Hi,(adsbygoogle = window.adsbygoogle || []).push({});

I've worked out a problem and don't understand why it's wrong... I think I might be going about this incorrectly.

Here it is:

http://www.synthdriven.com/images/deletable/01.jpg [Broken]

I started this by attempting to solve for E. In the beginning of the problem, I'm told that this is an infinite plate, "a uniformly charged vertical sheet of infinite extent", and have determined the following formula:

[tex]\vec{E}=\frac{\sigma}{2\epsilon_{o}}[/tex]

(This is what was derived in class for an infinite surface, I can go over that if anyone would like... sigma=Q/A)

I'm taking the "areal charge density" to be sigma.

[tex]\vec{E}=\frac{\sigma}{2\epsilon_{o}}=\frac{0.12\times10^{-6}}{2(8.85\times10^{-12})}=6779.66N/C[/tex]

Using the following formula, I determined the force, (where q is my given point charge):

[tex]F=E\timesq=(6779.66)(0.11\times10^{-6})=0.000746N[/tex]

http://www.synthdriven.com/images/deletable/02.jpg [Broken]

According to my little force diagram, this F is equivalent to my x-component of the force labelled F. And the y-component of this force is zero...

The next force is the weight, mg. I wasn't sure whether or not to do this in grams or kilograms, so I kept it at grams (my professor converted from kg to g in a previous example in class... even though some problems are typically answered in kg)

[tex]W=mg=(1g)(9.8m/s^2)=9.8N[/tex]

This force will be only in the -y direction, obviously.

And the tension, T is as follows:

[tex]T_{x}=T\sin{\theta}[/tex]

[tex]T_{y}=T\cos{\theta}[/tex]

[tex]\sum{F_{NETx}}=F_{x}+W_{x}-T_{x}=0.00076N+0N-T\sin{\theta}=0[/tex]

[tex]T\sin{\theta}=0.00076N[/tex]

[tex]T=\frac{0.00076N}{\sin{\theta}}[/tex]

[tex]\sum{F_{NETy}}=F_{y}-W_{y}+T_{y}=0-9.8N+T\cos{\theta}=0[/tex]

[tex]T\cos{\theta}=9.8N[/tex]

[tex]T=\frac{9.8N}{\cos{\theta}}[/tex]

[tex]\frac{0.00076N}{\sin{\theta}}=\frac{9.8N}{\cos{\theta}}[/tex]

[tex]\frac{\sin{\theta}}{\cos{\theta}}=\frac{0.00076}{9.8}[/tex]

[tex]\tan{\theta}=\frac{0.00076}{9.8}[/tex]

[tex]\arctan{\frac{0.00076}{9.8}}=\theta[/tex]

[tex]=0.004443[/tex] degrees

What am I doing wrong? This seemed like the right way to go about it...

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Homework Help: Electric Field and Infinite Plane

**Physics Forums | Science Articles, Homework Help, Discussion**