3.2.1 Dissipation of energy

Dissipation: energy spreads into less useful stores

1. Energy is conserved: the total amount of energy in a closed system does not decrease.
2. During every real transfer, some energy enters the intended useful store and some is transferred to other stores.
3. 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

1. When surfaces move against each other, friction acts opposite to the motion.
2. Work is done against friction, transferring energy from kinetic or other mechanical stores to the thermal energy stores of the surfaces and their surroundings.
3. The temperature rises because the average kinetic energy of particles in the warmed materials increases.
4. 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.

Energy becomes less useful, not destroyed

1. 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.
2. 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.
3. 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.
4. In each example, the surroundings cool only by spreading that thermal energy through an even larger region, so the energy becomes still less concentrated.

Descriptions must name the transfer and destination

1. 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.
2. Write that energy is transferred or dissipated, not that it is used up, lost or destroyed.

Common errors change the physics

1. Do not say friction creates energy; friction transfers energy between stores.
2. Do not call all thermal energy wasted because heating can be the intended useful transfer in a kettle or heater.

Check your explanation against conservation

1. The sum of all useful and unwanted energy transfers equals the total energy supplied.
2. Any energy-accounting diagram must therefore have equal total input and total output energies, even when much of the output is dissipated.

3.2.2 Reducing unwanted energy transfers

Reducing unwanted transfers keeps energy useful

1. Reducing unwanted transfers increases the fraction of the input transferred usefully without creating energy.
2. The correct method depends on whether the unwanted pathway is friction, conduction, convection or infrared radiation.

Lubrication reduces heating by friction

1. Oil or grease separates moving surfaces so their microscopic irregularities make less direct contact.
2. The frictional force falls, so less work is done against friction and less energy enters thermal stores.
3. Bearings, gears and engine parts therefore run cooler and more input energy remains available for useful motion.

Building insulation slows cooling

1. Loft insulation traps air and reduces conduction through the roof.
2. Cavity-wall insulation traps air in small pockets, reducing conduction and preventing large convection currents.
3. Double glazing uses a trapped gas layer to reduce conduction and convection through windows.
4. Draught proofing reduces convection by stopping warm air escaping through gaps.
5. Reflective foil reduces net infrared transfer into an outside wall because its shiny surface has low emissivity.

Thickness and conductivity control the rate

1. Lower thermal conductivity means less energy is transferred each second for the same area, thickness and temperature difference.
2. Greater wall thickness increases the conduction distance, so the rate of energy transfer and rate of cooling decrease.
3. A thick layer of low-conductivity material therefore slows cooling more than a thin layer of a good conductor.

Insulation choices have costs and limits

1. Payback time compares installation cost with the annual saving:
2. $$payback\; time=\frac{installation\; cost}{saving\; per\; year}$$
3. Thicker insulation usually gives progressively smaller extra savings, so cost and available space affect the choice.

Keep the mechanisms distinct

1. Lubrication applies to moving surfaces, while insulation applies where there is a temperature difference.
2. A valid improvement must act on the named unwanted transfer pathway.

3.2.3 Efficiency

Efficiency compares useful output with total input

1. Efficiency has no unit because it is a ratio of energies measured in the same unit.
2. It lies between $0$ and $1$, or between $0\%$ and $100\%$.

Calculate efficiency from energy

1. Use:
2. $$\eta =\frac{E_{useful}}{E_{total}}$$
3. $E_{useful}$ and $E_{total}$ are both measured in joules $(J)$.
4. For a percentage, multiply the ratio by $100\%$.

Rearrange for missing energies

1. $$E_{useful}=\eta E_{total}$$
2. $$E_{total}=\frac{E_{useful}}{\eta }$$

Power gives the same ratio

1. When input and output are measured over the same time:
2. $$\eta =\frac{P_{useful}}{P_{total}}$$
3. Use two energies or two powers; never divide energy by power.

Useful output depends on purpose

1. Light is useful for a lamp, kinetic energy is useful for a motor and heating water is useful for a kettle.
2. Other outputs are unwanted for that purpose, even if the same transfer is useful in another device.

Check units and limits

1. Check that useful energy is no greater than total energy.
2. Use $E_{dissipated}=E_{total}-E_{useful}$ when the unwanted transfer is required.

3.2.4 Increasing efficiency

Increasing efficiency reduces unwanted transfers

1. Efficiency rises when a larger fraction of the same input reaches the intended useful store.
2. For a fixed input, $E_{total}=E_{useful}+E_{dissipated}$, so reducing dissipation increases useful output.

Reduce friction and resistance

1. Lubricating bearings and gears reduces friction, so less energy enters thermal stores and more reaches the kinetic output.
2. Ball bearings, smooth surfaces and aligned parts can reduce rubbing and deformation.
3. Lower-resistance conductors and secure electrical connections reduce unwanted heating by current.

Reduce drag and unwanted vibration

1. Streamlining reduces air resistance, so less energy is transferred to the thermal stores of the air and vehicle.
2. Reducing unnecessary vibration limits transfers as sound and heating.
3. A method improves efficiency only when it reduces an output that is unwanted for the device's intended purpose.

Insulation improves heating systems

1. Insulation around a hot-water tank reduces transfer to the surroundings, leaving more energy in the water's thermal store.
2. A lid reduces convection and evaporation, so less energy is required to heat the contents.
3. Loft insulation, cavity-wall insulation, double glazing and draught proofing reduce transfers from buildings.
4. Insulation is not always helpful because refrigerators and computers need controlled heat removal.

Improvements have limits

1. No real process reaches $100\%$ efficiency because some unwanted transfers are unavoidable.
2. An improvement may add mass, cost, maintenance or manufacturing impacts.
3. Extra insulation gives progressively smaller savings once transfer through that part is already low.

Judge changes from evidence

1. Compare devices under the same operating conditions and for the same useful task.
2. A measured efficiency increase should match a reduction in unwanted output or an increase in useful output.