Three negative point charges lie along a line as shown in the figure.

+2 votes

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Find magnitude and direction of the electric field this combination of charges produces at point P, which lies 6.00{\rm cm} from the -q_2 charge measured perpendicular to the line connecting the 3 charges. Assume q_1 = 5.50\mu C and q_2 = 2.50\mu C

I was able to get the x components and I know the net force for the y components is zero. I also know the direction would be toward -q2.

For q1 I got F=[kq1/(0.08^2)]*(0.06m/0.08m)^2 and for q2 F=kq2/(0.06^2)

Do I plug in all the numbers and then use the forces to find the electric fields of each charge and then add them together?

asked Jul 13, 2012 in Physics by physicsstudent (45 points)

2 Answers

+3 votes

There is no need to calculate the Force in this equation. There is enough information to just calculate the electric field.

The formula for electric field is as follows:

where ke = 8.988 * 10^9

q = charge

r = distance

However, since there is more than one charge, you would need to take the sum to get the total electric field.

This formula will reflect that:

Plug in the values into the formula, and take the sum and that should get  you the correct answer.

Note that the distance, r, will be different for q1, you can find this r by using pythagorean theorem.

Please comment if you need more help.

answered Jul 13, 2012 by pokemonmaster ~Expert~ (3,854 points)
edited Jul 13, 2012 by pokemonmaster
Your math must be off? Try checking your conversions. Feel free to post your steps and conversions so we can see what you're doing wrong.
I did, multiple times. Can you please tell me what I'm doing wrong?
I converted 6 cm to 0.06m and for d in the q1 direction I got 0.1m after doing the Pythagorean theorem. And for the charges, my conversion were 5.50*10^-6 C for q1 and 2.50*10^-6 C for q2.
For E1 I got 2(4.94*10^6) and for E2 I got 6.24*10^6. I took those two numbers and added them together to get the 1.61*10^7 N/C which is wrong.
i believe you need to add them up as vectors?  But this should only have the x component of the vector since it appears the y components will cancel.  I will post how you to find the vector sum in a separate answer.
+1 vote

Here is a page from my physics book on how to approach this type of problem:

You first find E1 and E2, then you calculate the vector components. In this case you only need the Ex, since the Ey will cancel out.

answered Jul 14, 2012 by kirby ~Expert~ (3,020 points)
Nevermind, I figured it out.
I was using the d for q1 as 0.08 instead of as 0.1m even though it was right when i drew it out.
Sorry about that. Thank you very much for all the help!
No problem, glad to have helped.