Thermal Physics

• The one sentence that unlocks the chapter: temperature is about how fast particles move on average, internal energy is the total of all the particles' kinetic and potential energies, and thermal energy transfer is what happens because two places are at different temperatures. Confuse those three and half the marks in this topic disappear.
• Section 2.1 in one line: arrangement and forces decide the state; average kinetic energy decides the temperature; collisions decide the pressure; and at constant temperature \(pV\) does not change. Mastery check: can you explain a pressure rise twice — once for heating, once for compression — using different words each time?

• I can state the distinguishing properties of solids, liquids and gases. 2.1.1.1
• I can name the changes of state between solids, liquids and gases, and I know that solid–gas transfers in either direction are outside this syllabus. 2.1.1.2
• I can describe the particle structure of each state qualitatively, relating properties to the forces between particles, the distances between them and their motion. 2.1.2.1
• I can describe the relationship between particle motion and temperature, including absolute zero at −273 °C where particles have least kinetic energy. 2.1.2.2
• I can describe gas pressure, and changes in gas pressure, as the force per unit area produced by particles colliding with a surface. 2.1.2.3
• I can explain qualitatively, in terms of particles, the relationship between pressure and temperature at constant volume, volume and temperature at constant pressure, and pressure and volume at constant temperature. 2.1.2.4
• I can recall and use \(p_1V_1 = p_2V_2\) and sketch the pressure–volume graph for a gas at constant temperature. 2.1.2.5
• I can explain applications and consequences of thermal expansion, including the liquid-in-glass thermometer. 2.2.1.1
• I can explain thermal expansion of solids, liquids and gases in terms of particle motion and arrangement, and state the relative order of magnitude of the three. 2.2.1.2
• I can convert between kelvin and degrees Celsius using \(T\,(\text{in K}) = \theta\,(\text{in }^{\circ}\text{C}) + 273\). 2.2.1.3
• I know that raising an object's temperature increases its internal energy. 2.2.2.1
• I can describe a temperature rise as an increase in the average kinetic energies of all the particles in the object. 2.2.2.2
• I can define specific heat capacity as the energy required per unit mass per unit temperature increase, and use \(c = \Delta E / (m\,\Delta\theta)\). 2.2.2.3
• I can describe experiments to measure the specific heat capacity of a solid and of a liquid. 2.2.2.4
• I can describe melting, solidification, boiling and condensation as energy transfer without a change in temperature. 2.2.3.1
• I know the melting and boiling temperatures of water at standard atmospheric pressure. 2.2.3.2
• I can describe the differences between boiling and evaporation. 2.2.3.3
• I can describe evaporation as the escape of more energetic particles from the surface of a liquid. 2.2.3.4
• I can describe how temperature, surface area and air movement over a surface affect evaporation. 2.2.3.5
• I can explain how evaporation causes cooling. 2.2.3.6
• I can describe latent heat as the energy required to change the state of a substance, and explain it in terms of particle behaviour and the forces between particles. 2.2.3.7
• I can describe experiments to distinguish between good and bad thermal conductors. 2.3.1.1
• I can describe thermal conduction in all solids in terms of atomic or molecular lattice vibrations, and additionally in terms of free delocalised electrons in metallic conductors. 2.3.1.2
• I can explain convection in liquids and gases in terms of density changes, and describe experiments that illustrate convection. 2.3.2.1
• I can describe thermal energy transfer by infrared radiation and state that it does not require a medium. 2.3.3.1
• I can describe the effect of surface colour and texture on the emission, absorption and reflection of infrared radiation. 2.3.3.2
• I can describe how the rate of emission of radiation depends on the surface temperature and the surface area of an object. 2.3.3.3
• I can describe experiments to distinguish between good and bad emitters of infrared radiation. 2.3.3.4
• I can describe experiments to distinguish between good and bad absorbers of infrared radiation. 2.3.3.5
• I can explain everyday applications using conduction, convection and radiation, including kitchen pans, heating a room by convection, the infrared thermometer, and thermal insulation used to keep a liquid at temperature and to reduce transfer in buildings. 2.3.4.1

Why it matters: everything later in the chapter is this section reused. Thermal expansion is separation increasing; specific heat capacity is average kinetic energy increasing; changes of state are forces being overcome; convection is a density change caused by expansion. Get the particle picture exact here and the rest is bookkeeping.

Thermal Expansion

Thermal expansion is the increase in the volume, and usually the length, of a substance when its temperature rises, caused by the particles gaining kinetic energy and moving over a greater range so that their average separation increases; gases expand most, liquids less and solids least for the same temperature rise.

Temperature

Temperature is the physical quantity that indicates the average kinetic energy of the particles of a substance; a higher temperature means a higher average kinetic energy and faster particle motion, and the lowest possible temperature is absolute zero at about -273 degrees Celsius, where particles have their least kinetic energy.

Gas Pressure

Gas pressure is the force per unit area exerted on a surface by gas particles colliding with it; each collision changes a particle's momentum and so exerts a force, and pressure is the total force from all collisions divided by the area of the surface.

Internal Energy

Internal energy is the total energy stored by all the particles of an object, made up of the kinetic energy of their random motion and the potential energy associated with the forces between them; raising an object's temperature increases its internal energy, but internal energy also depends on the mass and the state of the substance.

Melting

Melting is the change of state from solid to liquid, which occurs at a fixed temperature for a pure substance at constant pressure; the energy supplied separates particles and weakens the forces holding them in fixed positions rather than raising their average kinetic energy, so the temperature stays constant during the change.

Specific Heat Capacity

Specific heat capacity is the energy required per unit mass per unit temperature increase, given by c = change in energy divided by mass times change in temperature, and measured in joules per kilogram per degree Celsius or equivalently joules per kilogram per kelvin.

Boiling

Boiling is the change of state from liquid to gas that occurs throughout the whole liquid at a definite temperature for a given pressure, one hundred degrees Celsius for water at standard atmospheric pressure, with bubbles of vapour forming inside the liquid and the temperature remaining constant while the change takes place.

Evaporation

Evaporation is the escape of the more energetic particles from the surface of a liquid, changing it to a gas; it occurs only at the surface, can happen at any temperature below the boiling point, and cools the liquid left behind because the particles remaining have a lower average kinetic energy.

Infrared Radiation

Infrared radiation is electromagnetic radiation emitted and absorbed by all objects that transfers thermal energy without needing a medium, so it travels through a vacuum; the rate at which an object emits it increases with the surface temperature and with the surface area, and dull black surfaces are the best emitters and absorbers while shiny light surfaces are poor emitters and absorbers but good reflectors.

Convection

Convection is the transfer of thermal energy through a fluid by the bulk movement of the fluid itself; the heated fluid expands so its density decreases and it rises, cooler denser fluid sinks to take its place, and the resulting circulation is a convection current. Convection cannot occur in a solid because a solid cannot flow.

Conduction

Thermal conduction is the transfer of thermal energy through a substance without any bulk movement of the substance itself; in all solids the particles at the hotter end vibrate more and pass energy on to neighbouring particles through collisions, and in metals free delocalised electrons also carry energy rapidly through the structure, which is why metals are the best thermal conductors.

• 1. “Particles expand when heated.” Why it is wrong: heating changes how energetically particles move, not their size. An atom is not compressible or inflatable. Corrected model: the particles gain kinetic energy, vibrate or travel over a larger range, so their average separation increases and the object expands. Exam-safe: “The particles gain kinetic energy and vibrate more, so their average separation increases and the object expands; the particles themselves do not change size.”
• 2. “Temperature is the total energy in an object.” Why it is wrong: temperature takes no account of how much substance is present. A spark and a bonfire can be at similar temperatures with vastly different energy. Corrected model: temperature indicates the average kinetic energy of the particles; the total is the internal energy, which also depends on mass and state. Exam-safe: “Temperature indicates the average kinetic energy of the particles, whereas internal energy is the total kinetic and potential energy of all of them.”
• 3. “Heat and temperature mean the same thing.” Why it is wrong: one is an energy transfer, the other is a measure of average particle kinetic energy. Adding energy to melting ice raises no temperature at all. Corrected model: thermal energy is transferred because of a temperature difference; temperature is what that transfer may or may not change. Exam-safe: “Thermal energy is transferred from the hotter object to the cooler one because of the temperature difference between them.”
• 4. “Gas pressure is caused by particles resting against the wall.” Why it is wrong: gas particles are in continuous free motion and touch the wall only briefly. A resting particle produces no momentum change and therefore no force. Corrected model: pressure arises from the momentum changes of particles rebounding during collisions, averaged over an enormous number of impacts. Exam-safe: “The particles collide with the wall; each collision changes their momentum and exerts a force, and the total force per unit area is the pressure.”
• 5. “Pressure is proportional to volume.” Why it is wrong: doubling the volume halves the pressure. Proportionality would give a rising straight line through the origin, which is not what is observed. Corrected model: \(p \propto 1/V\), so \(pV\) is constant and the \(p\)–\(V\) graph is a falling curve. Exam-safe: “For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume, so \(p_1V_1 = p_2V_2\).”
• 6. “The temperature rises continuously while a pure substance melts or boils.” Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it increases particle potential energy instead. Corrected model: the temperature stays constant throughout melting and throughout boiling, giving the flat sections of a heating curve. Exam-safe: “The energy supplied separates the particles against the forces between them, so their average kinetic energy and therefore the temperature stay constant.”
• 7. “Boiling and evaporation are identical.” Why it is wrong: they differ in location, temperature condition, the presence of bubbles and their effect on the liquid's temperature. Corrected model: boiling occurs throughout the liquid at one definite temperature; evaporation occurs at the surface at any temperature and cools the liquid. Exam-safe: “Boiling occurs throughout the liquid at a definite temperature, whereas evaporation occurs only at the surface and at any temperature.”
• 8. “Evaporation occurs throughout a liquid.” Why it is wrong: a particle deep inside is surrounded on all sides by neighbours attracting it and has no free direction in which to escape. Corrected model: evaporation is a surface process; only surface particles can leave the liquid. Exam-safe: “Only particles at the surface can escape, so evaporation takes place at the surface of the liquid.”
• 9. “The coldest particles escape during evaporation.” Why it is wrong: slow particles do not have enough energy to overcome the attractive forces. If the slowest left, the liquid would warm up, not cool. Corrected model: the most energetic surface particles escape, lowering the average kinetic energy of those remaining. Exam-safe: “The most energetic particles escape from the surface, so the average kinetic energy of the remaining particles falls and the liquid cools.”
• 10. “Convection occurs in solids.” Why it is wrong: convection requires the material itself to move from place to place, and the particles of a solid are held in fixed positions. Corrected model: convection occurs only in fluids — liquids and gases. In a solid, energy travels by conduction. Exam-safe: “Convection cannot occur in the solid because it cannot flow; energy is transferred through it by conduction.”
• 11. “Cold moves through a conductor.” Why it is wrong: there is no such thing as cold energy. Only energy is transferred, and it always goes from the hotter region to the cooler one. Corrected model: an object feels cold because thermal energy flows rapidly out of your hand into it. Exam-safe: “Thermal energy is conducted from your hand into the metal, which is at a lower temperature, so your hand cools.”
• 12. “Black surfaces only absorb radiation.” Why it is wrong: emission and absorption depend on the same surface property, so a good absorber is necessarily a good emitter. Corrected model: a dull black surface is a good absorber and a good emitter. Exam-safe: “The dull black surface is a good emitter as well as a good absorber, so it loses energy by radiation quickly.”
• 13. “Shiny surfaces are good emitters.” Why it is wrong: it confuses reflecting with emitting. A shiny surface is excellent at reflecting incoming radiation, which is exactly why it emits poorly. Corrected model: shiny light surfaces are poor emitters, poor absorbers and good reflectors. Exam-safe: “The shiny surface reflects most of the incident radiation and is a poor emitter, so the container cools slowly.”
• 14. “A vacuum stops infrared radiation.” Why it is wrong: radiation is the one route that needs no medium at all — energy from the Sun crosses space to reach us. Corrected model: a vacuum removes conduction and convection; radiation is dealt with by the silvered surfaces. Exam-safe: “The vacuum reduces conduction and convection because there are almost no particles between the walls; the silvered surfaces reduce radiation.”
• 15. “Insulation prevents all energy transfer.” Why it is wrong: no material stops transfer completely. A vacuum flask only slows it; the drink inside does eventually cool. Corrected model: insulation reduces the rate of thermal energy transfer. Exam-safe: “The insulation greatly reduces the rate of energy transfer, so the liquid cools much more slowly, but it does not prevent cooling entirely.”

• Command-word cue “State the change of state” wants one word: melting, solidification, boiling, condensation or evaporation. Writing “it turns into a liquid” describes the outcome but does not name the process, so it does not answer a question that asked for the name.
• The four questions every practical answer must answer What did you change? (independent variable) — exactly one thing. What did you measure? (dependent variable) — with what instrument, to what precision. What did you keep the same? (controlled variables) — this is where fair-test marks live. Why might the answer be wrong, and in which direction? — state whether the error makes the result too large or too small.
• Why heat loss makes \(c\) come out too large — say it in one line The calculation assumes all of the electrical energy \(VIt\) entered the sample. In reality some of it left to the surroundings, so the sample received less than \(VIt\) and its temperature rise \(\Delta\theta\) is smaller than that full energy would have produced. Dividing an over-large energy by an under-sized temperature rise, \(c = VIt/(m\Delta\theta)\), gives a value that is larger than the true one.
• Five phrases that keep a Topic 2 answer precise “…so the average separation of the particles increases…” (expansion) “…collisions with unit area of the wall become more frequent…” (pressure) “…the fluid expands, so its density decreases, so it rises…” (convection) “…the energy separates the particles, increasing their potential energy rather than their kinetic energy…” (change of state) “…the most energetic particles escape from the surface…” (evaporation)
• Five phrases to delete from your answers “the particles get bigger” → say the average separation increases “hot air rises” on its own → add the expansion and density steps “the cold comes in” → say energy is transferred out “the vacuum stops radiation” → the silvering deals with radiation “heat is lost” when you mean the temperature falls → say thermal energy is transferred to the surroundings