8.10-8.11 - Dissipation and mechanical heating
Brakes and moving machine parts become warmer when forces do work against friction. Follow the energy to its thermal destinations and explain why it is conserved but less useful for further motion.
Dissipation without disappearance
When a moving bicycle wheel is braked, its kinetic energy store decreases. The brake pads, wheel and nearby air become slightly warmer, so their thermal energy stores increase. The wheel has less energy available for continued motion, but energy has not vanished.
Choose a system before describing the change. If the system includes the wheel, brake and nearby surroundings, energy is transferred between stores inside that boundary. If the system contains only the wheel, energy also crosses its boundary into the brake and surroundings. In either account, total energy is conserved when every transfer is included.
Energy is dissipated when it is transferred into stores that are more spread out and less useful for the intended purpose, usually thermal stores of objects and the surroundings.
In every real system change, some energy is dissipated. Friction, air resistance, deformation, vibration and sound can all help to spread energy. Sound waves transfer energy away, and when the sound is absorbed that energy eventually increases thermal stores in the surroundings.
"Less useful" does not mean "destroyed". It means that the energy is difficult to gather and transfer back into the intended store. Usefulness also depends on the purpose: an increase in the water's thermal store is useful in a kettle, while an increase in the kettle casing's thermal store is not useful for boiling the water.
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