# Deflection of a particle between two plates

1. Jul 22, 2009

### sw1mm3r

I am studying for a test tomorrow and our prof gave us a practice test and I am confused on how to solve this problem

A charged particle is accelerated through a potential of 15,000 volts, after which it passes between two deflecting plates. What voltage must be applied to these plates to deflect the particle through an angle of 10 degrees?

NOTE the answer is 882 V (IDK how to get that)

Plates are horizontal with length of 3 cm and are .5 cm apart.
Particle starts at center of the two plates and once it goes between the two plates it is deflected downward 10 degrees

2. Jul 22, 2009

### Staff: Mentor

Welcome to the PF. You should post some of the relevant equations, and at least think out loud a bit about how to approach this.

Do you know how to convert from eV to Joules? And so then to calculate the velocity of the electron as it enters the region between the plates?

Do you know how to calculate the force on an electron from an external electric field? How to calculate the electric field, given the voltage between two plates and their separation? And do you remember the kinematic equations of motion, given a constant force (acceleration)?

3. Jul 22, 2009

### sw1mm3r

Im sorry I posted the question b/c i am super confused and dont even know how to start solving it

4. Jul 22, 2009

### Staff: Mentor

You start solving it by answering the questions I posted. If you don't know the answers to my specific questions, use wikipedia.org or Wolfram Alpha or similar technical search engines (or your textbook) to help you answer them. If you answer my questions, you should be very close to answering this problem.

We don't do your work for you. Try figuring out the answers to my questions.

5. Jul 22, 2009

### sw1mm3r

i realize u guys dont solve it for me... but i have been to this site before where ppl ask similar questions to those that i had and ppl gave like step by step of how to solve it...... thx for ur help

6. Jul 22, 2009

### Staff: Mentor

I DID give you step-by-step directions in the form of my hints/questions. Answer the questions above, and you're almost done!

7. Jul 22, 2009

### sw1mm3r

ok so I got this V=(tan(10)(4)(15000)(.0025))/(.03)

8. Jul 23, 2009

### edgerandy

where is the 4 come from ?

9. Jul 23, 2009

### Staff: Mentor

Where did *any* of that come from? We need to see your steps, starting with the relevant equations, and including the units of each quantity and equation.

That's what they are going to be looking for on the test, BTW. Each formula, each equation, each step, and units on all quantities.

10. Jul 23, 2009

### edgerandy

is there something not right with ur hint ? since the problem is about a charged particle, not an electron

the way i approach this problem is using ratio of the vertical potential over horizontal potential equal tan 10* 3cm/.25 cm and it is not give me the answer 882 J
i have no clue where the equation this V=(tan(10)(4)(15000)(.0025))/(.03) come from

11. Jul 23, 2009

### Staff: Mentor

Hmm, interesting point. My hints are still valid, I think, but you're right, you'd need to also see if the particle's mass and how much charge it has make a difference. It might be that they cancel out, or it might be that they show up in the solution. Good point.