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Heat is given off when an object's thermal energy is transferred. Thermal energy (see below) can be transferred in three ways: by conduction, by convection, and by radiation.
| 1. Conduction |
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Conduction is the transfer of energy from one molecule to another. This transfer occurs when molecules hit against each other, similar to a game of pool where one moving ball strikes another, causing the second to move. Conduction takes place in solids, liquids, and gases, but works best in materials that have simple molecules that are located close to each other. For example, metal is a better conductor than wood or plastic. |
2. Convection |
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Convection is the movement of heat by a liquid such as water or a gas such as air. The liquid or gas moves from one location to another, carrying heat along with it. This movement of a mass of heated water or air is called a current. |
| 3. Radiation |
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Heat travels from the sun by a process called radiation. Radiation is the transfer of heat by electromagnetic waves. When infrared rays strike a material, the molecules in that material move faster. In addition to the sun, light bulbs, irons, and toasters radiate heat. When we feel heat around these items, however, we are feeling convection heat (warmed air molecules) rather than radiated heat since the heat waves strike and energize surrounding air molecules. |
More about Thermal (Heat) Energy
Heat is given off whenever energy is being used. You can tell if a television has been on by feeling if it is warm. When you run up a flight of stairs you feel warm because you are burning food energy.
What exactly is heat? Heat is the transfer or flow of energy from a hot object to one that is cooler. When you feel a warm object, you are actually feeling thermal energy, which is the movement of molecules that make up the object. An object has more thermal energy when it is warm than when it is cool.
The more thermal energy an object has, the faster its molecules move. These faster moving molecules bump into each other more frequently and spread out as they require more space (decreasing the density of the molecules). Think of people standing in an elevator. If they started moving around, they would start bumping into each other and need more space. This is essentially what happens when molecules get more energy and start moving around; they spread out. |
For the most part, the volume of an object increases as the amount of thermal energy it receives increases. In other words, the molecules in warmer objects are less densely packed than the molecules in cooler objects. (NOTE: Temperature is a measurement of how fast molecules move.) |
You can't see thermal energy, but you can detect evidence of heat transfer. You might see the air shimmering over a radiator (convection), put your hand on a warm spoon that's been sitting in a hot bowl of soup (conduction), or notice that the sun shine feels warm on your skin (radiation). If you need evidence of thermal energy or heat in your life, just feel your arm. Your body generates heat 24 hours a day! (Taken from KEEP Activity Guide "Exploring Heat").
A note about thermal energy and heat. In strict scientific terms, there is a distinct difference between heat and thermal energy. A way to think about this distinction is objects possess thermal energy, while heat is transferred from one object to another. Wherever possible, we have tried to remain true to these distinctions. However, since heat is the more familiar term we often use that to facilitate understanding.
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