Chapter 6: Problem 8
and volume, \(V\), of a gas in a cylinder are related by \(P V^{n}=C\) (where \(C\) is a constant) Show that $$ \frac{\mathrm{d}^{2} P}{\mathrm{~d} V^{2}}=\frac{n(n+1) P}{V^{2}} $$
Short Answer
Expert verified
\(\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = \frac{n(n+1) P}{V^2} \)
Step by step solution
01
- Express Pressure as a Function of Volume
Starting from the given equation, express pressure, \(P\), as a function of volume, \(V\):\[P V^n = C\] Solving for \(P\), one gets: \[P = \frac{C}{V^n}\]
02
- Differentiate Pressure with Respect to Volume
Differentiate \(P = \frac{C}{V^n}\) with respect to \(V\):\[\frac{\mathrm{d} P}{\mathrm{d} V} = \frac{\mathrm{d}}{\mathrm{d} V} \left( \frac{C}{V^n} \right)\]Using the power rule for derivatives: \[\frac{\mathrm{d} P}{\mathrm{d} V} = C \cdot \frac{-n}{V^{n+1}}\]Since \(C = PV^n\), substitute \(PC = V^n\) back in the equation: \[\frac{\mathrm{d} P}{\mathrm{d} V} = -n \left( \frac{P}{V} \right)\]
03
- Differentiate Again with Respect to Volume
Differentiate \(\frac{\mathrm{d} P}{\mathrm{d} V} = -n \left( \frac{P}{V} \right)\) with respect to \(V\):\[\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = \frac{\mathrm{d}}{\mathrm{d} V} \left( -n \frac{P}{V} \right)\]Using the product rule for differentiation: \[\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = -n \left( \frac{\mathrm{d} P}{\mathrm{d} V} \cdot \frac{1}{V} + P \cdot \frac{-1}{V^2} \right)\]
04
- Substitute \( \frac{\mathrm{d} P}{\mathrm{d} V} \)
Substitute \(\frac{\mathrm{d} P}{\mathrm{d} V} = -n \left( \frac{P}{V} \right)\) back into the equation: \[\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = -n \left( -n \frac{P}{V^2} - \frac{P}{V^2} \right)\]Simplify: \[\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = -n \left( \frac{P}{V^2} (n+1) \right) \]This further simplifies to: \[\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = \frac{n(n+1) P}{V^2}\]
05
- Conclusion
We have shown that \(\frac{\mathrm{d}^2 P}{\mathrm{d} V^2} = \frac{n(n+1) P}{V^2} \)
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Pressure-Volume Relationship
To understand the pressure-volume relationship, we start with the given equation: \[P V^n = C\]. This equation shows that the product of the pressure (P) and the volume (V) raised to a power (n) is a constant (C).
It's a key concept in thermodynamics and relates to how pressure and volume change in various states of a gas.
By rearranging the equation, we can express pressure as: \[P = \frac{C}{V^n}\].
This simple manipulation sets the stage for further differentiation steps done in the solution.
It's a key concept in thermodynamics and relates to how pressure and volume change in various states of a gas.
By rearranging the equation, we can express pressure as: \[P = \frac{C}{V^n}\].
This simple manipulation sets the stage for further differentiation steps done in the solution.
Second Derivative
The second derivative in calculus helps us understand how the rate of change of a function's rate of change behaves.
To find it for our pressure-volume relationship, we first differentiate pressure with respect to volume once and then differentiate the resulting expression once more.
Using the equation \[P = \frac{C}{V^n}\], we apply the first derivative, giving \[ \frac{\text{d}P}{\text{d}V} = -n \frac{C}{V^{n+1}} \]. Simplifying and using the original equation, it turns into \[ \frac{\text{d}P}{\text{d}V} = -n \frac{P}{V} \].
The second derivative involves differentiating this new expression again: \[ \frac{\text{d}^2P}{\text{d}V^2} \]. Using differentiation rules helps in achieving this final formula: \[ \frac{\text{d}^2P}{\text{d}V^2} = \frac{n(n+1)P}{V^2} \].
This illustrates how pressure changes not just in response to volume but also how that rate of change itself adapts.
To find it for our pressure-volume relationship, we first differentiate pressure with respect to volume once and then differentiate the resulting expression once more.
Using the equation \[P = \frac{C}{V^n}\], we apply the first derivative, giving \[ \frac{\text{d}P}{\text{d}V} = -n \frac{C}{V^{n+1}} \]. Simplifying and using the original equation, it turns into \[ \frac{\text{d}P}{\text{d}V} = -n \frac{P}{V} \].
The second derivative involves differentiating this new expression again: \[ \frac{\text{d}^2P}{\text{d}V^2} \]. Using differentiation rules helps in achieving this final formula: \[ \frac{\text{d}^2P}{\text{d}V^2} = \frac{n(n+1)P}{V^2} \].
This illustrates how pressure changes not just in response to volume but also how that rate of change itself adapts.
Product Rule
When differentiating a product of two functions, the product rule is essential.
The rule states: \[ \frac{\text{d}}{\text{d}x}(uv) = u \frac{\text{d}v}{\text{d}x} + v \frac{\text{d}u}{\text{d}x} \] where u and v are both functions of x.
In our problem, we face such a situation while differentiating \[ -n \frac{P}{V} \].
Applying the product rule, we get: \[ \frac{\text{d}}{\text{d}V}( -n \frac{P}{V}) = -n \frac{\text{d}P}{\text{d}V} \frac{1}{V} - n \frac{P}{V^2} \]. This step is crucial in simplifying and achieving the accurate form of the second derivative.
Understanding this concept ensures mastery in tackling more complex differentiation problems.
The rule states: \[ \frac{\text{d}}{\text{d}x}(uv) = u \frac{\text{d}v}{\text{d}x} + v \frac{\text{d}u}{\text{d}x} \] where u and v are both functions of x.
In our problem, we face such a situation while differentiating \[ -n \frac{P}{V} \].
Applying the product rule, we get: \[ \frac{\text{d}}{\text{d}V}( -n \frac{P}{V}) = -n \frac{\text{d}P}{\text{d}V} \frac{1}{V} - n \frac{P}{V^2} \]. This step is crucial in simplifying and achieving the accurate form of the second derivative.
Understanding this concept ensures mastery in tackling more complex differentiation problems.