Beginner · 10 min
Energy stores and transfers
Track where energy is stored and keep a complete energy account.
Identify the system
Energy can be stored in motion, in a stretched spring, in a raised object–Earth system, or internally in materials. It can be transferred by mechanical work, electrical work, heating, or radiation. Choose what belongs inside your system before making an energy account. Energy leaving one store does not disappear; it moves to other stores or crosses the system boundary.
Worked example
A toy's spring loses 12 J of elastic energy. The toy gains 9 J of kinetic energy and its parts and surroundings gain 3 J of internal energy. 12 J = 9 J + 3 J
Account for every change
For a closed energy account, total energy before equals total energy after. For a system receiving or losing energy, final stored energy equals initial stored energy plus energy entering minus energy leaving. A decrease in visible motion is not evidence that energy was destroyed. Friction often transfers energy into internal stores that are harder to notice directly.
Worked example
A model system initially stores 50 J. It receives 20 J and transfers 15 J out. Final stored energy = 50 + 20 − 15 = 55 J
Complete an energy account
- A moving cart begins with 60 J of kinetic energy and stops. Its brakes gain 45 J of internal energy. Calculate the remaining transfer to the surroundings.
- Draw arrows for the transfers and check that the final energy total equals the initial total.