Unit question: What is energy, and how does it move and change form in the world around us?

You use energy every second: your muscles use chemical energy to move, lights convert electrical energy to light, and a ball rolling downhill converts stored energy into motion. In this unit, we explore what energy is and how it follows predictable rules as it flows through systems.

Energy is the ability to do work or cause change.

Kinetic energy (KE) is the energy of motion. Any moving object has kinetic energy. More mass and faster speed both increase kinetic energy.

Potential energy (PE) is stored energy based on an object's position or condition: - Gravitational potential energy depends on height above the ground. The higher an object is, the more potential energy it has. - Elastic potential energy is stored in stretched or compressed materials (like a spring or a rubber band). - Chemical potential energy is stored in the bonds of molecules (like in food or fuel).

Mechanical energy is the total of kinetic and potential energy in a system:

$$ME = KE + PE$$

As a ball rolls down a hill, its potential energy decreases and its kinetic energy increases, but the total mechanical energy stays the same (ignoring friction).

Energy transformation means energy changing from one form to another. The major forms of energy:

- Chemical energy: stored in molecular bonds (food, fuel, batteries) - Thermal energy: the total kinetic energy of particles in a substance (heat) - Electromagnetic (light) energy: energy carried by light waves - Electrical energy: energy from moving electric charges - Sound energy: energy carried by vibrations through matter - Nuclear energy: stored in atomic nuclei

The law of conservation of energy states: energy cannot be created or destroyed. It can only change form or transfer from one object to another. The total energy in a closed system always stays the same.

For example: a campfire converts chemical energy (wood) into thermal and light energy. The total energy before and after is identical; it just changed form.