# Homework Help: Electric field / charged plates question.

1. Sep 21, 2008

### frasifrasi

I have this as an online assignment and think I have the wrong answer, but the site is telling me the result is wrong and takes a percentage off my grade each time i send a wrong submission.

I am just posting an image link because that way you guys can see the picture:

http://img142.imageshack.us/my.php?image=screenhunter003xp5qz5.png

Here is what I have so far:

t = length of plates / horizontal velocity

= 2.20 * 10-2 m / 18.0 m/s = 1.22 * 10-3 s

In the same time, the drop must deflect by 'd' distance along vertical direction. By second equation of motion

d = uy * t + (1/2) * ay * t2

uy = initial velocity along vertical direction = 0

ay = acceleration along vertical axis.

=> 0.300 * 10-3 = 0 * t + 0.5 * ay * (1.22 * 10-3)2

ay = 3.0 * 10-4 / 1.49 * 10-6 = 2.01 * 102 m/s2

__________________________________________________________

F = q * E

by Newton's second law F = m * a

=> m * ay = q * E

q = m * ay / E = 1.0 * 10-11 * 2.01 * 102 / 7.90 * 104

q = 2.54 * 10-14 C

But this is wrong and the site won't give me a feedback. If anyone can help me out, I would be very thankful. : )

2. Sep 21, 2008

### Defennder

Check your value for ay again.

3. Sep 21, 2008

### frasifrasi

I did, it is 2.01 *10^2. Sorry, when I pasted, all the carrot signs disappeared.

Is there anything wrong with any of the steps?

Thanks.

4. Sep 21, 2008

anyone?