# The probability density for an electron that has passed through an experimental apparatus is shown in the figure. If 4100 electrons pass through the apparatus, what is the expected number that will land in a 0.10 mm-wide strip centered at x = 0.00 mm? y-axis units are see image below `P(x)= [psi(x)]^2(mm^-1)`

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First we determine the probability `p` of the event that one electron will land in a specified strip. Then the expected number of such electrons will be `p*N,` where `N` is the total number of electrons.

By the definition of a probability density `P(x),` the probability of being between...

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First we determine the probability `p` of the event that one electron will land in a specified strip. Then the expected number of such electrons will be `p*N,` where `N` is the total number of electrons.

By the definition of a probability density `P(x),` the probability of being between `a` and `b` is equal to  `int_a^b P(x) dx.` Here `a=-0.1` mm and `b=0.1` mm. Because the given density is an even function,  `p = 2 int_0^0.1 P(x) dx.`

It is simple to write a formula for `P(x)` for `x` between 0 and 3, it is `P(x)=1/3 - x/9` (a straight line). Therefore

`p = 2 int_0^0.1 (1/3 - x/9) dx = 2 (x/3 - x^2/18)|_(x=0)^0.1 = 2*(0.1/3-0.01/18) approx 0.0656.`

And the expected number is about  `4100*0.0328 approx 269.` This number is dimensionless.

Note that this number is the most probable, but the neighboring numbers are also very probable.

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