Cambridge O Level Physics · Syllabus 5054 · Thermal Physics
Temperature
What is 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.
This definition is part of the Thermal Physics chapter in Cambridge O Level 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; and thermal energy travels from hot to cold by conduction, convection and infrared radiation.
In a solid, particles are close together in a regular arrangement, held by strong forces, 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 with negligible forces except during collisions, and move rapidly and randomly. Raising the temperature raises the average kinetic energy of the particles; at −273 °C — absolute zero — the particles have their least kinetic energy. Gas pressure is the force per unit area produced by particles colliding with a surface, and for a fixed mass of gas at constant temperature \(p_1V_1 = p_2V_2\).
Heating a substance normally increases the average separation of its particles, so the substance expands — most for gases, less for liquids, least for solids. The energy needed to raise the temperature of one kilogram of a substance by one degree is its specific heat capacity, \(c = \Delta E/(m\,\Delta\theta)\). When a pure substance melts or boils, the energy supplied separates particles against the forces between them instead of speeding them up, so the temperature stays constant during the change; that energy is the latent heat. Evaporation is different again: it happens only at the surface, at any temperature, and it cools the liquid left behind.
Common mistakes with Temperature
- 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.”
- 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.”
- 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.”
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 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
Questions students ask about Temperature
What is the difference between temperature and internal energy?
Temperature indicates the average kinetic energy of a substance's particles, while internal energy is the total kinetic and potential energy of all the particles combined. A hot spark and a large bonfire can have similar temperatures despite very different internal energy, because internal energy also depends on the mass and state of the substance. Raising an object's temperature always increases its internal energy, but internal energy also changes during a change of state, when the temperature stays constant.
Why does the temperature stay constant while a pure substance melts or boils?
On a heating curve the temperature is constant during melting and during boiling because the energy supplied is separating the particles against the forces between them, not speeding them up. Since temperature depends on the average kinetic energy of the particles, and that average is not increasing during the change, the temperature does not rise until melting or boiling is complete. That energy is the latent heat.
What is the difference between boiling and evaporation?
Boiling happens throughout the whole liquid, only at one definite temperature — 100 °C for water at standard atmospheric pressure — with bubbles of vapour forming inside it. Evaporation happens only at the surface, can occur at any temperature below the boiling point, and cools the liquid left behind because the most energetic particles escape, lowering the average kinetic energy of those that remain. The two are not interchangeable.
How do you set out a specific heat capacity calculation in the exam?
Specific heat capacity \(c\) is the energy required per unit mass per unit temperature increase, \(c = \Delta E/(m\,\Delta\theta)\). Substitute the mass in kilograms and the temperature change in degrees Celsius — or kelvin, since a change of 1 °C equals a change of 1 K — then rearrange for whichever quantity is missing. Always check which value belongs in the denominator before substituting.