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.

3.3.1 Energy resources: renewable and non-renewable

Energy resources differ in renewal, reliability and impact

1. Resources provide heating, transport and electricity but differ in reliability, response time, output, impact, location and cost.
2. Renewable resources often depend on weather conditions and have lower efficiency rates non-renewable energy resources.

Fossil fuels and nuclear fuel are non-renewable

1. Fossil fuels are coal, oil and natural gas.
2. They can be stored and burned when required, giving controllable output.
3. Combustion releases carbon dioxide and can release sulfur dioxide and particulates; extraction and transport can also damage habitats.
4. Nuclear fuel produces large continuous outputs with no carbon dioxide from fuel combustion during operation.
5. Nuclear stations are expensive to build and decommission, create long-lived radioactive waste and require strict safety systems.

Biofuel depends on sustainable replacement

1. Biofuel is made from recent living material such as crops, wood or organic waste.
2. Plants absorb carbon dioxide while growing, but cultivation, processing and transport can create net emissions.
3. Fuel crops can compete with food production and use land, water and fertiliser.
4. Biofuel is renewable only when new biomass grows at least as quickly as it is harvested.

Wind, water and sunlight are renewable flows

1. Wind has no fuel emissions during operation but is intermittent and needs exposed sites.
2. Hydroelectricity can start quickly and give large output, but dams flood land and alter rivers.
3. Tidal power is predictable, but suitable coastal sites are limited and barrages disrupt estuaries.
4. Solar energy produces electricity or heats water, but output varies with daylight, season and cloud.
5. Manufacture and construction still have environmental impacts even when operation uses no fuel.

Choose resources for the required use

1. Transport favours energy-dense fuels or stored electricity because vehicles carry their supply.
2. Heating can use gas, electricity, biomass, solar thermal energy or heat pumps.
3. Electricity networks combine resources because demand changes and no single source is ideal everywhere.

An energy mix balances constraints

1. Renewables reduce dependence on finite fuels, but variable output can require storage and backup.
2. Non-renewable stations can provide controllable output, but create fuel, emission or waste problems.
3. The best mix depends on geography, demand, technology, acceptable impacts and timescale.

3.3.2 Patterns and trends in energy use

Energy-use trends reflect demand and technology

1. Data may show total consumption, consumption per person, use by sector or the share from each resource.
2. A pattern describes the evidence, while an explanation identifies a cause.
3. Check the period, units and whether the axis shows an amount, rate or percentage.

Demand changes across time and place

1. Daily electricity demand rises when homes and businesses use many appliances at once and falls during low-activity periods.
2. Seasonal demand changes with temperature and daylight, affecting heating, cooling and lighting.
3. Industrialisation, population growth and higher incomes can raise total use through manufacturing, transport and buildings.
4. Efficiency, insulation, changes in industry and conservation policies can reduce demand or slow its growth.
5. Total use can rise even while use per person falls if population grows sufficiently.

The resource mix also changes

1. Fossil-fuel use can fall when emissions limits tighten, fuel prices rise or lower-carbon technologies become cheaper.
2. Renewable generation can rise as equipment, grids, storage and backup systems improve.
3. Nuclear generation changes slowly because stations take years to build, operate for decades and are costly to decommission.
4. Short-term changes may reflect weather, maintenance or fuel supply rather than a lasting trend.

Quantify changes before explaining them

1. $$\Delta E=E_{final}-E_{initial}$$
2. $$percentage\; change=\frac{E_{final}-E_{initial}}{E_{initial}}\times 100\%$$
3. Power is the rate of energy transfer:
4. $$P=\frac{E}{t}$$
5. Use $1kW=1000W$, $1MW=10^{6}W$ and $1kWh=3.6\times 10^{6}J$.

Interpret trends without overclaiming

1. Correlation alone does not prove causation; an explanation must be physically plausible and fit the evidence.
2. Interpolation within the measured range is more reliable than extrapolation beyond it because the trend may change.
3. Year-to-year fluctuations should not be called a long-term trend unless the overall pattern supports that conclusion.

Avoid common data errors

1. A strong conclusion states direction, magnitude and timescale before giving a supported cause.
2. Where evidence is limited, state that the data are consistent with an explanation rather than claiming certainty.

9.3.1 Reducing unwanted energy transfer through lubrication

Friction and the energy it wastes

1. No surface is perfectly smooth, because even polished metal carries microscopic bumps and hollows.
2. When two surfaces slide over each other these bumps catch, and the surfaces have to be forced past one another, which is where friction comes from.
3. The moving object therefore has to do work against friction, and doing work is a transfer of energy.
4. That energy is not destroyed, because it is dissipated into the thermal energy stores of the two surfaces and then spreads out into the surroundings.
5. The transfer counts as unwanted because the energy is spread so thinly that it cannot be recovered to do a useful job, so less of the input energy reaches the intended output.
6. Friction also wears the surfaces away and raises their temperature, which shortens the working life of bearings, gears, chains and pistons.
7. In a bicycle chain, a car engine or a stiff door hinge, every joule dissipated by friction is a joule the machine never delivers.

How lubrication reduces the transfer

1. A lubricant is a fluid, usually an oil or a grease, placed between two surfaces that move over each other.
2. The lubricant is drawn into the gap and forms a thin layer that holds the two solid surfaces apart.
3. With the bumps no longer catching directly on each other, the frictional force between the surfaces becomes much smaller.
4. Because the frictional force is smaller, less work is done against friction for the same amount of movement.
5. Less work done against friction means less energy is dissipated into the thermal energy stores of the parts and the surroundings.
6. A larger share of the input energy is then transferred usefully, so the machine becomes more efficient, runs cooler and wears more slowly.
7. The chain of reasoning to write out in an answer runs as follows.
   1. The lubricant separates the moving surfaces.
   2. The frictional force between them is reduced.
   3. Less work is done against friction.
   4. Less energy is dissipated by heating.
8. Lubrication never removes friction completely, because layers of the fluid still resist sliding over each other, although that resistance is far smaller than dry contact between two solids.
9. A lubricant also carries heat away from the contact and flushes out particles worn from the surfaces, which is why engine oil is pumped around a circuit and then changed.

Comparing rates of energy transfer

1. The energy wasted by a machine is often quoted as a rate, which is the energy transferred divided by the time taken.
2. The equation is $rate\; of\; energy\; transfer=\frac{energy\; transferred}{time\; taken}$.
3. A rate measured in joules per second, $J/s$, is the same quantity as a power measured in watts, $W$.
4. Rates given per minute or per hour have to be converted before they can be compared, by dividing by $60$ to change minutes into seconds and by $3600$ to change hours into seconds.
5. Comparing the wasted rate before and after lubrication as a ratio shows how much the change has achieved, and the ratio is simplified by dividing both numbers by their highest common factor.
6. A ratio of $4:1$ means the wasted rate has fallen to a quarter of its original value, so lubrication has cut the waste by a factor of $4$.
7. Proportional reasoning works forwards as well, so halving the frictional force over the same distance halves the work done against friction and therefore halves the rate at which energy is dissipated.