Cambridge IGCSE Physics · Syllabus 0625 · Thermal Physics
Temperature
What is Temperature?
Temperature is the physical quantity that indicates how fast the particles of a substance are moving and therefore how much kinetic energy they have; a higher temperature means faster particle motion, and the lowest possible temperature is absolute zero at -273 degrees Celsius, where the particles have their least kinetic energy.
This definition is part of the Thermal Physics chapter in Cambridge IGCSE Physics.
Temperature in context
Thermal physics runs along a single chain of reasoning. Particles are arranged and move in a way that depends on the forces between them; heating makes them move faster, which raises temperature; faster particles collide harder and more often, which creates gas pressure and drives expansion; energy supplied at a change of state pulls particles apart instead of speeding them up, so the temperature holds still; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.
Topic 2 of Cambridge IGCSE Physics 0625 contains 45 printed statements: 24 Core, taught to all candidates, and 21 Supplement, taught in addition to Core for candidates following the Extended route. The Core statements cover the kinetic particle model and states of matter, Brownian motion as evidence for that model, gas pressure and the qualitative gas relationships, the kelvin scale, thermal expansion and its applications, internal energy, changes of state and evaporation, and thermal energy transfer by conduction, convection and infrared radiation with basic everyday applications. The Supplement statements add inter-particle forces, force per unit area in gases, \(pV = \text{constant}\), the particle explanation of expansion, average kinetic energy and specific heat capacity with its two experiments, the boiling–evaporation comparison and evaporative cooling of an object, the mechanisms and limits of conduction, the radiation balance including the Earth's temperature, emitter and absorber experiments, and complex multi-route applications.
In a solid, particles are close together in a regular arrangement and vibrate about fixed positions. In a liquid they are still close but disordered, and they move around one another. In a gas they are far apart and move rapidly and randomly. Raising the temperature makes the particles move faster; at −273 °C — absolute zero — they have their least kinetic energy. Gas particles collide with the walls of their container, and those collisions are what produce gas pressure. The random jiggling of microscopic particles suspended in a fluid, known as Brownian motion, is direct evidence that the fluid is made of particles in constant random motion.
Heating a substance normally increases the average separation of its particles, so the substance expands; Extended candidates also compare how much each state expands. A rise in temperature increases an object's internal energy. When a pure substance melts or boils, the energy supplied separates the particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; condensation and solidification are the reverse, with particles coming closer together and energy being given out. Evaporation is different again: it happens only at the surface, at any temperature below the boiling point, and it cools the liquid left behind.
Common mistakes with Temperature
- 2. “Temperature is the total energy in an object.” Core 2.2.2.1 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 how fast the particles are moving; the total is the internal energy, which also depends on mass and state. Exam-safe: “Temperature indicates how fast the particles move, whereas internal energy is the total kinetic and potential energy of all of them.”
- 3. “Heat and temperature mean the same thing.” Core 2.2.2.1 Why it is wrong: one is an energy transfer, the other is a measure of particle motion. 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.”
- 5. “The temperature rises continuously while a pure substance melts or boils.” Core 2.2.3.1 Why it is wrong: it assumes supplied energy must always increase particle speed. During a change of state it separates the particles 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 they do not move any faster and the temperature stays constant.”
- 11. “Cold moves through a conductor.” Core 2.3.1.1 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.”
- 15. Converting a temperature difference to kelvin by adding 273. Core 2.1.3.2 Why it is wrong: the two scales have the same size of degree and differ only in where zero is. A difference is unaffected by where zero is. Corrected model: convert a temperature by adding 273; leave a temperature change exactly as it is. Exam-safe: “A rise of 50 °C is a rise of 50 K.”
- 16. “Pressure is proportional to volume.” Supplement 2.1.3.3 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 that never touches either axis. Exam-safe: “For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume, so \(pV\) has the same value before and after.”
- 17. “Insulation prevents all energy transfer” and “an object at a steady temperature has stopped emitting”. Supplement 2.3.3.4 Supplement 2.3.3.5 Why they are wrong: both treat transfer as something that can be switched off. No material stops transfer completely; insulation only slows it. And a steady temperature is a balance, not a stop: the object is still emitting, and absorbing just as much back. Corrected model: think in rates. Insulation reduces the rate of transfer. An object holds a constant temperature while the rate at which it transfers energy away equals the rate at which it receives energy. Exam-safe: “The insulation greatly reduces the rate of energy transfer, so the liquid cools much more slowly; it stops cooling only once it is transferring energy away at the same rate as it receives it.”
Examiner tips on Temperature
- 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) “…it transfers energy away at the same rate as it receives it…” (constant temperature, Extended)
- 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 “radiation needs air to travel through” → radiation is the one route that needs no medium “heat is lost” when you mean the temperature falls → say thermal energy is transferred to the surroundings