The principle
- The principle of conservation of energy states:
Energy cannot be created or destroyed; it can only be transferred from one store to another.
- For any closed system, the total energy across all the stores stays constant
- Energy is never truly lost, but it can be dissipated (spread out) into the surroundings by heating or radiation; once dissipated, it is wasted because the original task cannot reclaim it
Useful and wasted energy
- Every real transfer produces a mixture of:
- Useful energy in the store the designer wanted (the kinetic store of the moving fan, the thermal store of the food)
- Wasted energy spread around the surroundings (heating from friction, sound carried away into the air, infrared radiation off hot casings)
- The total of useful + wasted output always equals the input, which is conservation of energy in practice
Example: a tennis racket hitting a ball
- Before: the racket has energy in its kinetic store; the ball is roughly stationary
- During the impact:
- some energy moves usefully to the kinetic store of the now-flying ball (mechanical pathway)
- some moves to the thermal store of the strings and the ball at the contact point as they deform (heating pathway, wasted)
- some leaves as the audible "thwack" through the air (radiation/sound, wasted)
- After: the racket has slowed slightly, the ball is moving fast, and the rest of the kinetic energy is now spread between the thermal stores of the racket, the ball, and the surrounding air
Example: an electric kettle boiling water
- The mains supply drives current into the kettle's heating element (electrical pathway, into the thermal store of the element)
- The element warms the water through the metal-water boundary (heating pathway, useful, into the thermal store of the water)
- Some energy heats the kettle's plastic body (heating pathway, wasted, into the thermal store of the plastic)
- Some energy dissipates into the surrounding kitchen air (heating + radiation pathways, wasted)
- All four outputs add up to the electrical input, so energy is conserved
Example: a child on a trampoline
- At the top of a bounce, the child holds energy in their gravitational potential store; their kinetic store is briefly zero
- Falling back to the mat, the gravitational potential store empties into their kinetic store (mechanical pathway, useful)
- As the trampoline stretches under the impact, the child's kinetic energy moves into the elastic potential store of the springs and fabric (mechanical pathway, useful)
- The trampoline then springs back, pushing the child upward; the elastic potential store empties back into the child's kinetic store, which in turn re-fills their gravitational potential store as they rise
- Each cycle leaks a small amount of energy into the thermal store of the trampoline, the child's body and the air, so successive bounces are gradually smaller