##### Electric field, homework help

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Jan 18th, 2016

Thank you for the opportunity to help you with your question!

This thing really frustrated you for a long time: Here is a simple solution to help you solve this and others:

Could be you are missing on a very simple step:

Explore this example carefully, see the mistake then replace the numbers with your own figures:

Two point charges lie on the x axis. A charge of + 6.5 µC is at the origin, and a charge of -1.5 µC is at x = 10.0 cm.

What is the net electric field at x = -2.0 cm?
I found the electrical field for each charge using the equation

E= k(q)/ r^2.

My calculations were:
For the first charge- (8.99e9)(6.5e-6)/(.2^2) = 1460875 and (8.99e9)(1.5e-6)/(.2 + .1)^2 = 149833.33

For the second. I added the two to determine the net field- 1610708.33N/C.

This answer was incorrect. So then I used -1.5e-6, recalculated and got net of 1311041.67; also incorrect.

Can anyone tell me if I am I using the wrong equation or doing something else wrong?

You are doing something else wrong. The distance from charge 1 to the point where the field is desired is 0.02 m, not 0.2. Likewise for charge 2, it is 0.1 + 0.02m , not 0.1 + 0.2.

Here is how it is done:

http://farside.ph.utexas.edu/teaching/302l/lectures/node21.html

Please let me know if you need any clarification. I'm always happy to answer your questions.
Jan 18th, 2016

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Jan 18th, 2016
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Jan 18th, 2016
Aug 21st, 2017
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