The shape of the lesson
By the end of the lesson, learners can
- A scale that needs no substance: Many properties change with temperature, but two thermometers using different properties need not agree between their fixed points.
- Converting, and the meaning of absolute zero: The Celsius scale is the thermodynamic scale shifted: T / K = θ / °C + 273.15.
- Warming without a state change: Q = mcΔθ.
- Changing state at constant temperature: Q = mL.
- Scales: T / K = θ / °C + 273.15; absolute zero is 0 K = −273.15 °C.
One picture every idea returns to
The thermodynamic (Kelvin) scale does not depend on the substance used. Its zero, absolute zero (0 K = −273.15 °C), is where particles have least kinetic energy, and T / K = θ / °C + 273.15.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, One scale, no substance needed.. The thermodynamic scale is built so that it does not depend on any particular material. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: A scale that needs no substance | Many properties change with temperature, but two thermometers using different properties need not agree between their fixed points. Project tab 2 of the lab (Temperature scales) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Converting, and the meaning of absolute zero | The Celsius scale is the thermodynamic scale shifted: T / K = θ / °C + 273.15. | Cooperative Concept Mapping |
| 30 to 43 min | Check and plenary | The quick-check questions from the student lesson, answered on mini whiteboards one at a time; re-teach the one most get wrong, then a random call on the last. | Individual, then whole class |
| 43 to 45 min | Exit and homework | Set the worksheet below (3 questions, 32 marks) from the topic practice set. Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Converting, and the meaning of absolute zero, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Cooperative Concept Mapping
Teams receive the key terms on cards and arrange them on a large sheet, drawing and labelling the links. Compare maps between teams before showing the model map.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
No examiner trap is recorded for this lesson. Use the quiz key below to spot the distractor most of the class picks.
Temperature scales: topic worksheet
3 questions, 32 marks. Written in the style of the papers; no past paper question is reproduced. These questions cover the whole topic, so sibling lessons share them.
Two metal blocks A and B are placed in thermal contact and isolated from their surroundings, as shown in Fig. 1.1. Block A starts at 85 °C and block B starts at 25 °C.
(a) State what is meant by thermal equilibrium between two bodies. [2]
(b) For the two blocks in contact:
(i) state the direction of the net transfer of thermal energy and explain your answer, [3]
(ii) state what is true of the two temperatures once thermal equilibrium is reached. [1]
(c) Using T / K = θ / °C + 273.15:
(i) express the starting temperature of block A in kelvin, [1]
(ii) a block later has a thermodynamic temperature of 300 K; state this in °C. [1]
(d) State the Celsius value of absolute zero, and state what is special about it in terms of the energy of the particles. [2]
(e) State one reason the thermodynamic temperature scale is preferred to a scale defined by the expansion of a particular liquid. [2]
This question is about the thermodynamic (kelvin) temperature scale.
(a) State what is meant by absolute zero. [2]
(b) Write the relation between thermodynamic temperature T in kelvin and Celsius temperature θ. [1]
(c) Convert 27 °C to kelvin, and 200 K to degrees Celsius. [2]
(d) Explain why a temperature change of 1 °C is equal to a change of 1 K. [1]
(e) State what is meant by the thermodynamic scale being independent of the substance used. [2]
(f) The mean kinetic energy of gas molecules is proportional to T in kelvin. Explain why this relation would fail if θ in °C were used. [2]
Thermometers measure temperature through a physical property that varies with it.
(a) State two physical properties that can be used to measure temperature. [2]
(b) Explain what is meant by calibrating a thermometer using two fixed points. [2]
(c) Explain why two different thermometers may disagree at temperatures between the fixed points. [2]
(d) State one advantage of the thermodynamic scale over an empirical scale. [2]
(e) A resistance thermometer reads 100 Ω at 0 °C and 138 Ω at 100 °C. Assuming a linear response, find the temperature when its resistance is 119 Ω. [2]
Answer key and marking notes
- (a)Two bodies are in thermal equilibrium when there is no net transfer of thermal energy between them ✓, which happens when they are at the same temperature ✓
- (b)(i)Net energy is transferred from A to B ✓, because A is at the higher temperature ✓; net flow is always from higher to lower temperature ✓
- (b)(ii)The two temperatures become equal ✓
- (c)(i)T = 85 + 273.15 = 358 K ✓
- (c)(ii)θ = 300 − 273.15 = 27 °C ✓
- (d)Absolute zero is −273 °C (−273.15 °C) ✓; it is the temperature at which the particles have their minimum (least) kinetic energy ✓
- (e)The thermodynamic scale does not depend on the property or substance used ✓; different liquids would not agree at points between the fixed points, whereas the thermodynamic scale gives one substance-independent value ✓
- (a)The lowest possible temperature, the zero of the thermodynamic scale, at which particles have minimum energy ✓✓
- (b)T / K = θ / °C + 273.15 ✓
- (c)27 °C = 300 K ✓; 200 K = −73 °C ✓
- (d)The two scales have divisions of the same size ✓
- (e)It does not depend on a property of any particular material, unlike an empirical scale ✓✓
- (f)The proportionality holds only from absolute zero ✓; the Celsius zero is arbitrary (the ice point), so KE would not be proportional to θ ✓
- (a)Any two of: the volume of a liquid, electrical resistance, the e.m.f. of a thermocouple, the pressure of a fixed-volume gas ✓✓
- (b)Marking the readings at two known temperatures (such as the ice and steam points) ✓ and dividing the scale evenly between them ✓
- (c)Their chosen properties vary differently (non-linearly) with temperature ✓, so the empirical scales agree only at the fixed points ✓
- (d)It is independent of the substance and agrees everywhere ✓✓
- (e)θ = (119 − 100)/(138 − 100) × 100 = 19/38 × 100 ✓ = 50 °C ✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
To convert a temperature in degrees Celsius to the thermodynamic (kelvin) scale you:
- Aadd 273 (273.15)
- Bsubtract 273
- Cmultiply by 273
- Ddivide by 273
Absolute zero is:
- A0 K, equal to about −273 °C
- B0 °C, equal to 273 K
- C−273 K
- Dthe boiling point of water
An advantage of the thermodynamic temperature scale over one based on the expansion of a liquid is that it:
- Adoes not depend on the substance used
- Bgives larger numbers
- Cnever needs fixed points
- Dworks only near room temperature
A temperature change of 1 K is the same as a temperature change of:
- A1 °C
- B273 °C
- C−272 °C
- D0 °C
Quiz key
T / K = θ / °C + 273.15, so you add 273 to the Celsius value.
Absolute zero is 0 on the kelvin scale, which is −273.15 °C, the point of minimum particle kinetic energy.
The thermodynamic scale is substance-independent, so it does not matter which material a thermometer is made from.
Only the zero of the scale differs; the size of the unit is identical, so a change of 1 K equals a change of 1 °C.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.