# Homework Help: Electron cloud from uv pulse aimed at negative plate of a capacitor

1. Nov 14, 2009

### nicholasjgroo

1. The problem statement, all variables and given/known data

A very short pulse UV laser is used to liberate a number of electrons from the negative plate in the arrangement in (i) above. The electrons drift towards the positive plate under the influence of the electric field over a time scale much longer than the laser pulse length. Suppose that after the centre of the cloud has travelled approximately 10 mm the spatial distribution of the charge is described by a Gaussian distribution with a standard deviation of 0.5 mm. Write down an expression that relates the electric field to the charge density and calculate the change in electric field across the electron cloud if 5×10^10 electrons m^-2 were emitted during the laser pulse. Over what distance does the electric field effectively change?

2. Relevant equations

god knows, this guys notes dont make any sense

3. The attempt at a solution

2. Nov 16, 2009

### gabbagabbahey

I didn't know God took notes.

Anyways, you'll find that the more effort you show the more people here are willing to help you. If your notes are too messy to read, I suggest you start by reading the relevant sections of your textbook instead.

As a hint for the problem at hand: what is a Gaussian distribution? What does that tell you about the general form of the charge density?

3. Nov 16, 2009

### nicholasjgroo

ok, spent the last 2 hours on this and heres what i've got.

the equation i've got for electric is the charge density divided by the permitivity of free space

for the change in electric field across the cloud E=q/4(pi)epsilon nought r squared.

and for the last bit i dont have a clue, i know of gaussian distributions but i've never used them.

4. Nov 16, 2009

### nicholasjgroo

i'm thinking the electrons arent uniformly distributed they're like a blob and the distribution of the electrons is modelled by the gaussian distribution but i cant figure out what he means by 'over what distance does the electric field change'