Hi, I have an online homework assignment due Tuesday. I have a professor that is

Question

Hi,

I have an online homework assignment due Tuesday. I have a professor that is from China and I have a hard time understanding his class and then trying to understand the textbook. Out of the 30 exercises assigned, I have around 7 left which I am needing help.

Exercise: Two punctual charges Q1 = +9.0 µC and Q2 = -5.0 µC are localized in x=0 (Q1) and in x = 10 cm (Q2) along the x axis as shown in the figure. What is the x coordinate in where the net force acting that point on a third charge Q3 placed in this point is zero?

Image: http://img408.imageshack.us/img408/7733/newpicture1ft.png

Any input as to how to reach the answer will bre greatly appreciated.

Thanks!

Explanation / Answer

Electric Field is = Kq/r² K is a constant 9E9 we want to find a point where force is 0 so where the electric field is 0 0 = kq1/r² + kq2/(.1m - r)² 0 = (9E9 * 9E-6) /r² + (9E9 * -5E-6)/(.1 - r)² 0 = 81000/r² - 45000/(.1 - r)² plug this into a graphing calculator the root is at .0572949m now plug back in to check 0 = 81000 /.0572949² - 45000(.1-.0572949)² 0=0 so at the point .0572949m or .0573m rounded is where the electric field is 0 so that is where the force would be 0 if a third charge was placed at that pint

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