Series Expansion for V(r) in Quantum Final | Step-by-Step Solution

In summary, the conversation is about finding the series expansion of a function in quantum physics, specifically the function V(r)=-V0/(1+e^(r-R)/a) for r>R. The solution involves using the geometric series and taking into account that exp((r-R)/a) > 1 for r>R.
  • #1
quasar987
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Homework Statement


I am ashamed to ask this, but in my quantum final, there was a little mathematically-oriented subquestion that asked to show that the function

[tex]V(r)=-\frac{V_0}{1+e^{(r-R)/a}}[/tex]

(r in [0,infty)) can be written for r>R as

[tex]V_0\sum_{n=1}^{\infty}(-1)^ne^{-n(r-R)/a}[/tex]

The Attempt at a Solution


:blushing:
 
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  • #2
You know the series expansion of (1+x)^-1 for |x|<1, right? So use it (and don't tell me that exp{(r-R)/a} >1 for r>R, because I know that).
 
  • #3
Yeah ok!

----
 
  • #4
Or (really the same thing) the "geometric series"
[tex]\sum_{n=0}^\infty ar^n= \frac{a}{1- r}[/tex]
 

1. What is a series expansion in quantum mechanics?

A series expansion in quantum mechanics is a mathematical method used to approximate the behavior of a system by breaking it down into simpler, more manageable terms. It involves expressing a complex function, such as the potential energy function V(r), as an infinite sum of simpler functions.

2. Why is series expansion important in quantum mechanics?

Series expansion is important in quantum mechanics because it allows us to solve complex equations and understand the behavior of quantum systems. It also helps us to make predictions and calculations that are not possible using traditional analytical methods.

3. How is the series expansion for V(r) determined?

The series expansion for V(r) is determined by using a mathematical technique called Taylor expansion. This involves expressing the potential energy function as an infinite sum of derivatives evaluated at a specific point. Higher order terms in the series provide more accurate approximations of the function.

4. What is the step-by-step solution for the series expansion of V(r)?

The step-by-step solution for the series expansion of V(r) involves first expressing the potential energy function as a sum of simpler functions using Taylor expansion. Then, each term in the series is evaluated and simplified. Finally, the terms are combined to form the full series expansion.

5. How is the series expansion used in practical applications?

The series expansion for V(r) is used in practical applications to model and predict the behavior of quantum systems, such as atoms and molecules. It is also used in calculations for quantum mechanical processes, such as energy levels and transitions. Additionally, series expansion is used in numerical methods, such as perturbation theory, to solve complex quantum mechanical problems.

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