travels through space or material. This is obvious when you stand near a fire and feel its warmth or when you pick up the handle of a metal pot even though the handle is not sitting directly on the hot stove. Invisible energy waves can travel through air, glass, and even the vacuum of outer space. These waves have electrical and magnetic properties, so they are called electromagnetic waves. The transfer of energy from one object to another through electromagnetic waves is known as radiation. Different wavelengths of energy create different types of electromagnetic waves.
- The wavelengths humans can see are known as "visible light." These wavelengths appear to us as the colors of the rainbow. What objects radiate visible light? Two include the Sun and a light bulb.
- The longest wavelengths of visible light appear red. Infrared wavelengths are longer than visible red. Snakes can see infrared energy. We feel infrared energy as heat.
- Wavelengths that are shorter than violet are called ultraviolet.
Can you think of some objects that appear to radiate visible light, but actually do not? The moon and the planets do not emit light of their own; they reflect the light of the Sun. Reflection
is when light (or another wave) bounces back from a surface. Albedo
is a measure of how well a surface reflects light. A surface with high albedo reflects a large percentage of light. For example, a snow field has a high albedo (as high as about .9, or 90% reflection). Dark surfaces, such as asphalt pavement, have a low albedo (meaning they absorb most of the solar energy they receive, and therefore heat up).
One important fact to remember is that energy cannot be created or destroyed—it can only be changed from one form to another. This is such a fundamental fact of nature that it is a law: the law of conservation of energy.
In photosynthesis, for example, plants convert solar energy into chemical energy that they can use. They do not create new energy. When energy is transformed, some nearly always becomes heat. Heat transfers between materials easily, from warmer objects to cooler ones. If no more heat is added, eventually all of a material will reach the same temperature.
is a measure of how fast the atoms in a material are vibrating. High temperature particles vibrate faster than low temperature particles. Rapidly vibrating atoms smash together, which generates heat. As a material cools down, the atoms vibrate more slowly and collide less frequently. As a result, they emit less heat. What is the difference between heat and temperature?
- Temperature measures how fast a material’s atoms are vibrating.
- Heat measures the material’s total energy.
Which has higher heat and which has higher temperature: a candle flame or a bathtub full of hot water?
- The flame has higher temperature, but less heat, because the hot region is very small.
- The bathtub has lower temperature but contains much more heat because it has many more vibrating atoms. The bathtub has greater total energy.
Heat is taken in or released when an object changes state, or changes from a gas to a liquid, or a liquid to a solid. This heat is called latent heat
. When a substance changes state, latent heat is released or absorbed. A substance that is changing its state of matter does not change temperature. All of the energy that is released or absorbed goes toward changing the material’s state.
For example, imagine a pot of boiling water on a stove burner: that water is at 100 degrees C (212 degrees F). If you increase the temperature of the burner, more heat enters the water. The water remains at its boiling temperature, but the additional energy goes into changing the water from liquid to gas. With more heat the water evaporates more rapidly. When water changes from a liquid to a gas it takes in heat. Since evaporation takes in heat, this is called evaporative cooling. Evaporative cooling is an inexpensive way to cool homes in hot, dry areas.
Conversely, when water vapor condenses from a gas into a liquid, forming water droplets in the atmosphere, it releases heat to the surrounding air. This is called the latent heat of condensation
Substances also differ in their specific heat
, the amount of energy needed to raise the temperature of one gram of the material by 1.0 degree C (1.8 degrees F). Water has a very high specific heat, which means it takes a lot of energy to change the temperature of water. Let's compare a puddle and asphalt, for example. If you are walking barefoot on a sunny day, which would you rather walk across, the shallow puddle or an asphalt parking lot? Because of its high specific heat, the water stays cooler than the asphalt, even though it receives the same amount of solar radiation.
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