NEET Physics Notes Mechanics Thermodynamics First Law of Thermodynamics
First Law of Thermodynamics
First Law of Thermodynamics
According to this law, the heat given to a system ∆Q is equal to the sum of increase in its internal energy ∆U and the work done ∆W by the system against the surroundings.
Mathematically,
∆Q = ∆U + ∆W
Sign Convention
∆Q= + ve when heat supplied
∆Q= – ve when heat is rejected
∆U = + ve when temperature increases
∆U= – ve when temperature decreases
∆W= + ve when work is done by the system (expansion)
∆W= – ve when work is done on system (compression)
First law of thermodynamics is based on the energy conservation.
Thermodynamic Processes
A thermodynamic process is the process of change of state of a system involving change of thermodynamic variables, e.g. p, V, T etc.
When a system undergo a thermodynamic change work done either by system on surrounding or by surroundings on system is called external work
Isothermal Process
It is that process in which temperature remains constant. Here, exchange of heat with the surroundings is allowed.
As per first law of thermodynamics, since, ∆T = 0, hence, ∆U = 0 for an ideal gas and we have ∆Q = ∆W. Thus, heat supplied to the system in an isothermal process is entirely used to do work against external surroundings.
Adiabatic Process
It is that process in which there is no exchange of heat of the system with its surroundings. Thus, in an adiabatic process p, V and T change but ∆Q = 0 or entropy remains constant
During an adiabatic expansion ∆W = +ve, hence, temperature of gas falls, i.e. an adiabatic expansion is always accompained with cooling.
As per first law of thermodynamics, since, ∆Q = 0 in an adiabatic process hence,
∆U = -∆W
(iv) Free expansion is an adiabatic process in which ∆W = 0. Hence, in accordance with first law of thermodynamics ∆U = 0 i.e. the final and initial values of the internal energy are equal in free expansion.
Isochoric Process
It is that thermodynamic process in which volume remains constant.
In an isochoric process for a given mass of gas
Indicator diagram for an isochoric process is a straight line parallel to p-axis.
Graph (a) shows isometric heating graph in which pressure increases, temperature increases ∆Q is positive and ∆U is positive.
Similarly, Graph (b) shows isometric cooling graph in which pressure decreases, temperature decreases, ∆Q is negative and ∆U is negative.
Work done in an isochoric
Isobaric Process
It is that process in which pressure remains constant.
As in an isobaric process for a given mass of gas.
Indicator diagram! for an isobaric process is a straight line parallel to V-axis
Graph (a) represent isobaric expansion, graph (b) represent isobaric compression.
Work done in an isobaric process
