PHYSICS UNIT VIII THERMODYNAMICS CHAPTER 1 FIRST LAW OF THERMODYNAMICS COMPLETE KNOWLEDGE

UNIT = VIII

                             CHAPTER 1

      FIRST LAW OF THERMODYNAMICS

THERMAL EQUILIBRIUM AND TEMPERATURE 

Two systems are said to be in thermal equilibrium with each other, if they are at the same  temperature

   Thus, the temperature is a property,which determines whether the two systems will be in thermal equilibrium or not. In other words, temperature is a thermodynamic property of all the systems, such that any two systems having the same temperature must be in thermal equilibrium.

ZEROTH LAW OF THERMODYNAMICS

It states that if tuo systems A and B are in thermal equilibrium with a third system C, then A and B must be in thermal equilibrium with each other. 

  Fig. 1.01 shows two systems A and B separated by an adiabatic wall (a wall which does not allow heat flow). The two systems are placed in contact witha third system C with a Diathermic wall (a wall which permits heat flow) in between. Suppose that the system A, B and C are at different temperatures.Obviously,the three systems will not be in thermal equilibrium with one another. However, the systems A and C;and the systems Band C will exchange heat and after certain time, they will attain thermal equilibrium separately. If the adiabatic wall between the systems A and Bis removed at that time, it will be found that there is no exchange of heat between the systems A and B. Therefore, the systems A and B also acquire thermal equilibrium,when the systems A and Bare allowed to attain thermal equilibrium separately with the system C. 

A Few Definition

Thermodynamic system.= An assembly of extremely large number of particles having a certain value of pressure, volume and temperature is called a thermodynamic system. For example, a large collection of gas molecules is a thermodynamic system. 

Thermodynamic variables= The variables which determine the thermodynamic behaviour of a system are called thermodynamic variables. The quantities like pressure P), volume (V) and temperature (T) are thermodynamic variables. There are some other thermodynamic variables, such as internal energy (U), entropy (S), etc. All other thermodynamic variables can be expressed in terms of P, V and T. 

Equation of state.= A relation between pressure, volume and temperature for a System is called its equation of state. The state of a system is completely known in terms of its pressure, volume and temperature. 

ror example, for 1 mole of an ideal gas, the equation of state is 

   Pv = RT

In a simple system, such as a gas contained in a cylinder, any two variables out the three variables P, V and T determine the state of the system. The third variable can be known by using the equation of state

Thermodynamic process= A thermodynamic process is said to be taking place, if the thermodynamic variables of the system change with time. 

In practice, the following types of thermodynamic processes can take place. 

Isothermal process.= A thermodynamic process that takes place at constant temperature is called isothermal process. 

Isobaric process. = A thermodynamic process that takes place at constant pressure is called isobaric process. 

Isochoric process = A thermodynamic process that takes place at constant volume  is called isochoric process. 

INTERNAL ENERGY OFA GAS 

According to the kinetic theory of gases, the molecules of a gas do not attract each other and such a gas is called an ideal or a perfect gas. However, it is not tru in actual practice. Experiments show that the molecules of a gas exert mutual force of attraction on one another. Such a gas is called a real gas. We know that whenever two bodies are present at a finite distance apart, they possess potential energy Therefore, the molecules of a real gas possess intermolecular potential energy. If the volume of the gas increases, work will be performed by the gas against the intermolecular attraction and hence the potential energy of the gas will increase. On the other hand, to compress a gas, work has to be done on the gas against the intermolecular forces and hence the potential energy of the gas will decrease. 

Therefore, intermolecular potential energy of a real gas is function of its volume.

P- V DIAGRAM

The The perfect gas equation or the equation of state is a mathematical relation between the three thermodynamic variables P, V and T. To know and describe the system, the knowledge of only two thermodynamic variables (out of P, V and T) is sufficient. It is because, the third variable can be calculated from the equation of state of the system. 

 

from the state A (P,, V) to B (P2, V,). On the other hand, Fig. 1.02 (6) represents the indicator diagram for the system undergoing compression. The internmediate states through which the system passes between the initial state A and the final state B are represented by the points on the curve AB. 

Importance of P-V diagram. It can be mathematically proved that the work done by a system or on the system is numnerically equal to the area under the P-V diagram. Durning expansion, the work is done by the system and the area ABB'A'under thee Tdiagram is traced in clockwise direction. On the other hand, during compression, rorkis doneon the system and the areaABB A' under the P-V diagram is traced nticlockwise direction. since work done by a system is taken as positive and work anticlockwis ne on the system as negative, it follows thatif areaunder P-V diagram is traced in lckwise direction, then work done will be positive and it will be negative, if the area is tracedin anticlockwise direction. 

WORK DONE DURING EXPANSION 

Consider gas enclosed in a cylinder provided with a frictionless and weightless n.Suppose that the gas undergoes expansion from the initial state A tesponding to pressure Pand volume V, to the final state B, when the pressure Volume become P, and V, respectively. Let us calculate the work done by the gas. 

TWO SPECIFIC HEATS OF A GAS 

The specific heat of a substance is defined as the amount of heat required to raise the temperature of its unit mass through one degree centigrade (or kelvin). 

The above definition serves well for solids and liquids. It is because, when heat is supplied to a solid or a liquid, it goes into it to produce an increase in the temperature only without causing any practical change in its volume or pressure. Accordingly, a solid or a liquid undergoes a definite rise in temperature, when supplied a given amount of heat. However, it is not true with gases. When a gas is heated, the volume and pressure may also change with increase in temperature. Hence, a gas can have any value of specific heat depending upon the condition under which it is heated. So that specific heat of a gas has a definite meaning, it is defined in the following two ways: 

Specific heat of a gas at constant volume= It is defined as the amount of heat required to raise the temperature of 1 g of a gas through 1°Cat constant volume. It is denoted by C 

Molar specific heat at constant volume. = It is defined as the amount of heat required to raise the temperature of 1 mole of a gas through 1°C at constant volume. It is denoted by C

 Specific heat of a gas at constant pressure. = It is defined as the amount of heat required to raise the temperature of 1 g of a gas through 1°C at constant pressure. It is denoted by Cp 

Molar specific heat of a gas at constant pressure. = It is defined as the amount of heat required to raise the temperature of 1 mole of a gas through 1°C at constant pressure. It is denoted by C Obviously

ISOTHERMAL PROCESS 

An isothermal process is one, in which the pressure and volume of the system change hut temperature remains constant. 

carry out isothermal process, a perfect gas is contained ina cylinder having otducting walls and the gas is compressed or allowed to expand very slowly. 

When the gas expands, cooling will occur due to the absorption of heat by the as. But as the gas is allowed to expand slowly; therefore before the temperature of the gas falls, heat is conducted into the cylinder through the conducting walls from the surroundings. Hence, when a gas is allowed to expand slowly, the temperature of the gas will remain unchanged. 

On the other hand, when the gas is compressed, heat will be produced. Since it is compressed slowly, the heat developed in the gas leaves to the surroundings through the conducting walls before it could increase the temperature of the gas. Hence, if the gas is compressed slowly, the temperature of the gas will remain unchanged. 

the cylinder should have conducting walls and  

the gas should be compressed or allowed to expand very slowly. 

ADIABATIC PROCESS

An adiabatic process is one, in which pressure, volume and temperature of the system hamOC but there is no exchange of heat between the system and the surroundings.

For carrying out adiabatic process, a perfect gas is contained in a cylinder having insulating walls and the gas is ALLLOWED to expand or compressed very quickly. 

When the gas expands, cooling will occur due to absorption of heat by the gas. Since the process takes place quickly and the walls are insulating, no heat can enter the system from surroundingS. As a result, the temperature of the gas will fall, when it is allowed to expand adiabatically. 

On the other hand, when the gas is compressed quickly, the heat produced cannot escape to the surroundings through the insulating walls. As a result, the temperature of the gas will increase during an adiabatic compression. 

the cylinder should have insulating walls and 

the gas should be compressed or allowed to expand very quickly. 

Do you need a any help? 

Please Comment Now.