# Why Is the Boltzmann Factor Exponential?

• B
• Glenn G
In summary, the Boltzmann factor, e^(-E/kT), is linked to the probability of a particle transitioning from its current energy state to a higher energy state. This can be seen through the example of 4kT energy jump, where only around 183 out of 10,000 particles would make the transition. This exponential relationship was derived by Boltzmann through the concept of energy conservation in collisions, and it is the only one that fits for stable energy distributions over time. This approach can be generalized to all systems with different energy levels, but it is more complex and beyond the scope of high school physics.
Glenn G
Hi community, trying to get my head around the Boltzmann factor...

e^(-E/kT)

It states in a book I'm reading that this is linked to the probability of a particle moving from its current energy state to an energy state E above it? So if you were looking at an energy jump of 4kT that the probability of a particle making this jump is :

e^(-4kT /kT) = e^(-4) = 0.0183 so like saying if there were 10,000 particles at temp. T that around 183 would make the transition. Is this the correct interpretation? What I'm not sure about is is there any time scale associated with this?

OK so I sort of get the concept that 183 out of 10,000 particles giving you 0.0183 represents a probability but I don't get where the exponential relationship comes from. Where did Boltzmann get this exponential relationship from and convinced himself that this modeled proabilities on a microscopic scale.

I read part of a book by Teller (old one) and it was saying that the e^(-E/kT) was like the probability of finding a particle? (don't really get this) and that say two particles (E1, E2 initially) collide the probability of finding the 2 particles afterwards has to be the same as before the collision and that from the conservation of energy, this has to be conserved also and that the exponential relationship is the only one where energy conservation holds in a collision because energies add,
e^(- (E1' + E2') /kT) (E1' E2' are the energies after the collision)

whereas probabilities multiply
e^(-E1'/kT) * e^(-E2'/kT)

Not really firm on this interpretation, would appreciate any input.

There is an analysis on wiki I've seen that goes into a derivation of how the density of the atmosphere changes with height and it goes into deriving an exponential relationship (see part below)

I can follow this route to showing why the density of gas drops exponetially with height but I still don't see how this necessarily assumes that the exponential relations e^(-E/kT) is applicable to so many other scenarios.
Would welcome any input/help.
Glenn.

It is not the probability of transitions, it is the relative probability of finding something in this state at any given point in time.

If you only have the ground state and a state 4 kT higher, you'll find a fraction of 1/(1+0.0183) in the ground state and a fraction of 0.0183/(1+0.0183) in the higher state. With more than two states, extend the formulas accordingly: the numerator is the Boltzmann factor of the state, the denominator is the sum of the Boltzmann factors of all states.Deriving that is not easy. You want to find an energy distribution that is stable over time.

For gas atoms, you can use the collision process: For a given distribution, the collisions should not change the distribution over time. The exponential distribution is the only one that fits.

In statistical physics, you can generalize this approach to all systems with different energy levels, but that is beyond the scope of high school physics.

Glenn G

## 1. What is the Boltzmann factor interpretation?

The Boltzmann factor interpretation is a concept in statistical mechanics that relates the probability of a system being in a particular energy state to the energy of that state. It is based on the Boltzmann distribution function, which describes the probability of a particle being in a particular energy state at a given temperature. This interpretation is used to understand the behavior of systems consisting of many particles, such as gases, liquids, and solids.

## 2. How is the Boltzmann factor calculated?

The Boltzmann factor is calculated using the formula e^(-E/kT), where E is the energy of the state, k is the Boltzmann constant, and T is the temperature. This formula is based on the Boltzmann distribution function, which states that the probability of a particle being in a particular energy state is proportional to the Boltzmann factor.

## 3. What is the significance of the Boltzmann factor interpretation?

The Boltzmann factor interpretation is significant because it allows scientists to predict and understand the behavior of systems consisting of many particles. By calculating the Boltzmann factor, we can determine the probability of a system being in a particular energy state, which in turn helps us understand the overall behavior and properties of the system. This interpretation is widely used in the fields of physics, chemistry, and materials science.

## 4. How does the Boltzmann factor interpretation relate to entropy?

The Boltzmann factor interpretation is closely related to entropy, which is a measure of the disorder or randomness of a system. The Boltzmann factor is directly proportional to the entropy of a system, meaning that as the entropy increases, the Boltzmann factor also increases. This relationship is described by the famous equation S = k ln(W), where S is the entropy, k is the Boltzmann constant, and W is the number of possible microstates of the system.

## 5. Can the Boltzmann factor interpretation be applied to classical and quantum systems?

Yes, the Boltzmann factor interpretation can be applied to both classical and quantum systems. In classical systems, the energy states are continuous, and the Boltzmann factor is calculated using the classical distribution function. In quantum systems, the energy states are discrete, and the Boltzmann factor is calculated using the quantum distribution function. However, the underlying principle remains the same, and the Boltzmann factor interpretation is applicable to both types of systems.

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