How Do You Calculate the Electric Force on a Charge at the Corner of a Square?

[bastige]

Homework Statement

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/101850597/81d74d53637084fd2c6b7bf5e75220df.jpg
Photo of problem ^

Homework Equations

The Attempt at a Solution

http://rs220l32.rapidshare.com/files/101852192/Snapshot_2008-03-23_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]

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

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]

Square root of 2 is 1?

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]

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?

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]

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.

?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 .