- #1
fatpotato
- Homework Statement
- Find an expression for the total movement ##s## of a bacteria knowing that its movement follow a normal distribution.
- Relevant Equations
- Expected value ##\bar{x} = \int_{-\infty}^{\infty}x\cdot p(x)dx##
Gaussian function ##p(x) = \frac{1}{\sqrt{2\pi}} \exp{-\frac{x^2}{2}}##
Hello,
I have to find an expression for the total movement of a bacteria ##s##, knowing that the bacteria is placed (centered) on a two side ruler at position ##x=0## (so a negative ##x## value means the bacteria has moved to the left of the ruler) and that the probability it moves to ##x## is given by the Gaussian function :
$$p(x) = \frac{1}{\sqrt{2\pi}} \exp{-\frac{x^2}{2}}$$
I have to express the total displacement ##s## using the fact that ##\bar{x} = \int_{-\infty}^{\infty}x\cdot p(x) dx## is the average expected displacement. Of course, since ##p(x)## is centered, ##\bar{x}## equals to zero, which makes sense both mathematically and physically, but now I have trouble finding a connection between this and a total displacement.
I thought about using ##s = \int_{-a}^{a}x\cdot p(x) dx## where ##a## would be the total displacement at point ##a##, but obviously, since ##x\cdot p(x)## is odd, any integral of this form will yield zero, so I thought about taking only one side with ##s = \int_{0}^{a}x\cdot p(x) dx##, yet I am not convinced.
Evaluating this new integral gives the following result :
$$s = \int_{0}^{a}x\cdot p(x) dx = \frac{1}{\sqrt{2\pi}} \big( 1 - \exp{-\frac{a^2}{2}} \big)$$
This means that the total displacement lies in the interval ##[0; \frac{1}{\sqrt{2\pi}})##, but why would be the total displacement of ##s## anything other than ##s## itself? My technique would imply that if we want to look at the total displacement ##s## of a bacteria, we have to first look at "all the movements up to point ##a##".
Does this make any sense?
Thanks in advance.
Edit : Relevant equations formatting
I have to find an expression for the total movement of a bacteria ##s##, knowing that the bacteria is placed (centered) on a two side ruler at position ##x=0## (so a negative ##x## value means the bacteria has moved to the left of the ruler) and that the probability it moves to ##x## is given by the Gaussian function :
$$p(x) = \frac{1}{\sqrt{2\pi}} \exp{-\frac{x^2}{2}}$$
I have to express the total displacement ##s## using the fact that ##\bar{x} = \int_{-\infty}^{\infty}x\cdot p(x) dx## is the average expected displacement. Of course, since ##p(x)## is centered, ##\bar{x}## equals to zero, which makes sense both mathematically and physically, but now I have trouble finding a connection between this and a total displacement.
I thought about using ##s = \int_{-a}^{a}x\cdot p(x) dx## where ##a## would be the total displacement at point ##a##, but obviously, since ##x\cdot p(x)## is odd, any integral of this form will yield zero, so I thought about taking only one side with ##s = \int_{0}^{a}x\cdot p(x) dx##, yet I am not convinced.
Evaluating this new integral gives the following result :
$$s = \int_{0}^{a}x\cdot p(x) dx = \frac{1}{\sqrt{2\pi}} \big( 1 - \exp{-\frac{a^2}{2}} \big)$$
This means that the total displacement lies in the interval ##[0; \frac{1}{\sqrt{2\pi}})##, but why would be the total displacement of ##s## anything other than ##s## itself? My technique would imply that if we want to look at the total displacement ##s## of a bacteria, we have to first look at "all the movements up to point ##a##".
Does this make any sense?
Thanks in advance.
Edit : Relevant equations formatting