# Finding the electric field between two parallel plates

## Homework Statement

Hi everyone! This is the homework problem I'm struggling with!

Initial velocity = 107 meters per second
The particle is an electron, so:
Charge = -1.6 x 10-19 coulombs
Mass = 9.11 x 10-31 kilograms

## Homework Equations

Kinematic formula x = vot + 1/2 at2
Newton's law: Force = mass times acceleration
Magnitude of electric field = force over charge

## The Attempt at a Solution

I'm really at wit's end with this one guys :(

Basically, I've been trying to use kinematics from mechanics to solve this problem, but the way I've been doing it I haven't been able to get to the solution without knowing the length of the plates and the distance between them.

As you can see the professor gives us the answer to help guide us through the solution, but I just can't see how to get to it for this one problem.

My homework is for thursday so I have a lot more time to try and think of a solution, but I thought maybe you guys could maybe give me a tip about what step I may be missing.

This is the work I've done so far. I've assigned variables to represent the length of the plates and the distance between them. The only solution I was able to get was in terms of those variables. I assume there's a way to make them cancel out with something? Thanks for any help!

My work so far:

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## Answers and Replies

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gneill
Mentor
I think you're right that you need more information about the plate geometry.

haruspex
Homework Helper
Gold Member
Clearly there is not enough information. If you make the horizontal distance greater, all the given data can stay the same but the field required would be less.

Seriously?

Oh boy. I suspected that at first but I was sure that I was missing something. Should I write to the professor? He's over 80 years old so I don't want to offend him.

haruspex
Homework Helper
Gold Member
Seriously?

Oh boy. I suspected that at first but I was sure that I was missing something. Should I write to the professor? He's over 80 years old so I don't want to offend him.
Is it the prof's diagram? Quite sure there's no other data with it?
You could submit the answer you have... or do you have to submit it to a bot?

rude man
Homework Helper
Gold Member

## Homework Statement

Basically, I've been trying to use kinematics from mechanics to solve this problem, but the way I've been doing it I haven't been able to get to the solution without knowing the length of the plates and the distance between them.
Don't feel bad, I agree you need both h and d to obtain E.
So now someone else can prove us both wrong ...

Is it the prof's diagram? Quite sure there's no other data with it?
You could submit the answer you have... or do you have to submit it to a bot?
Yeah it's his diagram. There's nothing else, I just took a screenshot of the third question. We have to give it in class on paper

Last semester in mechanics he sometimes sent us emails to extend the date of a homework when he realized he had mistakes in them. I assume he might do the same again