Physics 3.7 - 3.10 - Dissipation and insulation

Where dissipated energy goes

Begin by choosing a system. For a bicycle braking, a useful system might include the bicycle, rider, brakes, road and nearby air. Energy can move between stores inside that system and can cross a smaller system boundary, but it is not destroyed.

Consider a bicycle that is brought to rest by its brakes:

1. The moving bicycle and rider begin with energy in their kinetic store.
2. Friction acts where the brake pads rub against the wheel.
3. The brake pads, wheel and nearby air warm, so their thermal stores increase.
4. That energy then spreads through a large amount of the surroundings. It still exists, but it is no longer available to keep the bicycle moving.

This is why a mechanical process can become wasteful when it causes an unwanted temperature rise. The temperature rise is evidence that the internal energy of the rubbing parts has increased; energy then transfers by heating from those warmer parts to the surroundings. Saying that the energy was "lost" is incomplete: it has left the useful route, not ceased to exist.

The word useful always depends on the intended outcome. Rotation is useful from an electric motor that drives a fan; warming the motor casing is unwanted. In a brake, however, slowing the bicycle is the intended outcome, even though the transferred energy still ends up dispersed in thermal stores.

All system changes involve some dissipation, so some energy ends up stored in less useful ways. A bouncing ball returns to a lower height because some of its initial energy increases the thermal stores of the ball, floor and surroundings; sound transfers some energy away before it too is eventually dissipated. A machine with rubbing parts gets warmer because friction transfers energy away from its intended mechanical output.

Reducing unwanted energy transfer

Reducing dissipation does not mean breaking conservation of energy. It means directing a larger share of the supplied energy towards the intended change and a smaller share towards unwanted warming.

Lubrication

Two dry solid surfaces touch at many tiny rough regions. When they slide, friction opposes the motion and energy is transferred to thermal stores, raising the temperature of the parts and surroundings.

A lubricant, such as oil, forms a layer between the moving surfaces. This reduces direct contact and lowers unwanted friction. Less energy is transferred to thermal stores, so more of the supplied energy remains available for useful movement. Lubrication is suitable only where friction is unwanted: reducing the friction between a tyre and the road or between brake pads and a wheel would make the system less safe.

The full causal chain is:

Lubricant separates moving surfaces, reducing unwanted friction. Less energy is dissipated in warming, leaving more available for the intended mechanical transfer.

Thermal insulation

Thermal insulation reduces the rate at which energy is transferred by heating across a boundary. It does not make energy and it does not guarantee that no energy will transfer. Insulating a hot-water tank or a building means that less energy reaches the cooler surroundings each second, so the hot object cools more slowly.

Good insulating materials have a low thermal conductivity. A material with a low thermal conductivity transfers energy through it slowly for the same thickness and temperature difference. Thickness also matters, which is why it must be separated from material choice in a fair comparison.

Building walls and rate of cooling

A warm building transfers energy to cooler surroundings. Its rate of cooling describes how quickly its temperature decreases. When comparing walls, assume the same wall area, the same inside and outside temperatures and no other change unless one is stated.

Two independent wall properties affect this rate: