Ch 1 · 14 Temperature · A Level Physics 9702
Topic 1 of 3 in Thermal Physics — A Level Physics 9702 — 58 lessons.
Introduction
What does a thermometer actually measure, and why does the kelvin scale start where it does? This topic makes temperature precise, then quantifies the energy needed to warm a substance or change its state. (Syllabus topic 14: sections 14.1–14.3.)
Key ideas — 14.1 Thermal equilibrium
Thermal energy is always transferred from a region of higher temperature to a region of lower temperature.
🧑🏫 Teacher's explanation
Imagine two cups of coffee: one boiling hot and one lukewarm. If you pour the hot one into the lukewarm one, heat flows from the hot coffee to the lukewarm one until they reach the same temperature. This 'flow' is thermal energy transfer. Once they reach that same temperature, they are in 'thermal equilibrium'—meaning no more net heat moves between them.
📖 Definitions
🔍 Worked example — Easy
A sample of liquid is at a temperature of 25.0 ^circC. Convert this temperature to Kelvin.
🔍 Worked example — Standard
An object is at a temperature of 310 K. Calculate its temperature in degrees Celsius.
🔍 Worked example — Exam-style
A metal rod is placed in contact with a large block of ice at 0 ^circC. After a long time, the rod reaches a steady temperature. What is the temperature of the rod in Kelvin?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State why the resistance of the platinum wire is a suitable property for measuring temperature. [1] (b) A temperature reading of 100 ^circC is taken. Calculate the temperature in Kelvin. [1] [2 marks]
Key ideas — 14.2 (a) Measuring temperature
Any physical property that varies with temperature can be used to measure temperature. Examples:
🧑🏫 Teacher's explanation
Imagine you are trying to measure how much water is in a glass using a ruler. If the ruler expands when it gets hot, your measurement might be slightly off. Temperature works similarly. We cannot 'see' temperature directly; instead, we look for a physical property that changes predictably when things get hot or cold. This is called a 'thermometric property'.
📖 Definitions
🔍 Worked example — Easy
Convert a temperature of 25.0 ^circC to Kelvin.
🔍 Worked example — Standard
A gas thermometer measures a temperature of 350 K. Calculate this temperature in degrees Celsius.
🔍 Worked example — Exam-style
An experimenter uses a resistance thermometer to measure a metal rod. The resistance of the metal increases as it heats up. If the temperature of the rod is 450 K, calculate the temperature in ^circC and state the property being used.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the physical property of the thermocouple that changes with temperature. [1] (b) The student measures a temperature of 32.0 ^circC. Calculate the temperature in Kelvin. [2] [3 marks]
Key ideas — 14.2 (b) The thermodynamic scale
The thermodynamic (kelvin) temperature scale does not depend on the property of any particular substance. It is based on the behaviour of an ideal gas (topic 15) and has an absolute zero.
🧑🏫 Teacher's explanation
Imagine you are measuring height. You could use a ruler (a physical property like the length of a mercury column), but that ruler might expand or contract depending on the weather. A 'thermodynamic' scale is like measuring height using a universal constant that never changes, regardless of the environment. It provides a 'true' baseline.
📖 Definitions
🔍 Worked example — Easy
Convert 25.0circC to Kelvin.
🔍 Worked example — Standard
A liquid is at 350 K. Calculate its temperature in degrees Celsius.
🔍 Worked example — Exam-style
A gas is heated from 10.0circC to 50.0circC. Calculate the temperature change Δ T in Kelvin.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State one advantage of using the thermodynamic temperature scale rather than a scale based on the resistance of platinum. [1] (b) A sample is heated from -40.0circC to 120circC. Calculate the temperature increase in Kelvin. [2] [3 marks]
Key ideas — 14.3 (a) Specific heat capacity
The specific heat capacity c of a substance is the energy required per unit mass to raise its temperature by one kelvin (or 1 °C):
🧑🏫 Teacher's explanation
Think about why a metal spoon in a hot cup of tea gets hot much faster than the tea itself. Even though they are in the same liquid, the metal requires less energy to increase its temperature. This is because different materials have different 'thermal inertia'—some are stubborn and resist temperature changes, while others change temperature easily.
📖 Definitions
🔍 Worked example — Easy
Calculate the energy required to raise the temperature of 0.5 kg of water from 20circC to 80circC. The specific heat capacity of water is 4200 J kg^-1 K^-1.
🔍 Worked example — Standard
An electric heater of power 50 W is used to heat 2.0 kg of oil. The temperature rises by 15circC in 4 minutes. Calculate the specific heat capacity of the oil.
🔍 Worked example — Exam-style
A 0.20 kg block of metal is heated from 25circC to 105circC. 1500 J of energy is supplied. Calculate the specific heat capacity of the metal.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — 14.3 (b) Specific latent heat
The specific latent heat L of a substance is the energy required per unit mass to change its state without a change in temperature:
🧑🏫 Teacher's explanation
Imagine you are heating a pot of water. As you add heat, the temperature rises steadily. However, the moment the water reaches 100^°C, the temperature stops rising, even though you are still adding heat. The energy isn't being used to make the molecules move faster (which would increase temperature); instead, it is being used to break the 'glue' holding the molecules together so they can escape from a liquid to a gas.
📖 Definitions
🔍 Worked example — Easy
Calculate the energy required to melt 0.50 kg of ice at 0^°C into water at 0^°C. Use L_f = 3.3 × 10^5 J kg^-1.
🔍 Worked example — Standard
A 2.0 kg block of ice at 0^°C is melted completely into water at 0^°C. How much energy is absorbed? Use L_f = 3.3 × 10^5 J kg^-1.
🔍 Worked example — Exam-style
A student heats 0.25 kg of liquid water at 100^°C until it has completely turned into steam at 100^circC. Calculate the energy supplied. Use L_v = 2.3 × 10^6 J kg^-1.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Example 1. How much energy is needed to turn 0.50 kg of ice at −10 °C into water at 20 °C? (c_ice = 2100, c_water = 4200 J kg⁻¹ K⁻¹, L_f = 3.3 × 10^5 J kg⁻¹.)
🔍 Worked example — Easy
Calculate the energy required to raise the temperature of 0.40 kg of copper from 20 ^circC to 100 ^circC. The specific heat capacity of copper is 390 J kg^-1 K^-1.
🔍 Worked example — Exam-style
An electric kettle with power P = 2.2 kW is used to boil water. How much mass of water is converted to steam in 4.0 minutes? Assume all electrical energy is transferred to the water and the specific latent heat of vaporisation L_v = 2.3 × 10^6 J kg^-1.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) No net transfer of thermal energy between regions at the same temperature. (2) Any two: volume/density of a liquid; volume of a gas at constant pressure; resistance of a metal; e.m.f. of a thermocouple. (3) It does not depend on the property of any particular substance, and it has absolute zero as its zero. (4) 233.15 K. (5) The energy required per unit mass to change a liquid to a gas without a change in temperature; boiling separates the molecules completely and the gas does work against the atmosphere as it expands. (6) c = 50 × 120/(0.20 × 12) = 2500 J kg⁻¹ K⁻¹.