Two systems are in thermal equilibrium when they have the same temperature and no net heat flows between them. Thermodynamics describes a chosen system, its surroundings and the boundary separating them.
Learning Outcomes
Heat flows spontaneously from higher to lower temperature.
Equal temperature gives thermal equilibrium and zero net heat flow.
A diathermic boundary allows heat transfer.
Open systems exchange mass and energy.
Closed systems exchange energy but not mass.
Isolated systems exchange neither mass nor heat.
Intermediate Level
Heat and Temperature
Quantity
Meaning
SI unit
Heat
energy transferred because of temperature difference
J
Temperature
degree of hotness and measure related to average molecular kinetic energy
K
Thermal Contact
When bodies at different temperatures are connected through a diathermic substance, heat flows from the hotter body to the colder body until their temperatures become equal.
Key equation
T1=T2⇒ Qnet=0
System, Surroundings and Boundary
Term
Meaning
System
matter or region being studied
Surroundings
everything outside the system
Boundary
surface separating system and surroundings
Types of System
System
Mass exchange
Energy exchange
Example
Open
yes
yes
open beaker
Closed
no
yes
sealed container that can be heated
Isolated
no
no
ideal thermos flask
Everyday Example
A hot metal block in cooler water transfers heat to the water. Equilibrium is reached when both have the same temperature, although their internal energies need not be equal.
Common Mistakes
Use kelvin in gas and molecular-energy equations.
Distinguish heat, temperature and internal energy.
Convert all quantities to SI units before calculation.
Short-Question Preparation
Q. Define thermal equilibrium.
A. The state in which systems have the same temperature and no net heat transfer occurs.
Q. What is a diathermic substance?
A. A substance or boundary that allows heat to pass through.
Q. Differentiate open, closed and isolated systems.
A. Open exchanges mass and energy; closed exchanges energy only; isolated exchanges neither.
Sticky Notes
Heat flows spontaneously from higher to lower temperature.
Equal temperature gives thermal equilibrium and zero net heat flow.
A diathermic boundary allows heat transfer.
Open systems exchange mass and energy.
Closed systems exchange energy but not mass.
Isolated systems exchange neither mass nor heat.
60-Second Revision
Two systems are in thermal equilibrium when they have the same temperature and no net heat flows between them. Thermodynamics describes a chosen system, its surroundings and the boundary separating them.
Summary
Two systems are in thermal equilibrium when they have the same temperature and no net heat flows between them. Thermodynamics describes a chosen system, its surroundings and the boundary separating them.
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Exam preparation
Exam Focus
Important points
Heat flows spontaneously from higher to lower temperature.
Equal temperature gives thermal equilibrium and zero net heat flow.
A diathermic boundary allows heat transfer.
Open systems exchange mass and energy.
Closed systems exchange energy but not mass.
Isolated systems exchange neither mass nor heat.
Common mistakes
Keep heat, temperature and internal energy distinct.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
Thermal Equilibrium and Thermodynamic Systems
Sticky Notes · Quick Revision
1.
Key Point 1
Heat flows spontaneously from higher to lower temperature.
2.
Key Point 2
Equal temperature gives thermal equilibrium and zero net heat flow.
3.
Key Point 3
A diathermic boundary allows heat transfer.
4.
Key Point 4
Open systems exchange mass and energy.
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6 concept questions
Easy → Medium → Hard
Explanation after each answer
Exam Practice
Important Question
Q1. Define thermal equilibrium.
Important Question
Q2. What is a diathermic substance?
Important Question
Q3. Differentiate open, closed and isolated systems.
