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Energy Distribution

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💡 Key Idea

The Core Idea: Most Probable Energy

Not all energies are equally likely.

The energy distribution function $f(E)$ describes the probability that a particle in a system (like a gas) has energy $E$. For a classical ideal gas at temperature $T$, the Maxwell-Boltzmann distribution gives $f(E) \propto \sqrt{E} \, e^{-E/k_B T}$, where $k_B$ is the Boltzmann consta...

↳ Energy distribution is not uniform; it peaks at a low energy but has a high-energy tail.

📖 Definition

What is an Energy Distribution Function?

A probability map for energies.

An energy distribution function $f(E)$ is a mathematical function that gives the probability density of finding a particle with energy $E$. It is defined such that $f(E) \, dE$ is the fraction of particles with energy between $E$ and $E + dE$. The function is normalized: $\int_0^\infty f(E) \, dE = 1$.

↳ It tells you how likely each energy value is for a particle in the system.

⭐ Important Fact

The Boltzmann Factor

The exponential that rules thermal physics.

The Boltzmann factor $e^{-E/(k_B T)}$ is the core of the distribution. It tells us that states with higher energy are exponentially less likely to be occupied. This factor appears in all classical statistical mechanics and explains why particles tend to occupy lower energy states at finite temperatures...

↳ Higher energy states are exponentially suppressed by the Boltzmann factor.

📖 Smart notes

What you'll study, topic by topic

1

Energy Distribution Function: Maxwell-Boltzmann Statistics

The energy distribution function, specifically the Maxwell-Boltzmann distribution, describes how particles in a classical ideal gas are distributed over different energies. It combines a density of states factor (√E) wit...

  • The Maxwell-Boltzmann energy distribution is $f(E) \propto \sqrt{E} \, e^{-E/(k_B T)}$.
  • The distribution is normalized: $\int_0^\infty f(E) \, dE = 1$.
  • The most probable energy is $E_{mp} = \frac{1}{2} k_B T$.

~8 min · full explanation, examples & memory tricks in the app

❓ Leveled MCQ practice

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18 questions laddered from warm-up to topper-level, each with an explanation. A taste:

What does the Maxwell-Boltzmann distribution describe for a classical gas?

Beginner
A The distribution of particle speeds in a gas at thermal equilibrium B The distribution of charge carriers in a semiconductor C The distribution of particle positions in a crystal lattice D The distribution of energy levels in a quantum harmonic oscillator
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The distribution of particle speeds in a gas at thermal equilibrium

The Maxwell-Boltzmann distribution gives the probability distribution of speeds (and hence kinetic energies) of particles in a classical ideal gas at thermal equilibrium.

In the Maxwell-Boltzmann distribution, the probability of a particle having energy $E$ is proportional to which factor?

Beginner
A $\sqrt{E} e^{-E/k_B T}$ B $e^{-E/k_B T}$ C $E^2 e^{-E/k_B T}$ D $e^{+E/k_B T}$
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$e^{-E/k_B T}$

The Maxwell-Boltzmann distribution has the Boltzmann factor $e^{-E/k_B T}$, which gives the relative probability of a state with energy $E$ at temperature $T$.

What is the most probable speed $v_p$ of a gas molecule in the Maxwell-Boltzmann distribution?

Intermediate
A $\sqrt{\frac{k_B T}{m}}$ B $\sqrt{\frac{8k_B T}{\pi m}}$ C $\sqrt{\frac{2k_B T}{m}}$ D $\sqrt{\frac{3k_B T}{m}}$
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$\sqrt{\frac{2k_B T}{m}}$

The most probable speed is found by maximizing the speed distribution function, yielding $v_p = \sqrt{2k_B T/m}$.

The average speed $\bar{v}$ of molecules in a Maxwell-Boltzmann gas is given by:

Intermediate
A $\sqrt{\frac{3k_B T}{m}}$ B $\sqrt{\frac{2k_B T}{m}}$ C $\sqrt{\frac{8k_B T}{\pi m}}$ D $\sqrt{\frac{k_B T}{m}}$
Show answer & explanation

$\sqrt{\frac{8k_B T}{\pi m}}$

The average speed is calculated by integrating $v$ times the speed distribution, giving $\bar{v} = \sqrt{8k_B T/(\pi m)}$.

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