OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Temperature: (a) thermal equilibrium
Thermal equilibrium is the state reached when two or more objects in thermal contact have no net transfer of thermal energy between them.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 4
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Thermal equilibrium is the state reached when two or more objects in thermal contact have no net transfer of thermal energy between them. Their temperatures are equal, so there is no resultant heat flow. For example, a thermometer placed in a beaker of water reaches thermal equilibrium with the water when the thermometer reading becomes constant; at that point the thermometer and water are at the same temperature. Thermal equilibrium is a dynamic condition: energy may still be exchanged at a microscopic level, but the average rates of transfer in each direction are equal, so the net transfer is zero. This concept underpins the definition of temperature and the operation of thermometers.
What earns the marks
- Thermal equilibrium occurs when two objects in thermal contact have equal temperatures and no net transfer of thermal energy.
- At equilibrium, the average rates of energy transfer in both directions are equal, so the net energy transfer is zero.
- A thermometer reaches thermal equilibrium with its surroundings when its reading becomes constant.
- Thermal equilibrium is distinct from steady-state conditions where a temperature difference is maintained by a continuous energy input.
Common mistakes and how to put them right
Misunderstanding: thermal equilibrium means no energy transfer at all.
Put it right: Energy is still transferred microscopically, but the net transfer is zero because the rates in each direction are equal.
Misunderstanding: thermal equilibrium requires the objects to have the same internal energy.
Put it right: Equilibrium requires equal temperatures, not equal internal energies; internal energy also depends on mass and material.
Misunderstanding: a steady-state system with a constant temperature difference is in thermal equilibrium.
Put it right: A steady state can have a constant temperature difference with a continuous net energy flow, so it is not in thermal equilibrium.
Exam tips
- Read the question carefully to distinguish between 'no net transfer' and 'no transfer at all'.
- Recall that a constant thermometer reading indicates thermal equilibrium with the object being measured.
- Eliminate options that confuse thermal equilibrium with steady-state conditions or with equal internal energies.
Try the lesson
The start of the Temperature: (a) thermal equilibrium lesson, made from checked teaching notes for this specification point.
Listen
Thermal equilibrium is the state reached when two or more objects in thermal contact have no net transfer of thermal energy between them. Their temperatures are equal, so there is no resultant heat flow. For example, a thermometer placed in a beaker of water reaches thermal equilibrium with the water when the thermometer reading becomes constant; at that point the thermometer and water are at the same temperature. Thermal equilibrium is a dynamic condition: energy may still be exchanged at a microscopic level, but the average rates of transfer in each direction are equal, so the net transfer is zero. This concept underpins the definition of temperature and the operation of thermometers. A strong answer earns marks for these points. Thermal equilibrium occurs when two objects in thermal contact have equal temperatures and no net transfer of thermal energy. At equilibrium, the average rates of energy transfer in both directions are equal, so the net energy transfer is zero. A thermometer reaches thermal equilibrium with its surroundings when its reading becomes constant. Thermal equilibrium is distinct from steady-state conditions where a temperature difference is maintained by a continuous energy input. A common slip: Misunderstanding: thermal equilibrium means no energy transfer at all. Instead, energy is still transferred microscopically, but the net transfer is zero because the rates in each direction are equal. Exam tip: Read the question carefully to distinguish between 'no net transfer' and 'no transfer at all'.
See it
Key words
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Thermal equilibrium
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Steady-state conditions
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See every definition
- Thermal equilibrium
- The state reached when two or more objects in thermal contact have no net transfer of thermal energy between them, their temperatures being equal.
- Steady-state conditions
- Conditions where a temperature difference is maintained by a continuous energy input, so a continuous net energy flow occurs.
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