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$$\text { Verify that } v_{\mathrm{rms}}=\sqrt{\bar{v}^{2}}=\sqrt{\frac{3 k_{\mathrm{B}} T}{m}}$$

Short Answer

Expert verified
We can verify the formula \(v_{rms}=\sqrt{\bar{v}^2}=\sqrt{\frac{3k_BT}{m}}\) by understanding the root mean square velocity, expressing the average velocity squared using the Maxwell-Boltzmann distribution, and evaluating the integral. After calculating the integral and simplifying the expression for \(\bar{v}^2\), we obtain \(\bar{v}^2 = \frac{3k_BT}{m}\). Taking the square root of \(\bar{v}^2\), we get the root mean square velocity: \(v_{rms} = \sqrt{\frac{3k_BT}{m}}\), which confirms the given formula.

Step by step solution

01

Understanding the root mean square velocity

The root mean square (rms) velocity, \(v_{rms}\), is a statistical measure of the average velocity of the particles in a system. It is calculated as follows: \(v_{rms} = \sqrt{\bar{v}^2}\), where \(\bar{v}^2\) is the average of the squared velocities. Note that it is not the same as the average velocity, which for a random distribution of particles would be zero.
02

Express the average velocity squared using Maxwell-Boltzmann distribution

The Maxwell-Boltzmann distribution describes the probability distribution of velocities of particles in a system. We can use it to find the average squared velocity, \(\bar{v}^2\), as a function of temperature, T, and particle mass, m: \(\bar{v}^2 = \int_{0}^{\infty} v^2 f(v)dv\), where f(v) is the Maxwell-Boltzmann distribution function: \(f(v)=\left(\frac{m}{2 \pi k_{B} T}\right)^{3 / 2} e^{-\frac{m v^{2}}{2 k_{B} T}} 4 \pi v^{2}\). Now we will calculate this integral.
03

Calculate the integral for average velocity squared

To find \(\bar{v}^2\), we need to calculate the following integral: \(\bar{v}^2 = \int_{0}^{\infty} v^2 \left[\left(\frac{m}{2 \pi k_{B} T}\right)^{3 / 2} e^{-\frac{m v^{2}}{2 k_{B} T}} 4 \pi v^{2}\right] dv\). Let's perform substitution: \(u = \frac{mv^2}{2k_B T}\) and \(2k_BT\ du = mv\ dv\). So the integral becomes: \(\bar{v}^2 = \int_{0}^{\infty} \frac{2k_BT}{m} \left[\left(\frac{m}{2 \pi k_{B} T}\right)^{3 / 2} e^{-u} 4 \pi u\right] du\), \(\bar{v}^2 = \left(\frac{m}{2 \pi k_{B} T}\right)^{1 / 2} 8 \pi k_{B} T \int_{0}^{\infty} u e^{-u} du\). To evaluate the integral, we can use integration by parts, where \(I = \int_{0}^{\infty} ue^{-u} du\). Let \(U= u\), then \(dU = du\), Let \(dV = e^{-u}du\), then \(V = -e^{-u}\). Using the integration by parts formula, \(I = UV - \int VdU\): \(I = -(ue^{-u})\Bigg|_0^{\infty} + \int_{0}^{\infty} e^{-u}du\), \(I = -[-(0)-(1)] = 1\). So, \(\bar{v}^2 = \left(\frac{m}{2 \pi k_{B} T}\right)^{1 / 2} 8 \pi k_{B} T (1)\), \(\bar{v}^2 = \frac{3k_BT}{m}\).
04

Calculate the root mean square velocity

Now, we can find the root mean square velocity, \(v_{rms}\), by taking the square root of \(\bar{v}^2\): \(v_{rms} = \sqrt{\bar{v}^2} = \sqrt{\frac{3k_BT}{m}}\). Thus, we have verified that the given formula for root mean square velocity is: \(v_{rms} = \sqrt{\frac{3k_BT}{m}}\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Maxwell-Boltzmann Distribution
The Maxwell-Boltzmann distribution is a vital concept in understanding the behavior of particles in a gas. This theoretical framework provides a probability distribution of speeds for particles in a gas at a given temperature. It allows us to determine the likelihood of finding particles at different speeds at thermal equilibrium. This distribution is particularly important for gases in thermodynamic systems, as it suggests how molecules spread out velocity-wise due to random motion.
In mathematical terms, the distribution function predicts the probability of particles having a certain velocity. This is expressed through the function:
  • \[ f(v) = \left(\frac{m}{2 \pi k_{B} T}\right)^{3 / 2} e^{-\frac{m v^{2}}{2 k_{B} T}} 4 \pi v^{2} \]
Here, \(m\) is the mass of the particles, \(k_B\) is the Boltzmann constant, and \(T\) is the temperature. The Maxwell-Boltzmann distribution tells us that as temperature increases, particles are more likely to move at higher speeds. Overall, this distribution aids in predicting the behavior of gas molecules and is foundational to kinetic theory.
Statistical Mechanics
Statistical mechanics is a field of physics that provides a microscopic explanation for thermodynamic phenomena. By using statistical methods, it connects macroscopic properties like pressure, temperature, and volume with microscopic behavior of atoms and molecules. In essence, it translates the actions and interactions of countless small particles into observable phenomena.
The root mean square velocity (\(v_{rms}\)) calculation from the Maxwell-Boltzmann distribution is an application of statistical mechanics. The expression \(v_{rms} = \sqrt{\bar{v}^{2}}\) is derived by statistically analyzing the velocity distribution among particles. This formula allows predictions on kinetic energy within gases, where an average behavior emerges from chaotic, individual particle motions.By leveraging statistical mechanics, we can grasp how complex systems like gases behave under varying conditions. It presents a bridge between the randomness at atomic levels and order at larger scales.
Thermodynamics
Thermodynamics is the science concerned with heat and temperature and their relation to energy and work. It examines how matter and energy interact across different systems, leading to major concepts such as the laws of thermodynamics and state functions.
Within the context of the root mean square velocity, thermodynamics tells us how temperature affects the average kinetic energy of gas molecules. The formula \(v_{rms} = \sqrt{\frac{3k_BT}{m}}\) directly relates temperature (a thermodynamic variable) to the motion of particles in a gas. As the temperature of a gas increases, the root mean square velocity of its particles also increases, resulting in higher particle speed and energy.
Understanding thermodynamics allows us to predict how changes, such as heating a gas, will affect its overall properties and behavior, crucial for applications in engines, refrigeration, and even meteorology.
Kinetic Theory of Gases
The kinetic theory of gases is a model that describes gases as collections of small particles in constant, random motion. This theory helps explain macroscopic properties of gases, such as pressure and temperature, through molecular activity.
According to the kinetic theory, gases consist of a large number of small particles (atoms or molecules) that are in constant, random motion. The pressure of a gas arises from collisions between the particles and the walls of the container. The root mean square velocity is a crucial element in this theory, providing a statistical average of the velocity of gas particles, and is derived from their kinetic energy.
  • The formula \(v_{rms} = \sqrt{\frac{3k_BT}{m}}\) is significant because it succinctly relates the properties of individual particles to the observable properties of the gas.
Kinetic theory not only offers insight into fundamental behaviors of gases but also lays the groundwork for understanding exotic states of matter and the behavior of fluids under extreme conditions.

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