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Study Notes for Class 11-Thermodynamics Concepts

1. Thermal Equilibrium

Thermal equilibrium is a state in which two or more bodies in thermal contact with each other cease to exchange heat energy, resulting in no temperature change over time. This occurs when the bodies reach the same temperature. Understanding thermal equilibrium is crucial as it forms the basis of the Zeroth Law of Thermodynamics and other concepts in thermodynamics.



 

Example: Consider two objects, one hot and the other cold, placed in contact. Initially, heat flows from the hot object to the cold one. Over time, the temperature of the hot object decreases, while the temperature of the cold object increases until they reach the same temperature. At this point, the objects are said to be in thermal equilibrium.

 

Key Concept: If two systems are individually in thermal equilibrium with a third system, they are in thermal equilibrium with each other. This forms the basis for temperature measurement, as all systems in thermal equilibrium must have the same temperature.

 

2. Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This law is fundamental because it allows us to define temperature as a measurable and comparable quantity.

 

Importance: The Zeroth Law enables the establishment of temperature scales. For example, if system A is in thermal equilibrium with system C and system B is also in thermal equilibrium with system C, then systems A and B must have the same temperature.

 

Temperature Measurement: The Zeroth Law is the principle behind the use of thermometers. A thermometer reaches thermal equilibrium with the body whose temperature is being measured, thus giving an accurate temperature reading.

 

Practical Example: Consider three containers of water, each at different initial temperatures. If the temperature of container A is equal to that of container B, and the temperature of container B is equal to that of container C, then by the Zeroth Law, the temperature of container A is also equal to that of container C.

 

3. Heat, Internal Energy, and Work

Thermodynamics involves the study of energy transfer, primarily in the form of heat and work, and how these affect the internal energy of a system.

 

Heat: Heat is a form of energy transfer between systems or bodies due to a temperature difference. It always flows from a body at a higher temperature to one at a lower temperature until thermal equilibrium is achieved.

 

Units: The SI unit of heat is the joule (J), but it is also commonly measured in calories (1 calorie = 4.184 joules).

 

Example: When you place a hot cup of coffee on a table, the heat from the coffee transfers to the surrounding air until the coffee cools down to room temperature.


Read Also: Electric Current and Electric Currents in Conductors

 

Internal Energy: Internal energy is the total energy contained within a system, arising from the kinetic and potential energies of the molecules. It depends on factors like temperature, pressure, and volume.

 

Change in Internal Energy: When a system undergoes a change due to heat added or work done on or by the system, its internal energy changes. The change in internal energy (ΔU) is a key concept in understanding thermodynamic processes.

 

Example: In a gas, the internal energy increases when heat is added, causing the gas molecules to move faster, thereby increasing the temperature.

 

Work: Work in thermodynamics refers to energy transfer when a force is applied over a distance. In a thermodynamic system, work is done when the system changes its volume under pressure.

 

Formula: W = P ΔV, where \(P\) is the pressure and ΔV is the change in volume.

 

Example: When gas in a piston expands, it does work on the piston, pushing it upwards. The work done by the gas is equal to the force exerted on the piston multiplied by the distance it moves.

 

4. First Law of Thermodynamics

The First Law of Thermodynamics is a statement of the conservation of energy. It asserts that the change in the internal energy of a closed system is equal to the heat added to the system minus the work done by the system.

 

Mathematical Expression: ΔU= Q - W, where:

ΔU is the change in internal energy.

Q is the heat added to the system.

W is the work done by the system.

 

Explanation: This law implies that energy cannot be created or destroyed, only transformed from one form to another. The internal energy of a system increases if heat is added or work is done on the system, and it decreases if the system does work or loses heat.

 

Applications:

Adiabatic Process: In an adiabatic process, no heat is exchanged (Q = 0), so the change in internal energy is equal to the negative of the work done ΔU = -W.

 

Isothermal Process: In an isothermal process, the temperature remains constant, so the internal energy does not change ΔU= 0. Therefore, the heat added to the system equals the work done by the system Q = W.

 

Isochoric Process: In an isochoric process, the volume remains constant, so no work is done W = 0. The change in internal energy is equal to the heat added ΔU = Q.

 

Isobaric Process: In an isobaric process, the pressure remains constant. The change in internal energy is equal to the heat added minus the work done ΔU = Q - W.

 

Real-Life Example: Consider a gas in a cylinder with a movable piston. If heat is added to the gas, it will expand, pushing the piston upwards. The gas does work on the piston, and its internal energy increases according to the First Law of Thermodynamics.

 

Efficiency of Heat Engines: The First Law is also fundamental in understanding the efficiency of heat engines, which convert heat energy into mechanical work. The efficiency of an engine is the ratio of the work output to the heat input.

 

Conclusion:

Thermal equilibrium, the Zeroth Law of Thermodynamics, and the concepts of heat, internal energy, and work are foundational principles in thermodynamics. The First Law of Thermodynamics encapsulates the conservation of energy, showing how energy transitions between different forms within a system. These concepts are essential for understanding more complex thermodynamic processes and have wide-ranging applications in various fields, including engineering, physics, and environmental science. By mastering these principles, students can gain a deeper insight into the behavior of physical systems and the laws governing energy transfer and transformation.

 

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