Thermal Equilibrium

Thermal Equilibrium

Middle School

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Learning Objectives

• Define thermal equilibrium and its connection to the temperature of an object.

• Explain the difference between diathermic walls and adiabatic walls in transferring heat.

• State the Zeroth Law of Thermodynamics and understand why it is important.

• Describe how a thermometer works using the concept of thermal equilibrium.

Key Vocabulary

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Thermal Equilibrium

This is the condition where no net heat flows between two systems in thermal contact.

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Diathermic Wall

A boundary that allows heat to pass through, enabling systems to reach the same temperature.

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Adiabatic Wall

An insulating boundary that prevents any heat transfer between the systems it separates.

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Zeroth Law

If two systems are in thermal equilibrium with a third, they are in equilibrium with each other.

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Thermodynamic Equilibrium

A state where a system has achieved mechanical, thermal, and chemical equilibrium simultaneously.

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Thermal Contact and Walls

Diathermic Wall

• ​A diathermic wall is a boundary that allows heat to flow through.

• ​​It does not allow matter to pass from one system to the other.

• ​An example is a metal wall between two containers filled with gas.

Adiabatic Wall

• ​An adiabatic wall acts as a perfect insulator, blocking the flow of heat.

• ​​Unlike diathermic walls, it does not allow any heat to be transferred through it.

• ​A thermos flask uses this type of wall to keep its contents hot or cold.

Multiple Choice

If you want two objects at different temperatures to reach thermal equilibrium, what kind of wall should separate them?

• A rigid wall

• A diathermic wall

• An adiabatic wall

• A flexible wall

​Achieving Thermal Equilibrium

• ​Heat naturally flows from a hotter object to a cooler one.

• The heat transfer continues until temperatures are equal.

• At equilibrium, heat stops flowing between the two objects.

Multiple Choice

What is the main condition required for two systems in thermal contact to be in thermal equilibrium?

• They must be at the same temperature.

• They must have ongoing chemical reactions.

• They must have the same volume.

• They must be separated by an adiabatic wall.

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The Zeroth Law of Thermodynamics

• This law provides the scientific basis for accurately measuring temperature.

• If object A is in thermal equilibrium with object C...

• ...and object B is also in thermal equilibrium with object C...

• ...then objects A and B are in equilibrium with each other.

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Multiple Choice

According to the Zeroth Law, if a thermometer (System C) reads the same temperature for a cup of water (System A) and a block of wood (System B), what can be concluded?

• System A and System B are in thermal equilibrium with each other.

• System A and B can never reach thermal equilibrium.

• System A is hotter than System B.

• System C has transferred all its heat to A and B.

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How Thermometers Work

• A thermometer’s function is a direct application of the Zeroth Law.

• Heat flows between the thermometer and an object until thermal equilibrium is reached.

• At this point, the net heat flow stops and temperatures are equal.

• A thermometric property, like expanding liquid, shows the final temperature reading.

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Multiple Choice

Why does a mercury thermometer give a stable reading after being in your mouth for a few minutes?

• The thermometer has reached thermal equilibrium with your body.

• The mercury has absorbed all the heat from your body.

• The mercury needs time to chemically react to your body's temperature.

• The glass of the thermometer stops the mercury from expanding further.

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Common Misconceptions about Heat

Misconception	Correction
‘Cold’ flows from a cold object to a hot one.	Heat energy flows from a warmer object to a cooler one.
Objects stop interacting at the same temperature.	Energy exchange continues, but the net flow of heat is zero.
A thermos destroys heat or creates cold.	A thermos uses insulation to slow the rate of heat transfer.

Multiple Choice

What is the primary reason that a metal partition between two gas containers at different temperatures is considered a diathermic wall?

• Because it insulates the containers from the outside environment.

• Because it is rigid and prevents the gases from mixing.

• Because it is a good conductor that allows heat to flow between the containers.

• Because it allows both matter and heat to be exchanged freely.

Multiple Choice

According to the Zeroth Law of Thermodynamics, what fundamental property determines whether heat will flow between two systems?

• Their total volume.

• Their chemical composition.

• Their temperature.

• Their internal pressure.

Multiple Choice

A block of iron (A) is found to be in thermal equilibrium with a block of copper (C). The same block of copper (C) is then found to be in thermal equilibrium with a block of aluminum (B). What can you predict about the state of blocks A and B if they are brought into contact?

• Heat will flow from A to B because iron is denser.

• Heat will flow from B to A because aluminum is a better conductor.

• No net heat will flow between A and B.

• It's impossible to predict without knowing their initial temperatures.

Multiple Choice

A sealed tank contains two sections, one with hot gas and one with cold gas, separated by a perfect insulator. The entire tank has uniform pressure. Is the system in thermodynamic equilibrium? Why or why not?

• No, because the gases might react with each other (no chemical equilibrium).

• No, because the temperatures are not uniform (no thermal equilibrium).

• Yes, because the insulator prevents any further changes.

• Yes, because there is no pressure difference (mechanical equilibrium).

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Summary

• Heat is energy that flows from a hotter object to a cooler one.

• Objects in thermal contact reach equilibrium at the same temperature.

• The Zeroth Law of Thermodynamics is the principle that allows thermometers to work.

• Diathermic walls allow heat to pass, while adiabatic walls block it.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 3

• 4