- #1

Istiak

- 158

- 12

- Homework Statement
- An electron of charge e is moving at a constant velocity u, along x-axis. It enters a region of constant electric field E, which is pointing perpendicular to x-axis. The electron moves in a parabola. Which of the following represents the focal length of the parabola? Neglect any effects due to gravity. (for a parabola of type ##x^2=4ay##, a is the focal length)

- Relevant Equations
- ##\int \vec F\cdot d\vec s = \frac{1}{2}mu^2##

Here I was going to use ##\int \vec F \cdot d\vec l = \frac{1}{2}mu^2##

What I got that is ##l=\frac{mu^2}{2eE}##. Here the question is what is ##l## (I took ##x## while doing the work but here I used ##l## instead of ##x##)? I was assuming that it's ##x## since I am calculating work in the parabola. So my equation stands ##a=\frac{m^2u^4}{16 y(eE)^2}##. But there's no option of it. But what I found for ##l## that satisfies. But my question is how ##l## is focal length?