Temperature and thermal equilibrium Cambridge International AS & A Level Physics revision
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In plain words
Put a hot spoon in cold water and energy flows from the spoon to the water until they are at the same temperature. Then it stops. Temperature is the thing that decides which way energy flows. To put a number on it we use anything that changes steadily as it warms; and so that the number doesn't depend on what we chose, physicists use a scale built on energy itself, the kelvin scale.
5 things to know
- Thermal energy is transferred from a region of higher temperature to a region of lower temperature. Regions at the same temperature are in thermal equilibrium, and there is no net transfer between them.
- A thermometer uses a physical property that varies with temperature: the density of a liquid, the volume of a gas at constant pressure, the resistance of a metal, or the e.m.f. of a thermocouple.
- The thermodynamic (kelvin) scale does not depend on the property of any particular substance. T in kelvin = θ in °C + 273.15.
- Absolute zero, 0 K, is the lowest possible temperature.
- A change of 1 K is the same size as a change of 1 °C.
Worked example
Convert 25 °C to kelvin, and 77 K to degrees Celsius.
- T = θ + 273.15 = 25 + 273.15 = 298.15 K.
- θ = T − 273.15 = 77 − 273.15.
- = −196.15 °C.
Tips and tricks
- A temperature difference is the same number in kelvin and in degrees Celsius. Only actual temperatures need the 273.15.
- Use kelvin in every gas equation.
It lands in your notebook with its questions as flashcards.
Temperature and thermal equilibrium: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Two objects are in thermal equilibrium. What must be the same for both?
No net energy flows between them.
What is 0 °C on the kelvin scale?
Add 273.15.
What is the lowest possible temperature?
Absolute zero.
Which property is used by a thermocouple thermometer?
A voltage appears that depends on the temperature difference between its junctions.
The temperature of a liquid rises by 20 °C. What is the rise in kelvin?
The two scales have the same size of degree.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 6 marks. Write your answers on paper, then check them.
Temperature and thermal equilibrium
Cambridge International AS & A Level Physics 9702 · 6 marks · papermunch.org
Name ______________________________ Date ______________
Convert 37 °C to kelvin, and 4 K to degrees Celsius.[2]
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310 K (310.15 K), and −269 °C (−269.15 °C).
State what is meant by thermal equilibrium.[2]
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Two regions are in thermal equilibrium when they are at the same temperature, so there is no net transfer of thermal energy between them.
Name two physical properties that can be used to measure temperature.[2]
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Any two: the density (or volume) of a liquid, the volume of a gas at constant pressure, the resistance of a metal, the e.m.f. of a thermocouple.
Answers: Temperature and thermal equilibrium
- 1. 310 K (310.15 K), and −269 °C (−269.15 °C).
- 2. Two regions are in thermal equilibrium when they are at the same temperature, so there is no net transfer of thermal energy between them.
- 3. Any two: the density (or volume) of a liquid, the volume of a gas at constant pressure, the resistance of a metal, the e.m.f. of a thermocouple.