Imagine a world where nothing moves, nothing glows, nothing makes a sound, and nothing grows. That world would be a world without energy. Energy is one of the most important concepts in all of science. It is the reason a ball bounces, a light bulb glows, your body stays warm, and music plays from a speaker. In this unit, we will explore what energy is, the different forms it takes, how it moves from one place to another, and why it can never be created or destroyed.

All energy falls into two broad categories. Kinetic energy is the energy of motion. Any object that is moving, whether it is a speeding car, a vibrating guitar string, or the molecules bouncing around inside a hot cup of cocoa, has kinetic energy. The faster something moves, or the more mass it has, the more kinetic energy it possesses.

Potential energy is stored energy. It is energy that an object has because of its position, shape, or chemical makeup. A book on a high shelf has gravitational potential energy because it could fall. A stretched rubber band has elastic potential energy. The food you eat contains chemical potential energy that your body converts into motion and heat.

Energy exists in many forms, and each form is related to either motion or position:

Thermal energy is the total kinetic energy of all the tiny particles (atoms and molecules) inside a substance. The faster the particles move, the hotter the substance feels.

Light energy (also called radiant energy) is energy carried by electromagnetic waves. The sun is our biggest source of light energy.

Sound energy is produced when an object vibrates, creating waves that travel through air, water, or solids.

Electrical energy comes from the movement of electrically charged particles called electrons. It powers nearly everything in our homes.

Chemical energy is stored in the bonds between atoms in molecules. Food, batteries, and gasoline all contain chemical energy.

Mechanical energy is the combination of kinetic and potential energy in a moving or positioned object. A swinging pendulum has mechanical energy.

## Energy Transformations

One of the most fascinating things about energy is that it constantly changes from one form to another. Scientists call this an energy transformation (or energy conversion). A car engine transforms chemical energy in gasoline into mechanical energy (motion) and thermal energy (heat). A flashlight transforms chemical energy in the battery into electrical energy, then into light energy. Your body transforms chemical energy from the food you eat into mechanical energy (so you can move) and thermal energy (which keeps your body at about 37 degrees Celsius).

In every transformation, energy is never created and never destroyed. It simply changes form. This principle is called the law of conservation of energy, and it is one of the most fundamental laws in all of science.