Dissipation: energy spreads into less useful stores
Definition
Dissipation
Dissipation is the spreading of transferred energy into the thermal energy stores of the surroundings, which makes the energy less useful for further transfers.
- Energy is conserved: the total amount of energy in a closed system does not decrease.
- During every real transfer, some energy enters the intended useful store and some is transferred to other stores.
- Energy that is dissipated is still present, but it is spread among many particles in the surroundings and is difficult to transfer usefully again.
Mechanical processes warm objects and surroundings
- When surfaces move against each other, friction acts opposite to the motion.
- Work is done against friction, transferring energy from kinetic or other mechanical stores to the thermal energy stores of the surfaces and their surroundings.
- The temperature rises because the average kinetic energy of particles in the warmed materials increases.
- Air resistance produces the same overall effect because moving objects do work against drag and transfer energy to the thermal energy stores of the air and the object.
Key Idea
A mechanical process becomes wasteful when heating caused by friction or drag transfers energy away from the intended useful store.
Energy becomes less useful, not destroyed
- A moving bicycle eventually stops because energy from its kinetic store is transferred by friction in the brakes, tyres and bearings, and by air resistance, to thermal stores.
- A filament lamp transfers electrical energy usefully by radiation as visible light, while much more is transferred by infrared radiation and heating to the surroundings.
- In an electric motor, electrical energy is transferred usefully to a kinetic store, while resistance in the wires and friction in moving parts increase thermal stores.
- In each example, the surroundings cool only by spreading that thermal energy through an even larger region, so the energy becomes still less concentrated.
Example
- A motor receives 500 J500\ \text{J}500 J of electrical energy and transfers 350 J350\ \text{J}350 J to the kinetic energy store of a load.
- The dissipated energy is the remainder:
- Edissipated=500 J−350 J=150 JE_{\text{dissipated}}=500\ \text{J}-350\ \text{J}=150\ \text{J}Edissipated=500 J−350 J=150 J
- The 150 J150\ \text{J}150 J is mainly transferred to thermal stores by electrical resistance, friction and sound.
Descriptions must name the transfer and destination
- A complete explanation identifies the initial energy store or input, the intended useful transfer, the unwanted transfer mechanism and the thermal store that gains energy.
- Write that energy is transferred or dissipated, not that it is used up, lost or destroyed.
Exam technique
- For a braking question, link the chain directly: friction does work, the brakes and surroundings warm, and energy from the kinetic store is dissipated into thermal stores.
- When asked why dissipated energy is less useful, state that it is spread through the surroundings and is difficult to transfer back into a useful store.
Common errors change the physics
- Do not say friction creates energy; friction transfers energy between stores.
- Do not call all thermal energy wasted because heating can be the intended useful transfer in a kettle or heater.
Common Mistake
- Do not write that energy disappears when an object stops; its kinetic store decreases while thermal stores increase.
- Do not confuse energy dissipation with a failure of conservation of energy.
Check your explanation against conservation
- The sum of all useful and unwanted energy transfers equals the total energy supplied.
- Any energy-accounting diagram must therefore have equal total input and total output energies, even when much of the output is dissipated.
Self review
- What is energy dissipation?
- Why does friction raise the temperature of surfaces?
- Why is energy in the thermal stores of the surroundings less useful?
- Describe the energy transfers when a moving bicycle brakes.
