(b) Energy transfers
- Syllabus
- 2024
- Topic
- —
- Level
- —
Learning objectives
4.2Energy stores and transfersDescribe energy transfers involving energy stores:• energy stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear• energy transfers: mechanically, electrically, by heating, by radiation (light and sound)4.3Conservation of energyUse the principle of conservation of energy4.4Efficiency equationKnow and use efficiency = useful energy output ÷ total energy output × 100%.4.5Energy transfer devicesDescribe a variety of everyday and scientific devices and situations, explaining the transfer of the input energy in terms of the above relationship, including their representation by Sankey diagrams4.6Thermal energy transferDescribe how thermal energy transfer may take place by conduction, convection and radiation4.7ConvectionExplain the role of convection in everyday phenomena4.8Radiation emission and absorptionExplain how emission and absorption of radiation are related to surface and temperature4.9Thermal transfer practicalPractical: investigate thermal energy transfer by conduction, convection and radiation4.10Reducing unwanted energy transferExplain ways of reducing unwanted energy transfer, such as insulation
Track energy stores and transfer pathways
Energy is stored in a system and is transferred when something changes. Describe an event by naming the store that decreases, the transfer pathway, and the store that increases; a store is not itself a pathway.
| Energy store | Typical situation |
|---|---|
| chemical | fuel, food or a charged battery |
| kinetic | a moving object |
| gravitational | an object raised in a gravitational field |
| elastic | a stretched or compressed spring |
| thermal | a hotter object or surroundings |
| magnetic | interacting magnets |
| electrostatic | separated electric charges |
| nuclear | atomic nuclei |
| Transfer pathway | What identifies it |
|---|---|
| mechanically | a force moves through a distance and does work |
| electrically | moving charges transfer energy through a circuit |
| by heating | energy passes because of a temperature difference |
| by radiation | light or sound carries energy from one system to another |
Example: as a dropped object falls, its gravitational store decreases and its kinetic store increases by mechanical transfer. When it stops on the floor, energy is transferred into thermal stores of the object and surroundings.
Avoid phrases such as 'kinetic energy is transferred into heat energy'. Kinetic and thermal are stores; mechanically, electrically, by heating and by radiation are transfer pathways.
Account for energy using conservation
The principle of conservation of energy states that energy cannot be created or destroyed. In a closed system, the total amount of energy remains constant even though energy moves between stores.
For any device or event, account for all the starting energy. Part may reach the intended useful store and the rest may be transferred into other stores, often thermal stores of the device and surroundings. The complete energy account is: total input energy = useful output energy + energy transferred to other stores.
Example: when a rough ramp raises a block, some input energy increases the block's gravitational store. Friction transfers the remainder into thermal stores of the block, ramp and surroundings. Adding those changes gives the original input energy.
Energy described as 'wasted', 'lost' or 'dissipated' has not been destroyed. It has spread into less useful stores in the surroundings and is harder to use again.
Calculate energy-transfer efficiency
Efficiency is the fraction of the total energy output that is transferred usefully. Multiply the fraction by 100% to express it as a percentage.
\text{efficiency}=\frac{\text{useful energy output}}{\text{total energy output}}\times100%
Use energy values in the same unit. Identify the useful output from the purpose of the device, divide it by the total output, then multiply by 100%. To find total output instead, divide the useful output by the efficiency written as a decimal.
Example: 1800 J is transferred usefully at 16% efficiency. Write 16% as 0.16, so total output = 1800 J ÷ 0.16 = 11 250 J. The energy transferred to other stores is 11 250 J − 1800 J = 9450 J.
Efficiency has no unit and cannot exceed 100%. Do not substitute 16 into a decimal-form calculation: 16% means 0.16.
Represent device energy transfers with Sankey diagrams
A device transfers input energy into one or more outputs. The useful output is the transfer that serves the device's purpose; other outputs are transferred to less useful stores, commonly thermal stores of the surroundings.
| Device or situation | Input transfer or store | Useful output | Other output |
|---|---|---|---|
| lamp | electrically from a battery or supply | light radiation | heating of the lamp and surroundings |
| electric motor | electrically | mechanical transfer that increases a kinetic store | heating and sound radiation |
| winch lifting a load | chemical or electrical input | mechanical transfer that increases the load's gravitational store | thermal stores of the winch and surroundings |
A Sankey diagram uses one input arrow that splits into labelled output arrows. Arrow width represents energy: the useful and other output widths must add to the input width. For a 25% efficient device, the useful arrow is one quarter of the input width and the other-output arrow is three quarters.
Arrow length does not show energy amount; width does. Label each input and output and keep the widths proportional, rather than drawing equal arrows for unequal transfers.
Distinguish conduction, convection and radiation
When two regions are at different temperatures, there is a net transfer of energy from the hotter region to the colder region. Conduction, convection and radiation describe different mechanisms for that transfer.
| Mechanism | How energy is transferred | Where it can occur |
|---|---|---|
| conduction | neighbouring particles transfer energy through collisions and vibrations; mobile electrons also transfer energy rapidly in metals | solids, liquids and gases, strongest in good conductors |
| convection | warmer, less-dense fluid moves and carries energy while cooler, denser fluid replaces it | liquids and gases only |
| radiation | electromagnetic waves, mainly infrared for thermal transfer, carry energy | through matter or a vacuum |
A metal spoon in hot water warms mainly by conduction. Water circulating in a heated pan transfers energy by convection. Energy from the Sun reaches Earth by radiation because space is a vacuum.
Convection is bulk movement of a fluid, not movement through a solid. Conduction and convection require particles, but radiation does not.
Explain how convection currents form
A convection current forms because heating changes the density of a liquid or gas, causing warmer and cooler regions to move in a repeating circulation.
- A region of fluid receives energy by heating. 2. Its particles move faster and spread farther apart, so the fluid expands and becomes less dense. 3. The warmer, less-dense fluid rises. 4. Cooler, denser fluid sinks or moves in to replace it. 5. Continued heating repeats the movement and forms a current.
| Everyday phenomenon | Role of convection |
|---|---|
| room heated by a radiator | warm air rises from the radiator and cooler air moves towards it, circulating energy around the room |
| water heated in a pan | warmer water rises from the base while cooler water sinks, spreading energy through the water |
| rising air in a cloud | warmer, less-dense air rises and can carry droplets or hailstones upwards |
Particles do not expand and 'heat' does not rise. The heated fluid expands because its particles become farther apart; the resulting lower-density fluid rises.
Relate thermal radiation to surface and temperature
All objects emit and absorb thermal radiation. The net energy transfer depends on both the object's temperature and the properties of its surface.
| Surface | Absorption | Emission | Reflection |
|---|---|---|---|
| dull black | good absorber | good emitter | poor reflector |
| shiny white or silver | poor absorber | poor emitter | good reflector |
A hotter object emits thermal radiation at a greater rate than the same object at a lower temperature. If an object emits energy faster than it absorbs energy from its surroundings, it cools; if it absorbs faster than it emits, it warms.
A black solar collector absorbs radiation effectively. A shiny silver surface on an insulated cup reduces both absorption and emission of infrared radiation, helping to limit transfer between the drink and its surroundings.
Choose the property that matches the situation: absorption matters for incoming radiation, emission for outgoing radiation, and reflection for radiation turned away. Black is not always 'hotter'; the temperatures and energy transfers determine the change.
Investigate thermal energy transfer fairly
A thermal-transfer investigation must change one factor, measure a clear outcome, control competing factors and repeat measurements. The method should reveal conduction, convection or radiation rather than merely show that temperature changes.
| Mechanism | Comparative method | Measurement and conclusion |
|---|---|---|
| conduction | clamp equal-sized metal strips at the same depth in hot water; place identical wax or temperature indicators the same distance from the heated end | record time for the indicator to change; a shorter time shows faster conduction |
| convection | heat water gently near one side and introduce a small visible tracer near the heated region | observe the tracer rise above the heater and cooler water move in to complete the current |
| radiation | place equal volumes of water at the same starting temperature in dull-black and shiny-silver containers, equally distant from the same infrared heater | record temperature at equal time intervals; the larger rise shows greater absorption |
Keep dimensions, distance from the source, starting temperature, volume, heating time and heater output constant where relevant. Repeat each measurement, identify anomalies and compare means. Allow hot apparatus to cool before repeats and avoid contact with hot water or heaters.
A fair result supports a comparison only when the changed surface, material or heating position is the sole systematic difference. A single reading cannot establish reliability.
Reduce unwanted thermal energy transfer
Insulation reduces the rate of unwanted thermal energy transfer by targeting conduction, convection, radiation or evaporation. The useful design depends on which pathways are available.
| Design feature | Transfer reduced | Why it works |
|---|---|---|
| thick layer of foam, wool or plastic | conduction | the material has low thermal conductivity and increases the distance for transfer |
| small pockets of trapped air | conduction and convection | air is a poor conductor and cannot circulate when trapped |
| lid or cover | convection and evaporation | it prevents warm fluid from escaping and limits replacement by cooler fluid |
| shiny silver surface | radiation | it is a poor emitter and absorber and a good reflector of infrared |
| vacuum between walls | conduction and convection | there are no particles to transfer energy by either mechanism |
The same mechanisms can keep a hot object hot or a cold object cold: they slow energy transfer across the temperature difference. A well-designed container combines features because blocking only one pathway leaves others available.
Insulation does not create energy or stop transfer completely. Explain each feature through the pathway it slows; phrases such as 'traps heat' do not identify the mechanism.