Chapter 8: Problem 1145
If \(r\) denotes the ratio of adiabatic of two specific heats of a gas. Then what is the ratio of slope of an adiabatic and isothermal \(\mathrm{P} \rightarrow \mathrm{V}\) curves at their point of intersection ? (A) \((1 / \gamma)\) (B) \(\gamma-1\) (C) \(\gamma\) (D) \(\gamma+1\)
Short Answer
Expert verified
The ratio of the slopes of adiabatic and isothermal P-V curves at their point of intersection is \(\gamma\).
Step by step solution
01
Write down the equations of adiabatic and isothermal P-V curves
For an adiabatic process, we have the equation:
\(PV^{\gamma}=K_{1}\), where K1 is a constant
For an isothermal process, we have the equation:
\(PV = K_{2}\), where K2 is a constant
02
Differentiate the equations with respect to V
Now we need to find the slope of both curves at any point, so we will differentiate them with respect to V:
For adiabatic: \(\frac{dP}{dV} = -\frac{\gamma K_{1}}{V^{\gamma+1}}\)
For isothermal: \(\frac{dP}{dV}=-\frac{K_{2}}{V^{2}}\)
03
Calculate the ratio of slopes
Now, let's find the ratio of slopes of adiabatic and isothermal processes:
\(\frac{dP_{ad}/dV}{dP_{iso}/dV} = \frac{-\frac{\gamma K_{1}}{V^{\gamma+1}}}{-\frac{K_{2}}{V^{2}}}\)
04
Simplify the ratio
Simplifying the ratio, we get:
\(\frac{dP_{ad}/dV}{dP_{iso}/dV}=\frac{\gamma K_{1}V^{2}}{K_{2}V^{\gamma+1}}\)
At the point of intersection of both curves, \(K_{1} = K_{2}\) and \(PV^{\gamma}=PV\) which gives us:
\(V^{\gamma-1}=1\)
This implies that:
\(V^{\gamma+1}=V^{2}\)
Now substituting this value in the ratio:
\(\frac{dP_{ad}/dV}{dP_{iso}/dV}=\frac{\gamma K_{1}V^{\gamma+1}}{K_{2}V^{\gamma+1}}\)
\(\Rightarrow \frac{dP_{ad}/dV}{dP_{iso}/dV}=\frac{\gamma K_{1}}{K_{2}}\)
Since, at the point of intersection, \(K_{1}=K_{2}\), our final answer becomes:
\(\frac{dP_{ad}/dV}{dP_{iso}/dV}=\gamma\)
The correct option is (C) \(\gamma\).
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Specific Heat Ratio
The specific heat ratio, often denoted as \( \gamma \), is a crucial factor in the study of thermodynamics as it relates to gases. It is defined as the ratio of the specific heat at constant pressure \( C_p \) to the specific heat at constant volume \( C_v \). Understanding \( \gamma \) is essential in analyzing processes such as adiabatic and isothermal, as it tells us how a gas behaves when heat is added or removed.
- In an adiabatic process, no heat is transferred to or from the system, which means that the specific heat ratio directly influences how the pressure and volume change.
- The value of \( \gamma \) is greater than 1 for gases, with typical values ranging for air around 1.4, indicating that gases expand more at constant pressure compared to constant volume.
Slope of Curves
In thermodynamics, the slope of a curve tells us about the rate of change of one variable with respect to another. For P-V (pressure-volume) curves, understanding the slope is critical for both adiabatic and isothermal processes.
- In an adiabatic process, the slope is derived from the equation \( PV^{\gamma} = K_1 \). Differentiating gives the slope \( \frac{dP}{dV} = -\frac{\gamma K_1}{V^{\gamma+1}} \), showing dependence on both \( \gamma \) and the volume \( V \).
- For an isothermal process described by \( PV = K_2 \), the slope is simpler, \( \frac{dP}{dV} = -\frac{K_2}{V^2} \), indicating a direct relation to \( V \).
Differentiation of Equations
Differentiation is used to find slopes of curves representing physical processes, such as the pressure-volume relationships in gas laws like the adiabatic and isothermal equations. Differentiating these equations provides the rate at which pressure changes with respect to volume.
- For the adiabatic equation \( PV^{\gamma} = K_1 \), differentiation with respect to \( V \) yields \( \frac{dP}{dV} = -\frac{\gamma K_1}{V^{\gamma+1}} \).
- For the isothermal equation, \( PV = K_2 \), differentiation gives \( \frac{dP}{dV} = -\frac{K_2}{V^2} \).
Point of Intersection
The point of intersection between adiabatic and isothermal curves is a critical concept when comparing processes involving gases. At this intersection, both curves satisfy their respective equations equally for a given pressure and volume.
- At the intersection, \( PV^{\gamma} = K_1 = K_2 = PV \).
- This point conditionally implies \( V^{\gamma-1} = 1 \), simplifying to \( V^{\gamma+1} = V^2 \), which allows for simplification in comparing slopes.
P-V Curves
P-V curves, or pressure-volume curves, are graphical representations of the relationships between pressure (P) and volume (V) for a gas under different processes. These curves are foundational to understanding thermodynamic processes like adiabatic and isothermal.
- An adiabatic P-V curve does not transfer heat and is steeper due to the energy conservation equation \( PV^{\gamma} = K_1 \).
- An isothermal P-V curve represents constant temperature processes with the equation \( PV = K_2 \), indicating a hyperbolic curve behavior.