November 24, 2012 2:03 pm

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Here’s a riddle: How is heat like a river? Answer: They both flow “downhill.” You will never see a river flow up a mountain. A river only goes one way, down. Heat also goes one way. Heat only flows from something that is hotter to something that is colder.
You might think that putting an ice cube in your lemonade makes the lemonade cooler. What really happens is that the lemonade makes the ice cube warmer. Heat goes out of the warmer lemonade into the colder ice cube. Heat only flows one way.
Heat is an important kind of energy. Heat from the Sun makes life on Earth possible. Heat makes your home warm. Heat cooks your food. Heat makes cars go. You use more heat than any other kind of energy.
Things that make heat are called heat sources. The Sun is a heat source. Fire is a heat source. Melted rock and metal inside Earth are sources of heat. Heat sources make other things hot.
Things get hot because tiny bits of matter called atoms and molecules move around. Everything is made of atoms or molecules. They are much too small for you to see.
Atoms and molecules are always moving around. They move around slowly in something that is cold. They move around quickly in something that is hot. Heat energy comes from moving atoms and molecules.
An iron frying pan is made of iron atoms. Heat energy from a fire makes the iron atoms move faster. The atoms bang into one another. The faster the atoms whiz around, the hotter the frying pan gets. The hot frying pan in turn makes things like strips of bacon hot.
Heat energy can move in three ways. These three ways are called conduction, convection, and radiation. Conduction carries heat through things that are solid. The frying pan heats the bacon through conduction.
The frying pan sits on a flame. Heat from the flame makes atoms at the bottom of the pan move faster. The atoms on the bottom bang into the atoms above them. This makes those atoms move faster and bang into more atoms. As atoms bang into other atoms, the heat energy travels from the bottom of the frying pan to the top. The heat energy enters the bacon and cooks it to a crisp.
Convection heats liquids and gases. A radiator heats air in a room by convection. Air near the radiator gets hot. Hot air rises. Cooler air replaces the heated air. This air gets hot and also rises. The air keeps going around and around from cooler to hotter. The way the air moves is called a convection current.
Radiation can send heat across empty space. This is how heat from the Sun travels to Earth. Heat rays from the Sun strike the surface of Earth and make it warm.
Temperature and heat are not the same thing. Temperature is a measurement of heat. Temperature tells whether something is hot or cold.
You measure temperature with a thermometer. A thermometer has a scale, such as Fahrenheit or Celsius. A thermometer hanging outdoors measures the temperature of the air. Air that is 32° Fahrenheit (0° Celsius) does not have much heat energy. At that temperature, water can freeze into ice. Air that is 100° Fahrenheit (38° Celsius) can make you feel very warm. Better put some ice into your lemonade and try to cool off!
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