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Energy Transfers and Transformations

Energy cannot be created or destroyed, but it can be transferred and transformed. There are a number of different ways energy can be changed, such as when potential energy becomes kinetic energy or when one object moves another object.

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Earth Science, Physics

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Water Boiling Pot

There are three types of thermal energy transfer: conduction, radiation, and convection. Convection is a cyclical process that only occurs in fluids.

Photograph by Liu Kuanxi
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Energy, or the power to do work, cannot be created or destroyed. However, energy can change form. It can also move between objects.

A common example of energy moving between objects, called energy transfer, is the transfer of kinetic energy from a moving object to a motionless object. Kinetic energy is the energy of motion. When a bat hits a ball, some of the bat's kinetic energy moves to the ball. However, the energy stays in the same form.

Thermal energy, or heat, is related to energy from temperature. When something is heated, its temperature rises. That's because its molecules move faster. Temperature measures the "hotness" or "coldness" of an object. "Heat" refers to thermal energy moving from a hotter system to a cooler one. Thermal energy transfers in three ways: conduction, convection, and radiation.

Conduction is when thermal energy moves between molecules that are touching each other. If you place a metal spoon in boiling water, the end not touching the water gets hot. This happens because metal is an excellent conductor. This means heat travels through it easily. Some materials, such as wood, are poor conductors. Heat does not travel through them easily. They are known as insulators.

Convection only happens in liquids and gases. When water is boiled on a stove, molecules at the bottom of the pot are closest to the heat. They get thermal energy first. They move faster and spread out. This means there are fewer molecules at the bottom of the pot. These molecules rise. They are replaced at the bottom by cooler, denser water. These steps repeat. They create a current of molecules sinking, heating up, rising, cooling down, and sinking again.

The third type of heat transfer is called radiation. It is very important to life on Earth. With radiation, a heat source does not have to touch the object being heated. Radiation can even move heat through the emptiness of space. Nearly all thermal energy on Earth comes from the sun. It travels in the form of energy waves, such as light. Materials on Earth take in these waves. They use them for energy or reflect them into space.

Energy Can Change Form

Energy can also change form. This is called transformation. For example, a ball on a hill has stored energy because of its position. This energy is called gravitational potential energy. It is the ability of an object to do work because of its position in a gravitational field. The higher on the hill this ball is, the more gravitational potential energy it has. When it rolls down the hill, that potential energy changes into kinetic energy.

The ball's kinetic energy is changed into heat by the opposing force of friction. Friction is the force resisting objects sliding against each other. The ball stops at the bottom of the hill because friction transforms all its kinetic energy into heat. As with energy transfers, the amount of energy stays the same in transformations.

Energy on a Sand Dune

In nature, energy transfers and transformations happen constantly. Look at sand dunes in a coastal environment.

Thermal energy shines from the sun. It heats the land and ocean. However, water heats more slowly than land. The temperature difference creates a convection current. This current appears as wind.

This wind has kinetic energy. It transfers kinetic energy to sand by carrying it short distances. If the moving sand hits something, it stops because of the friction created. Its kinetic energy is changed into heat energy. As sand builds up, these impacts can create dunes.

Sand dunes provide a special environment. Plants grow there. They use light energy to change water and carbon dioxide into energy. That energy 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 sugar's energy into kinetic and heat energy.

Energy transfers and transformations happen constantly. They allow life to exist.

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Director
Tyson Brown, National Geographic Society
Author
National Geographic Society
Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks

October 19, 2023