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## Homework Statement

There is a beam of electrons in the space starting at x=0 and ending at x=20cm. The cross section of the beam is A.The velocity of the electrons is a function of position x and let it be v(x).Now i need to calculate the volume charge density as a function of x.

## Homework Equations

## The Attempt at a Solution

What i did is this:

I took an imaginary plane perpendicular to the beam.In a small time interval dt the amount of charge crossing the plane of area A is J.So dividing this J with v(x) should give the density. But J is not specified.Is there any other way to solve this?