Energy is the ability to do work or cause change. It comes in many forms: kinetic (motion), potential (stored), thermal (heat), electrical, chemical, nuclear, light, and sound.

The law of conservation of energy states that energy cannot be created or destroyed — only transformed from one form to another. A roller coaster at the top of a hill has potential energy. As it descends, potential energy converts to kinetic energy.

Electrical circuits require a power source, a conductor, and a load (like a light bulb). Current flows through a closed circuit. A switch opens or closes the circuit. Series circuits have one path; parallel circuits have multiple paths.

Waves transfer energy without transferring matter. Mechanical waves (sound) need a medium. Electromagnetic waves (light) can travel through empty space. All waves have wavelength, frequency, and amplitude.