# Figure 56 sweat evaporating from skin removes energy

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Figure 5.6: Sweat evaporating from skin removes energy and cools the body. Figure 5.7: Dew forms when water vapor in air condenses into droplets. evaporation - change from liquid to gas at a temperature below the boiling point. condensation - change from gas to liquid at a temperature below the boiling point.

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100 U NIT 2 P ROPERTIES OF M ATTER Figure 5.8: Convection currents in water. The hot water at the bottom of the pot rises to the top and replaces the cold water. convection - the transfer of heat through the motion of fluids such as air and water. Convection What is convection? Convection is the transfer of heat through the motion of fluids such as air and water. If you have warmed your hands over a fire, you have felt convection. Heat from the flame was transferred to your hand by the upward movement of air. Natural convection Convection occurs because fluids expand when they heat up. Since expansion increases volume, but not mass, the density of a warm fluid becomes lower than the density of surrounding cooler fluid, causing the warmer fluid to float upward. In a pot on the stove, hot water circulates to the top and cooler water sinks to the bottom. This circulating flow is called natural convection . Forced convection In many buildings and houses, a boiler heats water that is then pumped throughout the structure to distribute the heat. Since the heat is being carried by a moving fluid, this is another example of convection. However, the flow is created by pumps, which makes this an example of forced convection . Natural convection also occurs in the same system. The heat from a hot radiator warms the air in a room by natural convection. The warmer air rises and cooler air is drawn from the far side of the room. The cooler air is then warmed and rises, creating circulation that spreads heat through the room.
101 5.1 L IQUIDS AND G ASES C HAPTER 5: S TATES OF M ATTER The atmosphere of Earth Air is not “nothing” Air feels “light” because it is 1,000 times less dense than water. Air may seem like “nothing” but all the oxygen our bodies need and all the carbon needed by plants comes from air. As a tree grows, you will not see soil disappear to provide mass for the tree. After oxygen and hydrogen (from water), the most abundant element in a tree is carbon. All of those carbon atoms come from carbon dioxide (CO 2 ) in the air. Air is a mixture of gases Air is the most important gas to living things on the Earth. The atmosphere of Earth is a mixture of gases (Figure 5.9). Molecular nitrogen (N 2 ) and oxygen (O 2 ) account for 97.2 percent of the mass of air. Argon and water vapor make up most of the rest. Atmospheric pressure Gravity creates pressure because fluids have mass and therefore weight. The Earth’s atmosphere has a pressure due to the weight of air. The density of air is low, but then the atmosphere is more than 80,000 meters deep (Figure 5.10).

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• Fall '10
• ALLISON

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