Question

"Two point charges, q1 and q2, are located .3m apart on the x-axis, as show in the figure above (see Twiddla link below). Charge q1 has a value of -3.0X10^(-9)C. The net electric field at point P is zero.
d) Determine the x-coordinate of the point on the line between the two charges at which the electric potential is zero.
e) How much work must be done by an external force to bring an electron from infinity to the point at which the electric potential is zero? Explain your reasoning."

I tried part d, but I think I made stuff up and didn't do it right.

Answer 1

For part e, my initial incline is that it is a conceptual problem, not something that's calculated, because it involves infinity. I this correct?

The diagram: http://www.twiddla.com/1504676

Any and all help is greatly appreciated! :)
a few moments ago

Answer 2

I've been thinking about part e, and my guess is that the answer is infinity joules, but I don't know how to "explain my reasoning" other than to say that since d = infinity anything else being multiplied by it will be inconsequentially minimal and therefore doesn't "affect" infinity.

Answer 3

Any ideas?

Answer 4

Can you explain those answers? :) I'm not sure how to solve it myself, and so I don't know where you're coming from.

Answer 5

electric field = k q/r^2
k = 9 * 10^9
putting the value for both the charges and the put their sum equal to zero
kq1/r^2 = kq2/(3+r)^2

where r = 1m
q1 = -3 * 10^-9

putting these values in the expression you will get the value of q2, which will be 48 * 10^-9

for potential
V = kq/r
again puttting for both charges and the sum is equal to zero

kq1/r = kq2/(3-r)
on putting values you will get the value of r, which is equal to 1/5 0r 0.25m on the right of q1 charge....hence co-ordinate will be -0.75m
last time i made a mistake

for e . there is no potential difference between the infinity and that point hence external work done is zero

Answer 6

Okay, I think I understand...

Answer 7

I'm not entirely sure how you did part d still.

Answer 8

The first step is getting the value for the second charge q2.
We do this by using the fact that the electric field is zero at the point P, and the equation for electric field is kq/r^2.
We add together the electric field from each charge q1, q2 with respect to point P and decide that this sum must be 0 by the information given. We plug in our information with q2 remaining to solve for q2!


The second step, then, is finding the point where electric potential is zero. Assume there is some point between q1 and q2. It lies a distance of r from q1 and thus 3-r from q2 (just completing the interval, 3-r + r = 3; original length of interval). This time, we use electric potential V = kq/r. We add the potential between each point to get zero. Then plug in our information we have, leaving r as a variable to solve for!

Answer 9

And @amonoconnor if there is any part that you are still uncertain of, please specify so we can work on that particular spot! :)

Answer 10

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