Energy is all around us. Energy can be classified as potential or kinetic. Potential energy is stored energy. Kinetic energy is moving energy. There are many types of potential and kinetic energies. Thermal energy is a type of kinetic energy because it is the energy of vibrating or moving molecules.
The molecules that make up all the matter in the universe are continually moving. Even the molecules that make up a block of solid ice are vibrating. The hotter matter is, the more the atoms and molecules move. The atoms and molecules found in solids vibrate in place. The molecules in liquids move around slowly, and the molecules in gases zip around like crazy.
The hotter the temperature, the faster molecules move. Moving particles makes water boil and produce steam. The molecules in the water are moving faster and faster until they move so quickly they become a gas.
Thermal energy is the energy a molecule has because of its motion. The faster a molecule is moving, the more thermal energy it has. Thermal energy is sometimes called internal energy.
Heat is not the same as thermal energy. Heat is the transfer of thermal energy from one object to another. Heat always moves from a hot object to a cold object. For example, when you put an ice cube in your lemonade, thermal energy from the lemonade is transferred to the ice cube, so your lemonade gets colder.
Thermal energy is energy from motion, so it is transferred between objects by motion. In the case of the lemonade, the molecules in the lemonade are moving around quickly because the lemonade is warm. When the ice cube is added to the lemonade, its molecules are vibrating but not moving around. Every time a molecule of warm lemonade hits a molecule of ice, it transfers some of its energy to the ice molecule. As a result, the lemonade molecule loses some of its energy. The loss of energy causes the lemonade to cool down.
There are three ways thermal energy can be transferred. They are also called types of heat transfer. Convection is the transfer of thermal energy through the movement of liquid or gas molecules. Both liquid and gas molecules rise as they get hotter and fall as they cool. When cool molecules touch a hot surface, they get hotter. The increase in thermal energy makes them rise. Cool molecules rush in to fill the space these molecules have just left. When the rising molecules lose contact with the hot surface, they become cooler and fall back down.
Meanwhile, the cool molecules that are now touching the hot surface rise. This pattern creates a circle pattern. Scientists call this circular movement convection currents. You can see convection currents in boiling water and air movement in the atmosphere.
Another type of heat transfer is conduction. When one object touches another object, thermal energy is transferred from the warmer object to the colder object. The earlier example of ice in a warm lemonade is an example of conduction. Burning your hand on a hot pan is another example of conduction.
Radiation is different from convection and conduction because it does not involve molecules. Thermal energy transferred by radiation comes from electromagnetic waves. The Sun’s heat travels to Earth through electromagnetic waves. Not only does the Sun radiate these electromagnetic waves, but you do too! Everything on Earth emits electromagnetic waves. The amount of electromagnetic waves an object radiates depends on its thermal energy, so when you are hot, you are radiating more electromagnetic waves. Electromagnetic radiation from humans can be seen on infrared cameras.
As you can see, thermal energy is constantly being transferred between molecules. Often, the three types of heat transfer are happening at the same time. For example, fire transfers heat through radiation, convection, and conduction. Most of a fire’s heat is released through radiation, but when the burning wood of fire touches air molecules, thermal energy is transferred by conduction. Then, the hot air molecules rise transferring thermal energy by convection. The three types of heat transfer are happening simultaneously.
Thermal energy comes from motion, so all matter in the universe has thermal energy. We can increase the thermal energy of an object by increasing the motion of its molecules. Have you ever gone for a run on a cold day? As you ran, your body got warmer because running caused the molecules that make up your body to vibrate faster. The faster the molecules vibrate, the more thermal energy they have. The mechanical energy of running becomes thermal energy.
Electrical energy transforms into heat energy through the movement of electrons. As the electrons move along wires, they transfer some of their energy to the environment around them into thermal energy. Some electrical devices, like an electric heater, are designed to heat the environment. Other electrical devices, like a television, release heat just because of the moving electrons.
Light energy becomes heat energy because when light waves hit molecules, it causes them to vibrate. More vibrations increase the molecules’ thermal energy. As a result, sunlight makes us feel warm.
Chemical energy can also be converted into thermal energy. Food moves through our bodies while we digest. The movement of the food particles increases the thermal energy of the molecules involved.
The conversion of thermal energy into other types of energy doesn’t happen as easily as other energy conversions. Thermal energy is converted to different energy types in endothermic reactions. These reactions require more energy than they produce. Heat, or the transfer of thermal energy, is necessary to make these reactions happen.
While it is difficult to convert thermal energy into other types of energy, thermal energy is continuously moving from one object to another through convection, conduction, and radiation.