How Do You Determine Electric Fields and Forces Between Charges?

[derekbeau]

I have two questions I am having trouble with, if you could please help me that would be great.

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Two point charges, q1 = 4.0×10^6 C and q2 = -1.0×10^6 C, are located on the x-axis at x1 = -1.0 cm and x2 = 3.0 cm

Determine the electric field at the origin

I have no idea how to do this
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Two protons are separated by a distance of 1.0×10^10 m (roughly the diameter of an atom). Calculate the electric force and gravitational force on one proton due to the other proton. The electric force is much larger. What is the ratio of the electric force to the gravitational force?

I thought id find the electric force by:
F = [k(1.6 x 10^-19)(1.6 x 10^-19)] / (10^-20) = 2.304 x 10^-8

And find the gravitational force by:
F = [(g m1 m2) / (10^-20)] = 2/742 x 10^-33

I don't know if those are right, but even if they are i don't know how to format the answer
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I just need some thorough explanation on these two questions, i got all my other ones done.

Thanks

 

[bross7]

Q1
Because you were given that both point charges are along the x-axis essentially you are just being asked to find the net electric field resulting from the two point charges. Using your equation for the electric field you should be able to find the field for each point and from this the net field. Remember that the distance to the origin is not the same for both charges.

Q2
It's been a while but I believe you are right on. You calculated the electric and gravitational forces and indeed the electric force should be considerably stronger.
When setting up a ratio of the two forces it would be better to write it in the form Fe/Fg or Fg/Fe. Either way you will get the ratio of one force to the other.

 

[derekbeau]

ok for Q1 i did

F = kQ / d^2 = [k(4 x 10^ -6)] / 1
F = kQ / d^2 = [k(-1 x 10^-6)] / 9

Then i added these two together but didnt get the correct answer

 

[derekbeau]

nvm i got them