OCR Specification focus:
‘Thermal equilibrium occurs when objects in contact have no net heat flow and equal temperature.’
Thermal equilibrium is a fundamental concept in thermodynamics, describing the condition where two or more bodies in thermal contact exchange no net heat. It underpins temperature measurement and the definition of temperature scales.
Understanding Thermal Equilibrium
When two objects of differing temperatures are brought into contact, energy is transferred as heat from the hotter object to the cooler one. This transfer continues until both reach the same temperature. At that point, no net heat flow occurs, and the system is said to be in thermal equilibrium.
Thermal Equilibrium: The condition in which two or more objects in thermal contact have no net heat transfer between them because they are at the same temperature.
The absence of net heat flow does not imply that molecular motion stops — only that the average energy transfer between the objects balances out. Individual particles continue to move randomly, but on average, there is no overall energy exchange.
Thermal Contact and Energy Transfer
For thermal equilibrium to occur, objects must be in thermal contact, allowing energy transfer by conduction, convection, or radiation.
Conduction occurs in solids through molecular vibrations or free electron movement.
Convection occurs in fluids (liquids and gases) where warmer, less dense regions move upward and cooler regions sink.
Radiation allows energy transfer through electromagnetic waves and can occur even across a vacuum.
These processes continue until thermal equilibrium is established — when temperature uniformity is achieved throughout the system.
Key Principles in Thermal Equilibrium
Direction of Heat Flow:
Heat energy always flows spontaneously from a region of higher temperature to one of lower temperature until equilibrium is reached.
Heat conduction carries energy from the hot end to the cold end of a solid, reducing the temperature difference. This process continues until both ends reach the same temperature, at which point there is no net heat flow. The clean, labelled arrow highlights the one-directional energy transfer during equilibration. Source.
Equality of Temperature: Once equilibrium is achieved, all objects in the system share the same temperature, providing a measurable physical state.
No Net Energy Exchange: Although microscopic energy exchanges still occur, the rate of energy transfer in each direction is equal.
Dependence on Physical Contact: Without thermal contact, equilibrium cannot be established, as there is no mechanism for heat exchange.
The Zeroth Law of Thermodynamics
The concept of thermal equilibrium forms the foundation of the Zeroth Law of Thermodynamics, which formalises how temperature is defined and compared.
Zeroth Law of Thermodynamics: If two systems are each in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
This law provides the logical basis for temperature measurement.
A thermometer in equilibrium with object B reads the same temperature when moved to object C if B and C are themselves in equilibrium. This illustrates the Zeroth Law of Thermodynamics: thermal equilibrium is transitive, so equal temperatures are consistent across pairings. The diagram underpins why thermometers can compare temperatures reliably. Source.
This law provides the logical basis for temperature measurement. It implies that temperature is a transitive property — if object A and object B are each in equilibrium with a thermometer (object C), then A and B are also in equilibrium with each other and must share the same temperature.
Temperature and Thermometers
The Zeroth Law allows thermometers to serve as practical tools for determining whether two systems are in thermal equilibrium. When a thermometer is placed in contact with a body:
If its reading changes, the thermometer and the body are not yet in equilibrium.
Once the reading stabilises, thermal equilibrium has been achieved, and the thermometer’s temperature equals that of the body.
Thermometers operate on the principle that certain physical properties vary predictably with temperature, such as:
Volume expansion of liquids (e.g. mercury or alcohol thermometers).
Electrical resistance in resistance thermometers.
Electromotive force in thermocouples.
These physical responses allow calibration of the thermometer on established scales (°C or K).
Microscopic View of Thermal Equilibrium
At the molecular level, thermal equilibrium is governed by the distribution of molecular energies. In any substance:
Molecules are in constant random motion.
Collisions between particles result in energy exchanges.
Over time, these exchanges lead to an even distribution of average kinetic energy, corresponding to uniform temperature.
This microscopic perspective supports the idea that temperature reflects the average kinetic energy of particles in a system. When two bodies reach thermal equilibrium, their average kinetic energies become equal.
Energy Transfer and Internal Energy
The internal energy of a system is the total of all the microscopic kinetic and potential energies of its particles. When heat is transferred between bodies, their internal energies change until equilibrium is established.
EQUATION
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Energy Flow and Equilibrium
ΔQ = 0 at thermal equilibrium
ΔQ = Heat energy transferred (J)
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This means that, at equilibrium, there is no net change in energy — any energy leaving one body is exactly balanced by the energy entering another.
Achieving Thermal Equilibrium in Systems
The process of reaching equilibrium depends on several factors:
Temperature difference: Larger differences cause faster initial heat transfer.
Thermal conductivity: Materials with high conductivity (e.g. metals) reach equilibrium more quickly.
Surface area and contact quality: Greater contact promotes faster energy exchange.
Mass and specific heat capacity: Larger or higher-capacity bodies take longer to change temperature.
These variables influence how quickly equilibrium is achieved but not the final outcome — equal temperature throughout the system.
Importance of Thermal Equilibrium in Physics
Thermal equilibrium is essential to multiple areas of physics and engineering:
It defines temperature as a measurable quantity.
It forms the basis for the absolute temperature scale (Kelvin).
It underpins concepts such as heat capacity, phase transitions, and thermodynamic processes.
It ensures that energy conservation principles apply within isolated systems.
Understanding thermal equilibrium allows physicists to model real systems accurately, from simple laboratory experiments to complex planetary atmospheres, by recognising when and how energy exchange ceases.
FAQ
The rate at which two objects reach thermal equilibrium depends on:
Temperature difference: A greater temperature difference increases the rate of heat flow.
Thermal conductivity: Materials like metals transfer heat faster than insulators.
Surface area and contact quality: Larger or smoother contact surfaces enhance conduction.
Mass and specific heat capacity: Smaller masses or lower heat capacities reach equilibrium faster.
External factors such as airflow or insulation can also influence how quickly equilibrium is achieved by altering heat loss or gain to the surroundings.
No — if two objects are at different temperatures and in thermal contact, there will always be a net heat flow from the hotter to the cooler object.
However, if they are not in thermal contact (for instance, separated by a perfect vacuum and fully insulated from radiation), no heat can be exchanged. In that case, while their temperatures differ, they cannot affect each other’s state, so the concept of “thermal equilibrium between them” doesn’t apply.
No. Thermal equilibrium refers to equal average kinetic energy per particle, not identical energies for each individual particle.
Particles in any material have a range of energies due to continuous collisions and interactions. At equilibrium, the distribution of energies remains constant over time, meaning energy exchange still occurs, but overall averages do not change.
This steady distribution is described statistically by the Maxwell–Boltzmann distribution for gases.
A thermometer must reach thermal equilibrium with the system it measures to ensure both share the same temperature.
If equilibrium hasn’t been achieved, heat will continue to flow between the thermometer and the object, causing the thermometer’s reading to change over time.
Only when the reading stabilises does it accurately represent the object’s temperature, confirming that no net heat transfer occurs.
Thermal equilibrium governs many familiar phenomena:
Room temperature stabilisation: Air and objects in a room exchange heat until they reach a common temperature.
Human body regulation: Skin and surrounding air exchange heat, seeking equilibrium; clothing slows this process.
Planetary climates: Earth’s surface and atmosphere achieve a near-equilibrium between solar energy absorbed and infrared energy emitted.
These examples show that equilibrium principles explain temperature stability and heat flow in both small-scale and large-scale systems.
Practice Questions
Question 1 (2 marks)
Two metal blocks, A and B, are placed in thermal contact. Initially, block A is at 80 °C and block B is at 20 °C. After some time, both blocks reach the same temperature of 50 °C.
(a) Explain what is meant by thermal equilibrium.
Mark Scheme:
1 mark: States that thermal equilibrium occurs when no net heat transfer takes place between bodies in contact.
1 mark: States that this is because the bodies have reached the same temperature.
Question 2 (5 marks)
A student investigates the concept of thermal equilibrium using a thermometer and two metal objects, X and Y.
The thermometer is first placed in contact with object X until the reading becomes constant. It is then placed in contact with object Y, and the reading does not change.
(a) Explain, using the Zeroth Law of Thermodynamics, what conclusion the student can draw about objects X and Y.
(b) Discuss the energy transfers that take place between the thermometer and each object until equilibrium is reached, referring to molecular motion.
(c) Explain why thermal equilibrium is essential for accurate temperature measurement.
Mark Scheme:
(a)
1 mark: States that the Zeroth Law of Thermodynamics says if two bodies are each in thermal equilibrium with a third body, they are in equilibrium with each other.
1 mark: Therefore, objects X and Y must be at the same temperature.
(b)
1 mark: Explains that heat energy flows from the hotter object to the cooler one until their average molecular kinetic energies are equal.
1 mark: Mentions that energy transfer continues until no net energy exchange occurs between the thermometer and the object.
(c)
1 mark: States that the thermometer must reach thermal equilibrium with the object to give a true reading of its temperature.
1 mark: Explains that if equilibrium is not reached, the thermometer would still be changing temperature, giving an inaccurate reading.
