, always remains a gas. A liquid can change state up or down, depending on its temperature. Note: Do not be confused about chemical changes brought on by heat. If something becomes so hot it begins burning, this is not a change of state, but a chemical reaction. Changing gasoline into a vapor to make it burn is a change of state, but the actual burning is not. Figure 5-2. This electric stove burner transfers heat to the pot and water by direct contact. Figure 5-4. Water, like most substances, can occur in three states: solid (ice), liquid (water), or gas (water vapor or steam). Change of state occurs when heat is either added or removed. Water freezes, becomes ice Water boils, becomes vapor Ice melts, becomes water Water vapor condenses, becomes water Figure 5-3. Radiation is the transfer of heat through waves of radiant energy, such as when sunlight travels through millions of miles of empty space to heat a car interior. The sun's heat transfers through empty space or radiation. Radiant energy heats the car interior, even on a cool day.