Energy cannot be created or destroyed. This means the total amount of energy in the universe has always been and will always be constant. However, energy can change form and even transfer between objects.

A common example of energy transfer is the transfer of kinetic energy—the energy of motion—from a moving object to a stationary object. When a golf club is swung and hits a golf ball, some of the club's kinetic energy transfers to the ball. In this type of energy transfer, energy moves from one object to another but stays in the same form. A kinetic energy transfer is easy to observe and understand, but other important transfers are not as easy to visualize.

Thermal energy has to do with the internal energy of a system from its temperature. When a substance is heated, its temperature rises because its molecules move faster and gain thermal energy. Temperature measures the "hotness" or "coldness" of an object. The term heat refers to thermal energy being transferred from a hotter system to a cooler one. Thermal energy transfers occur in three ways: conduction, convection, and radiation.

Conduction is when thermal energy is transferred between molecules in contact with one another. If you place a metal spoon in a pot of boiling water, the end not touching the water gets very hot. This happens because metal is an excellent conductor. Heat travels easily through the material. The vibrations of molecules at the end of the spoon touching the water spread throughout the spoon, until all the molecules are vibrating faster. The whole spoon gets hot. Some materials, such as wood and plastic, are poor conductors. Heat does not travel through them easily. They are known as insulators.

Radiation Transfers Heat through Space

Convection only occurs in liquids and gases. When water is boiled on a stove, water molecules at the bottom of the pot are closest to the heat source. They gain thermal energy first. They move faster and spread out. This creates a lower density of molecules, or quantity of molecules in that volume, at the bottom of the pot. These molecules rise. They are replaced at the bottom by cooler, denser water. The process repeats, creating a current of molecules sinking, heating up, rising, cooling down and sinking again.

The third type of heat transfer—radiation—is critical to life on Earth. With radiation, a heat source does not have to touch the object being heated. Radiation can transfer heat even through the vacuum of space. Nearly all thermal energy on Earth comes from the sun. It radiates to the surface of our planet. It travels in the form of energy waves, such as visible light. Materials on Earth absorb these waves to use them for energy or reflect them back into space.

In an energy transformation, energy changes form. A ball sitting on a hill has gravitational potential energy, which is the ability for an object to do work due to its position in a gravitational field. The higher on the hill this ball is, the more gravitational potential energy it has. When a force pushes it down the hill, that potential energy transforms into kinetic energy. The ball loses potential energy and gains kinetic energy.

In a frictionless universe, the ball would continue rolling forever. On Earth, however, the ball's kinetic energy is transformed into heat by the opposing force of friction. The ball stops at the bottom of the hill. Just as with energy transfers, energy is conserved in transformations.

Energy Moves from One Form to Another

In nature, energy transfers and transformations happen constantly, such as in a coastal dune environment.

Thermal energy radiates from the sun, heating the land and ocean. However, water heats up more slowly than land. This temperature difference creates a convection current, which appears as wind.

This wind possesses kinetic energy, which it transfers to sand by carrying it short distances. If the moving sand hits something, it stops due to the friction created. Its kinetic energy is then transformed into thermal energy, or heat. Once enough sand builds up, these impacts can create sand dunes.

These newly formed sand dunes provide a special environment. Plants grow there, using light energy to transform water and carbon dioxide into chemical energy, which is stored in sugar. When an animal eats the plant, it uses the stored energy to heat its body and move around. This transforms the chemical energy into kinetic and thermal energy.

Though it may not always be obvious, energy transfers and transformations happen constantly. They are what enable life to exist.