You know that thermal energy is the same as the kinetic energy of the particles in a substance. You also know that understanding thermal energy is very important to our existence, because we could not survive if the planet was too hot or too cold. In this activity, you will learn how thermal energy can be transferred  from one substance to another.

You are already familiar with two important terms: heat and temperature. If you learn more about the scientific definitions of these two terms, you will better understand the various ways in which thermal energy can be transformed.

When people use the term heat, they often mean thermal energy. Scientists use the term heat to refer to thermal energy that moves from a warmer object to a cooler one. You can understand this idea when you look at our example of water again. If you hold an ice cube in your hand, the ice cube melts because your hand is warmer than the ice cube. Thermal energy from your hand transfers to the ice. Heat, then, is really a measure of the kinetic energy of a material. The heat, or thermal energy, of a substance changes according to how fast the atoms are moving as well as how many molecules are moving.

 

 

 

People use temperature to measure heat. When heat is transferred from a substance with higher kinetic energy to a substance with lower kinetic energy, the temperature of the first substance decreases while the temperature of the second substance increases.

The difference between heat and temperature might be clearer if you consider a few examples:

• If you have a glass of water and a hot tub full of water at the same temperature, the hot tub will have more heat energy because there is more water in it. That is, there are more water molecules in the hot tub, so there is more thermal energy there than in the glass.
• If you have a glass of water and a hot tub full of water at the same temperature and you pour the glass of water into the hot tub, no heat energy is transferred. If you pour a glass of cold water into the hot tub, a transfer of energy occurs from the hotter water to the cold water until they are the same temperature.

Therefore, heat is thermal energy transferred from one substance to another. Temperature is a measure of the kinetic energy of the substance.

 

 

You know that thermal energy transfers from hot to cold. Heat transfer has three forms: conduction, convection, and radiation.

Conduction

When two substances come into direct contact with each other, the molecules of the warmer substance run into the molecules of the colder substance. As the molecules move, the kinetic energy of the warmer substance slows and the kinetic energy of the cooler substance increases. An example of conduction occurs when you put a pot on a hot burner: It doesn't take long for the pot to warm up!

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Convection

When air warms, air molecules gain kinetic energy. This means the air molecules move faster. As they move faster, the warm air molecules rise above cooler air molecules. Let's take a look at the pot example again. As you just saw, when the stove is first turned on, heat is transferred by conduction between the element on the stove through the bottom of the pot to the water. However, eventually the water starts bubbling. These bubbles are the hot water rising to the surface, thereby transferring heat from the hot water at the bottom of the pot to the cooler water at the top by convection. At the same time, the cooler, more dense water at the top will sink to the bottom, where it is subsequently heated.

Radiation

Although conduction and convection transfer heat through kinetic energy, radiation transfers energy through invisible waves (electromagnetic waves). Let's take one last look at the pot example from above. If you heat up the stove as much as you can, it will glow red. This is a visible sign of radiation. In those areas where the stove and the bottom of the pot are not in contact, radiation transports heat from the stove to the heat.

Probably the most obvious example of radiation is our sun. Heat from the sun comes to us on these waves, and when the waves come into contact with a substance, the atoms or particles in the substance start to move more quickly. As you know, this increase in kinetic energy means the substance will warm up.