Click here to go to the NEW College Discussion Forum

My AP Physics teacher gives us these damned worksheets with insainly difficult problems, and I need help.

An electron is fired at a speed of v0- 5.6 x 10^6 m/s at an angle of theta0= -45(degrees) between two parallel conducting plates that are D = 2.00 mm apart as shown below. If the potential difference between the plates is (delta)V = 100 V, determine:

A. How close, d, the electron will get to the bottom plate and,

B. where the electron will strike the upper plate.

Any help, even if you only have the time/expertise to hint at the answer or solve only part of the problem would be greatly appreciated.

By Jszab (Jszab) on Sunday, May 02, 2004 - 04:59 pm: Edit

I am assuming that bottom is minus, and top plus. []You shoul have mentioned it.

You should mention about gravity, but it will not have big influence on result.

Follow the steps, I will try to explain it to You. Not to write down :P. ;)

1. When the electron is fired between the plates, the electric force starts to pull [or push] him up.

2. Because, when one plate is pulling the electron with a force inversly proportional to distance from the plate, and second pushing, with the same principle, the overall result is that:
Force affecting between plates has the same direction and magnitude [exception - ends. In this case we do not have neccesary data to do it with ends. So omit it as too difficult ;] ]

next in a minute

By Transferkid (Transferkid) on Sunday, May 02, 2004 - 04:59 pm: Edit

I'll give you a hint:

From the deltaV you can determine the E-field( V=E*D), and from the E-field, you can determine the electrostatic force on the electron(F=q*E, with q the elementary charge of the electron).
From here on, your problem is in fact a mechanics problem. You can determine the acceleration of the electron due to the electrostatic force, and hence an equation of velocity as a function of time( v(t)=int(a)dt), and therefore an equation for the displacement of the electron as a function of time(x(t)=int(v)dt, naturally). Once you've done that, the answers to A and B can come easily.

By Jszab (Jszab) on Sunday, May 02, 2004 - 05:05 pm: Edit

3.The force started to acting on the electron, so it will move towards upper plate.

This force - gravity force = Force that we seek for
W = eED >> use it to find this force.

4. Now sketch trajectory, it will look as: [rotate your page by 180 degrees] as trajectory of thrown ball, in gravity field.

By Jszab (Jszab) on Sunday, May 02, 2004 - 05:14 pm: Edit

Now solve, as for kinetics gravity problem [for the first task, or make from dynamics, and energy.]

Second, is easiest to determine horizontal velocity, and time of "flight" , and thus distance, simply by taking V/t.

I hope You understand it now .
It is the same problem, as any other in dynamics. Nothing new.