For a diatomic gas, $C_{V} =\frac{5}{2}R ;C_{P} =\frac{7}{2}R $ $\Delta Q = n C_{P} \Delta T = n\left(\frac{7}{2}R\right)\Delta T$ $\Delta U = n C_{V} \Delta T = n\left(\frac{5}{2}R\right)\Delta T$ According to first law of thermodynamics, $\Delta W = \Delta Q - \Delta U = n R \Delta T$ $\therefore \:\:\: \Delta Q : \Delta U : \Delta W = \frac{7}{2} : \frac{5}{2} : 1 $ or $7 : 5 : 2.$
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We know that the process when the pressure remains the same is known as isobaric process. So for the Isobaric process, Change in heat energy is, ΔQ = nCpΔT (where Cp is molar heat capacity at a constant pressure). Change in the internal energy is, ΔU = nCvΔT (where Cv is molar heat capacity at a constant pressure) So, from the thermodynamics law, we know, Q = ΔU + W Where W is work, ΔU is internal energy, and Q is heat. Work done by the closed system is defined as: W = ∫pdV Where, Δ means change over the whole process, whereas the d denotes a differential. As the pressure is constant, this means that W = PΔV. After applying the ideal gas law, this becomes, W = nRΔT Where R represents the gas constant and the n represents the number of moles. W = PΔV = nRΔT Therefore, ΔQ : ΔU : ΔW = Cp : Cv : R So for the diatomic gas, number of degrees of freedom is f = 5 So now we get, Cv = \(\frac{fR}{2}\) = \(\frac{5R}{2}\) Also, Cp=Cv+R ⇒ Cp = \(\frac{5R}{2}\)+R ⇒ Cp = \(\frac{7R}{2}\) Hence now we have, Cp = \(\frac{7R}{2}\), Cv = \(\frac{5R}{2}\) Thus, the ratio is: ΔQ:ΔU:W = \(\frac{7R}{2}\) : \(\frac{5R}{2}\) : R ∴ΔQ : ΔU : W = 7 : 05 : 02 Therefore, the correct answer is option ‘B’
Thermodynamics in physics is a branch that deals with heat, work and temperature, and their relation to energy, radiation and physical properties of matter.
Important Terms
System
A thermodynamic system is a specific portion of matter with a definite boundary on which our attention is focused. The system boundary may be real or imaginary, fixed or deformable.
There are three types of systems:
Isolated System – An isolated system cannot exchange both energy and mass with its surroundings. The universe is considered an isolated system.
Closed System – Across the boundary of the closed system, the transfer of energy takes place but the transfer of mass doesn’t take place. Refrigerators and compression of gas in the piston-cylinder assembly are examples of closed systems.
Open System – In an open system, the mass and energy both may be transferred between the system and surroundings. A steam turbine is an example of an open system.
Thermodynamic Process
A system undergoes a thermodynamic process when there is some energetic change within the system that is associated with changes in pressure, volume and internal energy.
There are four types of thermodynamic process that have their unique properties, and they are:
Adiabatic Process – A process in which no heat transfer takes place.
Isochoric Process – A thermodynamic process taking place at constant volume is known as the isochoric process.
Isobaric Process – A process in which no change in pressure occurs.
Isothermal Process – A process in which no change in temperature occurs.
Laws of Thermodynamics
Zeroth Law of Thermodynamics
The Zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a separate third body, then the first two bodies are also in thermal equilibrium with each other.
First Law of Thermodynamics
The First law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing three kinds of transfer of energy, as heat, as thermodynamic work, and as energy associated with matter transfer, and relating them to a function of a body's state, called internal energy.
Third law of thermodynamics states, regarding the properties of closed systems in thermodynamic equilibrium: The entropy of a system approaches a constant value when its temperature approaches absolute zero.
