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# Physical Science Study Guide for McGraw-Hill's ASVAB (page 4)

By Dr. Janet E. Wall
McGraw-Hill Professional

### Heat

Heat is created by moving molecules. The faster the molecules move, the more heat is generated. Heat is basically kinetic energy (review the section on energy earlier in this chapter). Heat is measured by a thermometer.

Heat transfers or flows from warmer objects to colder objects when they are in contact. Transfer takes place in three ways: radiation, conduction, and convection.

Radiation   The first method of heat transfer is radiation, which takes place via invisible waves through the air or even through a vacuum. The energy from the sun warms us through radiation, just as a fire in a fireplace warms our hands. A microwave oven heats food via radiation.

Conduction   The second method of heat transfer is conduction, which refers to heat transferred between atoms bumping into each other in a substance. What happens if you put a spoon in boiling water? Eventually the handle of the spoon gets too hot to hold. The heat from the water causes the molecules in the spoon to move and collide, creating heat that eventually is conducted up the spoon handle to your hand.

Some materials are good conductors of heat, and some are not. Metals are good conductors of heat, with gold, silver, and copper being the best conductors. Copper is used more because it is cheaper and more plentiful. Some materials are called insulators because they are not very good at conducting heat. Materials made of cotton or wool, for example, are not good conductors of heat. If you wrap yourself in a wool blanket, rather than conducting the heat away from your body, it traps the heat and you get warmer. Air is a good insulator, as are plastic, wood, rubber, and tile (think of the tiles on the space shuttle that protect the shuttle from the heat of friction created when the shuttle strikes the atmosphere during its return to Earth).

Think about heat conduction when you get into a car on a hot sunny day when the windows are closed. Vinyl or leather seats will feel very hot, as they are better conductors of heat then fabric seats.

Convection   Heat also transfers via a process called convection. Convection generally occurs in gases or liquids. For example, as the temperature of a pan of water on the stove increases, the molecules start to move around more quickly. As they do this, the distance between the molecules increases and the liquid expands. The denser, cooler water then sinks to the bottom of the pan and forces the warmer liquid upward. A circulation of the colder and warmer liquid begins. On a much larger scale, this is the process that helps to create currents in the oceans.

The same process creates currents in the atmosphere. When land is heated by the sun, it heats the air that is in contact with it. The warmer air expands and rises, and the cooler, heavier air sinks down. The rising air creates upward motion, which is used by birds and hang gliders to stay aloft. Sometimes these upward currents are called thermals. Thermals can reach many thousands of feet up and will give people in airplanes a bumpy ride.

Convection also creates land and sea breezes at a shoreline. Land heats faster than water. As the land heats up, it warms the air above it. The air expands and rises. It is then replaced by cooler air flowing in from over the water. This process creates a wind current that goes from the water to the land. We call this current a sea breeze. In the evening, the reverse happens. The land cools faster than the water, so the air current flows in the opposite direction. This creates what is called a land breeze—a breeze from the land to the sea.

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