Energy Stores & Transfers
Where energy lives and how it moves
What is Energy?
Energy is a property that objects and systems possess. It's the capacity to do work — the ability to make something happen. We measure it in joules (J).
Energy cannot be created or destroyed; it can only be transferred from one place to another, or from one form to another. This is the foundational law of physics.
💡 Think of it this way: Energy is like money. You don't create money when you spend it; you transfer it from your account to someone else's. The total amount of money in the world stays the same — it just moves around.
Systems in Physics
A system is simply an object or group of objects you choose to study. It's your boundary — what's inside is the system, what's outside is the surroundings.
Why does this matter? Because when you define a system, you decide where energy "starts" and where it "stops" for the purpose of your analysis. You're not concerned with energy transfers beyond your boundary.
Example: If you're studying a falling apple, your system is the apple + Earth. You don't care about the Sun's gravitational field on the apple — it's outside your system.
The Eight Energy Stores
Energy can sit in eight different "storage vaults":
1. Kinetic Store — Energy of anything moving. A car cruising at 60 mph has kinetic energy. The faster it goes, the more it has.
2. Gravitational Potential Store — Energy of height in a gravitational field. A book on a high shelf has more gravitational PE than one on the floor.
3. Elastic Potential Store — Energy in stretched or compressed things. A stretched rubber band, a compressed spring, or bent metal all hold elastic PE.
4. Thermal Store — Heat energy. Every object has thermal energy; hotter objects have more of it.
5. Chemical Store — Energy locked in bonds between atoms. Food, fuel, and batteries all store chemical energy.
6. Nuclear Store — Energy inside atomic nuclei. Radioactive materials and nuclear reactors tap into this.
7. Electrostatic Store — Energy in electric fields between charged particles. Two magnets holding apart store energy in their electrostatic fields.
8. Magnetic Store — Energy in magnetic fields. Two magnets pushing away from each other store magnetic energy.
Energy Transfer Pathways
Energy moves between stores via four pathways:
Mechanical pathway:
Electrical pathway: