Magnitude of electric force

bastige

1. The problem statement, all variables and given/known data
Four point charges are placed at the four corners of a square. Each side of the square has a length L.
Find the magnitude of the electric force on q2 due to all three charges q1 , q2 , q3
and q4. Given L=1 and q= 1.38 μC. Answer in N


http://rs209gc2.rapidshare.com/files...f5e75220df.jpg
Photo of problem ^

2. Relevant equations



3. The attempt at a solution
http://rs220l32.rapidshare.com/files...3_19-17-38.jpg
Photo of work ^

dynamicsolo

Since you give units of N and microcoulombs in your statement, you are presumably working in SI units. Right off, I'd say check your value for k: I believe it is off by three orders of magnitude... (In your statement, I think you want the net force on q2 from the other three charges, so you don't want to type 'q2' again...)

Also, watch your forces and components. For starters, what is the distance along the diagonal of the square? What is the force between the diagonally opposed charges?

bastige

Well the diagonal is also 1. K should = 8.99E9. I still get the wrong answer.

dynamicsolo

If the side of a square is 1, how long is the diagonal?

bastige

A^2 + B^2 =c^2
1^2+1^2=c^2
2=c^2
1=c

Snazzy

Square root of 2 is 1?

bastige

hmm...good point, guess i shouldn't do this in my head. well, its 1.41421. Where Do i need to use it? I didn't have r in my final formula? where did i go wrong?

dynamicsolo

The value for c you just calculate is what you will use in Coulomb's Law for the force between the diagonally opposite charges. Since the formula calls for r^2, you could use r^2 = 2 there.

bastige

??????I'm sorry, I'm physics illiterate

dynamicsolo

You have the equation for the force

kqq/(r^2) , which is Coulomb's Law.

In the image you show for your work, the force for the "southeast" diagonal should be using 2 in the denominator, instead of 1, since the distance squared between the two charges q2 and q4 is 2 .