# Homework Help: Deflection of an Electron and Capacitor problems

1. Jan 22, 2004

### Moxin

k, Here's the problem:

An electron is accelerated horizontally from rest in a television picture tube by a potential difference of 24600 V. It then passes between two horizontal plates 6.08 cm long and 1.42 cm apart that have a potential difference of 236 V. At what angle theta (in degrees) will the electron be traveling after it passes between the plates?

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Being that I have no clue how to EVEN APPROACH this problem, I just did random stuff. I have a feeling this problem involves the arctan function.. so, first I took the arctan of 1.42/6.08 - Wrong. Then the Arctan of E (which I calculated to be 16164) over the potential difference which accelerated the particle - Wrong. I tried variations of those two - Wrong. I honestly don't have the slightest clue where to go next

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My next problem goes:

A 2.76 microF capacitor is charged to 1230 V and a 6.21 microF capacitor is charged to 480 V. These capacitors are then disconnected from their batteries, and the positive plates are now connected to each other and the negative plates are connected to each other. What will be the potential difference across each? What will be the charge on the 2.76 microF capacitor? What will be the charge on the 6.21 microF capacitor?

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Once again, I have no clue how to approach. This is absolutely depressing. I spent an hour try'na figure out each of these problems (like I do with the rest of the 15 or so, unfortunately, Booo to bad books and even worse professors). Anyways, any suggestions.. ?

2. Jan 22, 2004

### ShawnD

The electron is starting from rest and being deflected in 2 perpendicular directions. Break the problem into 2 parts, 1 part per direction.

The electron starts with lots of potential energy caused by the voltage and ends with lots of kinetic energy; in both directions. Just find the final velocity in each direction then use trig to find the angle.

Ep = Ek

Last edited: Jan 22, 2004
3. Jan 22, 2004

### himanshu121

Apply
$$\frac{mv_f^2-mv_i^2}{2}=eV$$
Which is shorter way

4. Jan 22, 2004

### Moxin

Re: Re: Deflection of an Electron and Capacitor problems

Can you clarify how I'd apply that further himanshu

Well, before you edited and expounded step by step, I had followed what you said but still couldnt get it right...

I found in the book an example that had an equation velocity = sqrt(-2qV/m)

I applied this equation to both the horizontal direction by using 24600 V, then the vertical direction by using 236 V.. I came out with 93008330 m/s for the horizontal and 9109830 for the vertical component.. used arctan(vertical/horizontal) and came out with an angle of 5.594 degrees................and its still wrong

I dunno what else to try

5. Jan 22, 2004

### Moxin

And any suggestions on the 2nd problem ? ?

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