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Thermodynamics – Definition, Laws, Equilibrium, Sample Problems

  • Last Updated : 05 Aug, 2021

Energy transferred from one body to another body due to temperature differences is known as heat. Heat can be produced by doing mechanical work. For example, when we rub our hands, they become warm. It is because work done to move one hand over the other hand is converted into heat. This beat warms our hands. In the 19th century. Count Rumford and James Prescott Joule studied the relationship between work and beat le found that the same amount of work done always produces the same amount of heat.

What is Thermodynamics?

The study of the relationship between mechanical work, heat, and other forms of energy and energy transfer is known as thermodynamics. 

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Thus, thermodynamics is the study of heat, temperature, and the conversion of mechanical work into heat and vice-versa In thermodynamics, the main focus is on the macroscopic quantities of the system such as pressure, volume, temperature. mal energy entropy, enthalpy, etc, which have an impact on the internal state of the system. Thus, thermodynamics provides a macroscopic description of the system.



Thermodynamic Systems

A collection of an extremely large number of atoms or molecules confined within certain boundaries such that it has certain values of pressure (P), volume (V) and temperature (T) is called a thermodynamic system. 

Anything outside the thermodynamic system to which energy or matter is exchanged is called its surroundings. Taking into consideration the interaction between a system and its surroundings, a system said to be an open system if it can exchange both energy and matter with its surroundings may be divided into three classes 

  1. Open system: A system is said to be an open system if it can exchange energy and matter with its surroundings. See figure 1(A).
  2. Closed system: A system is said to be a closed system if it can exchange only energy (not matter) with its surroundings. See figure 1(B).
  3. Isolated system:  A system is said to be isolated if it can neither exchange energy nor matter with its surroundings. See figure 1(C).

Figure 1

Thermodynamic Variables 

A thermodynamic system can be described by specifying its pressure, volume, temperature, internal energy, enthalpy, and the number of moles. These parameters or variables are called thermodynamic variables. 

Thus, the variables which are required to specify the state of the thermodynamic system are called thermodynamic variables. Entropy is a measurement of energy present in a system or process but is not available to do work. It is also defined as the measure of disorder in the system. Enthalpy is the measurement of the total energy of a thermodynamic system.

Types of thermodynamic variables

  1. Intensive variables: The variables which are independent of the size of the system are called intensive variables. e.g. Temperature, pressure, and specific beat capacity. 
  2. Extensive variables: The variables which depend on the size or mass of the system are called extensive variables. e.g. Volume, energy, entropy, heat capacity, and enthalpy.

Thermal Equilibrium 

When two bodies having different temperatures are placed in contact, then the energy flows from a body at a higher temperature to a body at a lower temperature. The flow of energy continues from one body to another to attain the same temperature. When both the bodies in contact have the same temperature and there is no energy for the body till both bodies are between them, then these bodies are in thermal equilibrium. Thus, two bodies or systems in contact are said to be in thermal equilibrium if both are at the same temperature. 

  • A thermodynamic system is said to be in thermodynamic equilibrium if its variables like pressure, volume, temperature, number of particles, etc. do not change with time. 
  • Any isolated system is in thermodynamic equilibrium.

Thermodynamic Processes

Any process in which the thermodynamic variables of a thermodynamic system change is known as the thermodynamic process.

  • Quasi-Static Process (Quasi-Static means nearly static): A process in which the system departs only infinitesimally from the equilibrium state is known as quasi-static process In this process, the change in pressure or change in volume or change in temperature of the system is very very small.
  • Isothermal Process: A process in which pressure and volume of the system change at constant temperature are called the isothermal process. In this case, P and V change but T constant. i.e. dT (change in temperature) = 0. 
  • Adiabatic Process: A process in which pressure, volume, and temperature of the system change, but there is no exchange of heat between the and its surroundings is called the adiabatic process. In this case, P. V and T change but Q = 0. The system should be compressed or allowed to expand suddenly so that there is no time for the exchange of heat between the system and its surroundings. Since these two conditions are not fully realized in practice, so no process is perfectly adiabatic.
  • Isochoric Process:  A thermodynamic process that takes place at constant volume is called the isochoric process. It is also known as the isovolumic process. In this case, dV = 0.
  • Isobaric process: A thermodynamic process that takes place at constant pressure is called the isobaric process. In this case, dP = 0.
  • Cyclic process: A cyclic process consists of a series of changes that return the system to its initial state.

Sample Problems

Problem 1: Define Isolated system.



Solution:

A system is said to be isolated if it can neither exchange energy nor matter with its surroundings.

Problem 2: What do you mean by thermodynamic variables?

Solution: 

A thermodynamic system can be described by specifying its pressure, volume, temperature, internal energy, enthalpy and the number of moles. These parameters or variables are called thermodynamic variables. Thus, the variables which are required to specify the state of thermodynamic system are called thermodynamic variables. 

Problem 3: What do you mean by the adiabatic process?

Solution:  

A process in which pressure, volume and temperature of the system change, but there is no exchange of heat between the and its surroundings is called adiabatic process. In this case, P. V and T change but Q =0.

Problem 4: Define extensive and intensive variables.

Solution: 



Intensive variables: The variables which are independent of the size of the system are called intensive variables. e.g. Temperature, pressure and specific beat capacity.

Extensive variables: The variables which depend on the size or mass of the system are called extensive variable. e.g. Volume, energy, entropy, heat capacity and enthalpy.

Problem 5: What do you mean by Quasi-Static Process?

Solution: 

A process in which the system departs only infinitesimally from the equilibrium state is known as quasi-static process In this process, the change in pressure or change in volume or change in temperature of the system is very very small .

Problem 6: Can water be boiled without heating?

Solution: 

Yes, water can be boiled inside a closed insulated vessel by increasing the pressure on the surface of water.




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