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**Stuck on two electric field questions....**

Hi guys,

I have these two questions which I am unable to answer.

This is the first one:

Calculate the magnitude of the electric field at one corner of a square 1.26 m on a side if the other three corners are occupied by 2.80×10-6 C charges .

Hint: Draw the diagonal through the square from the lower left with one of the 3 charges to the upper right with the point where the electric field is wanted; the resulting field due to the 2 other source charges on either side of the diagonal points along the diagonal (why ?); calculate the magnitude for the vector sum; add this value to the field due to the charge on the lower left which also points along the diagonal (simply add the numbers).

I don't really even know how to start....(there is an example like this in my text book 'physics'by Giancoli, but it is quite different from this question)

the second one:

A positive point charge Q1 = 3.43×10-5 C is fixed at the origin of coordinates, and a negative charge Q2 = -7.27×10-6 C is fixed to the x axis at x = +1.97 m. Find the location of the place along the x axis where the electric field due to these two charges is zero.

E = kQ/r^2

I set up E = 0 = 308700/1.97^2 - 65430/1.97^2

I set it to equal zero, but obviously that is incorrect.

I think I'm getting confused because one of the charges is set at the origin, it threw me off a little bit. So I know that for the first part the "r" should not be 1.97....

Any help is appreciated.... thanks....

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