Introduction to thermodynamics, First Law of thermodynamics

 Introduction to thermodynamics

Fundamental concepts:

System: it is any portion of the universe in which we are interested and the interface between a system and its surroundings is called boundary

Properties of System:

The physical properties of system may be divided into:

1.     Extensive properties:

-         They are properties which depend on the amount of substance present in the system such as mass, volume, internal energy, surface area.

-         These properties change with the quantity of matter present in the system.

2.     Intensive properties:

-          They are properties that are characteristic of the substance and independent on the amount of substance present in system such as pressure, temperature, density, viscosity.

Thermodynamic equilibrium:

-         This equilibrium divided into three different types of equilibrium:

a.     Mechanical equilibrium:

It is equilibrium that reached when no macroscopic movements within the system with respect to the surroundings.

b.     Chemical equilibrium:

For a system consist of more than one substance, if the composition does not change with time.

c.      Thermal equilibrium:

When the temperature of the system is uniform and is the same as the temperature of surroundings.

Processes of changes of state:

Change of state of a system can be made in number of ways as follow:

1.     Adiabatic process:

It is the process in which the change is carried out under such conditions that no change of heat is allowed between the system and surroundings

2.     Isothermal process:

It is the process in which the change is carried out in such a way that temperature remains constant.

3.     Isobaric process:

It is the process in which the change is carried out at constant pressure.

4.     Isochoric process:

It is the process in which the change is carried out at constant volume

State function:

-         It is the function that is independent of the path and depends only on the initial and final states of the system such as energy and entropy.

-         Functions that are dependent on the path called path functions such as work.

 

Properties of state function:

-         When energy of a system is changed from state A to state B, the change in energy  ∆E is given by

                                                              ∆E= E_B - E_A

-          dE can be written as:

                              dE = _y dx + _x dy

Where x, y are any variables of state, for example any two of P, T, and V.

The equivalence between Heat and Work:

-         Work (w): is defined as any quantity of energy that flows across the boundary of a system and its surroundings during the process of change in its state (due to mechanical link)

-         If the work is done by the system the then, it takes positive value

-         But if work is done on the system then, it takes negative value

 

-         Heat (q): is defined as any quantity of energy that flows across the boundary of a system during a change in its state due to temperature difference between the system and surroundings and flows from higher to lower temperature.

-         If the heat is added to the system the then, it takes positive value

-         But if the heat is given by the system then, it takes negative value.

 

-         Heat and work are called path functions.

First law of thermodynamics:

It states that “Energy can be transferred but never created or destroyed but converted from energy form to another energy form”

Internal energy:

-         It consists of kinetic energy, potential energy, molecular energy……

-         Internal energy is state function.

The first law of thermodynamics can be written as:

E = q - W

-     E: internal energy.

-         q: quantity of heat transferred.

-         W: work done.

That is for a system change from state A to higher energy state B through path 1 and if this system returned from B to A through path 2

Then the change in energy in the cyclic process E_{A            A} equal to the sum of E₁ and E₂

   E_{A       A} =  E₁ E₂

E₂ = - E₁

      E_{A       A} =  E₁ E₁ =0

So, the internal energy is state of function that it is not depend on the path but depends only on the initial and final states of the system.

And dE = 0

Pressure-Volume work:

Consider that we have a cylinder of cross-sectional area A contain a gas and fitted with weightless piston

P=          F = PA

F: is the total force acting on the piston

Suppose that as a result of expansion the piston moves a distance d₁

 dw = F. d₁ =PA .d₁

dw: is the work done by the gas on the surroundings

 dv =A d₁

dv: is the increase in volume due to movement of the piston

 dw = Pdv

and the total work in the expansion is obtained by

W=

             V₁: is the initial volume

             V₂: is the final volume

 W = P

= P (v₂ –v₁)

By substituting in the first law of thermodynamics

                                                   = q -

Special cases:

1.     At constant volume:

dv = 0

 = q

2.    If opposing pressure = 0

When the gas expand against zero pressure no work is done

 = q

            and this type of expansion called free expansion

3.    If opposing pressure is constant

 w = P (v₂ - v₁)

 = q - P (v₂ - v₁)

4.    If opposing pressure is variable

In this case P is variable and is function of volume.

For all calculations of work in gas expansion we have two cases

(1)             If v₂ > v₁

    P is positive value and,

    W is positive value

The positive value of work means that the system expands and the work is done by the system on the surroundings

(2)            If  v₁ > v₂

   W is negative value

The negative value of work means that the system compressed (or contracted) and the work is done by the surroundings on the system