Full chapter
Thermal processes
All 4 topics and the revision summary on one page.
01
Heating and conduction
A temperature difference drives a net transfer of energy from a hotter region to a cooler one. Conduction transfers that energy through interactions between particles.
Heating is an energy transfer, measured in joules. Temperature relates to average random particle kinetic energy. Heat and cold are not substances stored inside objects.
Thermal equilibrium: equal temperatures
Place a metal block at 60°C in thermal contact with another at 20°C. Energy transfers by heating from the hotter block to the cooler one. If the pair is isolated from the surroundings, their temperatures approach a common value.
At thermal equilibrium, the two regions have the same temperature and there is no net energy transfer by heating between them. Their particles continue moving and interacting; equilibrium does not mean that particles stop.
The starting temperatures alone do not determine the final temperature. The amounts and materials also matter, so do not automatically take the average of 60°C and 20°C. Equal final temperatures also do not imply equal total internal energies.
How a solid conducts
Particles at the hotter end of a solid have greater average kinetic energy in their vibrations. Through interactions with neighbouring particles, energy is transferred along the structure towards the cooler region.
The atoms or molecules remain near their usual positions while vibrating. The solid does not conduct by sending its hot particles in a stream from one end to the other.
In a metal, mobile electrons provide an additional way to transfer energy. Electrons move through the structure and exchange energy through interactions with its particles and other electrons. Their motion in both directions produces a net transfer from the hotter region towards the cooler one.
Energy passes through the solid towards the cooler region
The large circles mark fixed mean positions. Particles at both ends vibrate; the solid does not flow along the strip.
Vibrations in a solid
Blue double arrows: vibration about fixed positions. Neighbouring particles transfer energy through their interactions.
An additional mechanism in a metal
Small blue dots and arrows: mobile electrons and their motion. They provide an additional way to transfer energy through a metal.
The orange arrows show the direction of net energy transfer. Their lengths do not measure transfer rates.
Conductors and insulators have different rates
With comparable dimensions and temperature differences, a good thermal conductor transfers energy more readily than a thermal insulator. An insulator slows conduction; it does not make all energy transfer impossible.
A metal pan base helps transfer energy from a hotter hob to the cooler contents. A suitable insulating handle slows conduction towards a hand. The useful material depends on which transfer the design is meant to encourage or reduce.
Worked explanation
Metal and wood at the same room temperature
A metal block and a wooden block have both reached 20°C in a room. A hand is warmer than either block.
- Direction: energy leaves the warmer hand and enters the cooler block in both cases.
- Rate: the metal conducts energy away from the contact region more readily than the wood.
- Observation: the hand cools faster in contact with the metal, so the metal can feel colder even though both blocks started at the same temperature.
The sensation concerns the rate at which the hand loses energy. It is not evidence that the metal began at a lower temperature.
Name the net transfer and its mechanism. In solid conduction, particle interactions transfer energy without bulk flow of the solid. In metals, include mobile electrons as well as vibrating particles.
Optional check A metal block and a wooden block have both been in a 20 degrees C room long enough to reach that temperature. A warmer hand touches each. Why can the metal feel colder?
02
Convection in fluids
Convection transfers energy with the bulk movement of a liquid or gas. Natural convection develops when density differences set the fluid in motion.
A fluid can flow. Density is mass divided by volume. If a portion of fluid expands while its mass stays the same, its density decreases. Energy still transfers overall from hotter regions to cooler ones.
Follow a complete circulation
Heat a liquid locally near one side of the bottom of a vessel. Assume that the liquid expands when warmed over the temperature range used.
- Warming: the portion near the heater gains energy and expands.
- Density change: its mass is spread over a larger volume, so it becomes less dense than the surrounding cooler liquid.
- Rising: the surrounding denser fluid displaces the warmer, less dense portion upwards.
- Return flow: cooler liquid moves into the region left behind. The moving fluid carries energy and a circulation develops.
Heating one lower region can set up a circulation
This liquid expands when warmed over the temperature range shown. Blue arrows show bulk liquid motion; the orange arrow shows energy entering from the heater.
The returning cooler liquid completes the circulation. Energy is carried with the moving liquid.
The warmed portion does not have to lose mass or become weightless. Its smaller density follows from greater volume for the same mass. A complete explanation connects the temperature change to expansion, density and movement.
Why the heater position matters
Worked comparison
Heating water at the bottom or at the top
Consider water initially at 20°C and warming through a range in which it expands.
- Heating near the bottom: the newly warmed, less dense water is below denser water, so it rises and promotes circulation through the sample.
- Heating near the top: the warmed, less dense water is already above the cooler, denser water. Much less natural circulation develops through the whole sample.
Lower regions can still gain energy through other transfers. The top-heating case does not show that energy can never travel downwards.
A room heater placed low can start a similar circulation in air: warmed air expands and rises, while cooler air moves into the lower region. The process redistributes energy with the air rather than sending every warmed particle directly to every part of the room.
Convection needs a fluid that can move
Particles in an ordinary solid remain near fixed positions, so they cannot form this fluid circulation through the material. A liquid or gas can move bodily from one region to another.
Fluid can also be moved by a fan or pump. This forced movement can carry energy even when natural circulation is weak. It should not be explained by pretending that every airflow is caused by warmer fluid rising.
"Heat rises" is not a complete explanation. Under the stated conditions, the warmer fluid rises because it is less dense than its surroundings. Conduction and radiation can transfer energy in other directions, and a fan can direct fluid flow.
Optional check A sample of water initially at 20 degrees C is heated gently only near its top. Assume ordinary expansion over the temperatures used. Why is natural convection through the whole sample much weaker than when it is heated near the bottom?
03
Thermal radiation
Energy can transfer by electromagnetic radiation across a vacuum. No material medium is required between the source and receiver.
Electromagnetic waves can transfer energy. This pathway differs from conduction through particles and convection with moving fluid.
Ordinary warm objects emit thermal radiation, mainly infrared at everyday temperatures. They need not glow visibly. Radiation from the Sun reaches Earth through space, showing that air is not required to carry it.
Objects emit and absorb
An object can emit radiation and absorb radiation from its surroundings at the same time. Its net energy change depends on the difference between what it receives and what it emits.
A hot object in cooler surroundings usually loses energy overall. At thermal equilibrium with surroundings at the same temperature, emission and absorption balance, so there is no net radiative transfer even though both continue.
Radiation transfers energy across a vacuum
Both bodies emit radiation. The arrows show directions, with no numerical rates implied.
Surface colour and texture
Otherwise identical bodies at the same surface temperature, with the same exposed area and surroundings.
For this usual comparison, dull black is a better emitter. It is also a better absorber of the same incident radiation.
Surface temperature
Same surface finish, exposed area and surroundings; only the surface temperature differs.
A hotter surface emits radiation at a greater rate.
Exposed surface area
Same surface finish, surface temperature and surroundings; the exposed surface area differs.
A larger exposed area gives a greater total rate of emission under these conditions.
Net radiative transfer also depends on what is absorbed from the surroundings. Real cooling can include conduction and convection as well.
Three factors to compare
- Surface colour and texture
- For the usual comparison of similar opaque surfaces, a dull black surface is a better absorber and emitter of thermal radiation than a shiny metallic surface. The shiny surface reflects a greater fraction of incident radiation. Compare at the same temperature, exposed area and surroundings. Visible colour alone is not a universal rule for every material and wavelength.
- Surface temperature
- Keeping the surface finish and area unchanged, a hotter surface emits energy at a greater rate. To compare net loss, also keep the surroundings unchanged: an object's temperature alone does not tell you how much radiation it receives.
- Surface area
- At the same temperature and with the same finish, a larger exposed surface emits energy at a greater total rate. For absorption under equal illumination and orientation, a larger exposed area also intercepts more radiation. Use the area actually exposed, not a hidden part of the surface.
A comparison should state what is held fixed. Changing both the finish and temperature does not isolate the effect of finish; changing the surroundings can alter net transfer even when the object's surface is unchanged.
Investigate absorption with a fair comparison
Use two otherwise identical metal cans containing the same mass of water. Give one a dull black outer finish and leave the other shiny. Treat the thin finish as having a negligible effect on the total mass and thermal response apart from radiation.
- Keep the samples comparable: use the same can material, mass and geometry, the same water mass, and the same initial temperature.
- Keep the source comparable: expose both to the same lamp at equal distances and orientations for the same duration. Keep the surroundings and air movement the same.
- Measure consistently: place matching thermometers at the same immersion depth in the water, clear of the can walls and base. Use the same lids and any stirring procedure.
- Record the change: read temperature at the same elapsed times, allowing for the sensors' response. Repeat the comparison from matched starting conditions.
| Time / min | Dull black can / °C | Shiny can / °C |
|---|---|---|
| 0 | 20 | 20 |
| 2 | 24 | 22 |
| 4 | 28 | 24 |
| 6 | 31 | 25 |
Interpret the readings
Compare temperature rises over the same interval
After 6 min, the dull black can's water has risen by 31 - 20 = 11°C. The shiny can's water has risen by 25 - 20 = 5°C.
With the matched samples, the greater rise is evidence of greater net energy gain under these conditions. It is consistent with the dull black surface absorbing radiation more effectively.
The readings do not show that every joule received was retained. Both cans also transfer energy to their surroundings, and conduction carries energy from each warmed can into its water. The temperature-rise ratio is not automatically a ratio of radiation absorbed from the lamp.
A can closer to the lamp may receive more radiation for that reason alone. A thermometer touching the illuminated metal may read a locally hotter region rather than the water. Repeating readings does not fix unequal distances or misplaced sensors.
A cooling comparison needs similar care: match material, mass, shape, initial temperature, sensor placement and surroundings. Dull and shiny surfaces differ in radiative emission, but a simple cooling experiment in air also includes conduction and convection. Do not claim those routes have vanished.
Emission is not the same as net loss. Keep both incoming and outgoing radiation in the explanation. A shiny surface is a weaker emitter and absorber in the stated comparison, not a surface that transfers no energy at all.
Optional check Two opaque metal surfaces have the same exposed area and are both at 80 degrees C in the same cooler surroundings. One is dull black and the other shiny metallic. In the usual thermal-radiation comparison, which emits energy at the greater rate?
04
Controlling thermal transfer
Explain a thermal design by tracing each transfer route and identifying which feature changes it.
Conduction transfers energy through particle interactions; convection carries it with fluid; radiation transfers it by electromagnetic waves. A temperature difference sets the direction of net heating. A real design can involve all three processes.
Why a vacuum flask has several features
A vacuum flask has two walls separated by an evacuated space, reflective surfaces facing that gap, and a stopper at its opening. Each feature addresses a different route.
A flask reduces several routes of energy transfer
This schematic cutaway contains a warm drink in cooler surroundings. Match each numbered feature to its role below.
- Vacuum gap: reduces conduction and prevents convection across the gap.
- Reflective faces: reduce radiative transfer between the walls.
- Insulating stopper: reduces conduction and movement of fluid through the opening.
- Neck: the solid connection still conducts some energy.
- Small supports: hold the vessel in place but still provide conducting paths.
The same features also slow energy entering a cold drink from warmer surroundings.
- Evacuated gap
- Removing most gas particles greatly reduces conduction through the gap and prevents an ordinary fluid convection current there. Radiation can still cross the gap because it does not require particles.
- Reflective surfaces facing the gap
- Shiny surfaces are poor absorbers and emitters in the usual thermal-radiation comparison. They reduce radiative transfer across the gap that the vacuum alone cannot prevent.
- Insulating stopper
- A low-conductivity stopper slows conduction at the opening. Closing the opening also limits exchange of fluid between the contents and the surroundings.
- Neck and supports
- Solid parts connecting the inner and outer regions still conduct some energy. The stopper is not a perfect insulator, and the reflective surfaces do not remove all radiation.
Apply the same design twice
A hot drink and a cold drink
- A drink at 80°C in a 20°C room: net energy transfers out of the drink. The flask slows its cooling.
- A drink at 5°C in a 25°C room: net energy transfers into the drink. The same barriers slow its warming.
The second case does not require cold to flow out. Reverse the temperature difference and the net transfer direction reverses. The features still reduce the available transfer routes.
Sometimes the aim is faster transfer
A cooling device may use a conducting metal base and fins. Conduction spreads energy from the hotter component into the metal; the fins provide a larger exposed area for transfer to the surroundings.
Air moving past the fins carries energy away. A fan can strengthen this fluid movement when natural circulation is insufficient. Radiation also contributes, with its rate affected by the surface finish, temperature and area. Calling every feature an "insulator" would miss the purpose of this design.
Compare designs under the same conditions
To investigate the effect of a cup's insulating wrap, change that wrap while keeping the cup, amount of liquid, lid, initial temperature, surroundings and measurement method the same. Record temperature against elapsed time using the same sensor position and intervals.
If one cup starts at 80°C and another at 60°C in the same room, they have different driving temperature differences. Their final readings alone cannot isolate the effect of the wrap. Resetting the starting conditions addresses that problem; repeating the unequal comparison does not.
Identify where the remaining energy can go: through the cup and lid, into moving surrounding air, and by radiation. An improvement should reduce a named route, rather than claim that an extra layer must stop every process.
Match the feature to the mechanism. A vacuum suppresses routes across the gap that need a material medium; a shiny surface reduces radiative transfer; a low-conductivity solid slows conduction. None is a complete explanation for the whole flask by itself.
Optional check Air enters the space that was evacuated between the walls of a vacuum flask. The reflective finishes remain unchanged. What new explanation accounts for faster transfer across this gap?
Revision summary
Start with the temperature difference
Net energy transfers by heating from higher to lower temperature. At thermal equilibrium, temperatures are equal and there is no net heating between the regions. Particles still move. Equal temperature does not establish equal internal energy.
- Conduction
- In a solid, interactions between vibrating particles transfer energy without bulk flow of the material. Mobile electrons add a mechanism in metals. Good conductors transfer energy more readily under comparable conditions; insulators slow rather than eliminate it.
- Convection
- Energy is carried with moving liquid or gas. With ordinary expansion, warming increases volume for the same mass, decreasing density. Less dense fluid rises while cooler, denser fluid returns. Explain a complete circulation and its conditions.
- Radiation
- Electromagnetic waves transfer energy without a material medium. Objects emit and absorb, so net transfer depends on both. Radiation can cross a vacuum.
Retain all three radiation comparisons
- Colour and texture: dull black is a better absorber and emitter than shiny metallic in the usual comparison of similar opaque surfaces.
- Temperature: higher surface temperature gives a greater emitted rate when finish and area are unchanged.
- Area: larger exposed area gives a greater total emitted rate at the same temperature and finish; equal illumination is needed when comparing absorption.
Hold the other conditions fixed. An emitted rate alone is not a net loss, and visible colour alone is not a universal rule for every material and wavelength.
Explain a design one route at a time
In a vacuum flask, the gap suppresses conduction and convection across it; reflective faces reduce radiation; the stopper limits conduction and fluid exchange at the opening. Neck and support connections remain conduction routes. The same design slows warming of a cold drink and cooling of a hot drink.
For faster cooling, conducting metal spreads energy to exposed surfaces, a larger area supports greater transfer, and moving air carries energy away. The intended result determines which transfers to encourage.
Use measurements to support the explanation
In a surface comparison, match material, mass, geometry, starting temperature, source distance, orientation, surroundings, sensor placement and timing. Compare changes over the same interval. A larger rise in matched samples indicates greater net energy gain under those conditions.
Record temperatures and elapsed times with units. Repeat from the same starting conditions to assess variation, and correct specific causes such as a sensor touching the wrong region or unequal lamp distances. A simple cooling test in air has not removed conduction and convection.
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