# Electric Force physics question

1. Aug 28, 2009

### GreyGus

1. The problem statement, all variables and given/known data

Three positive charges are located in the x-y plane (see plot below), with Q1=3.10 μC, Q2=5.20 μC and Q3=6.30 μC. Note that the charges are located at grid intersections, and that the x and y coordinates are in cm.
dynamically generated plot
A. Calculate the magnitude of the electric force on Q1 due to Q2.

2. Relevant equations
F=kq1q2/r^2

3. The attempt at a solution
Fx=k(3.1*10^-6)(5.2*10^-6)/((4/100)^2)
Fy=k(3.1*10^-6)(5.2*10^-6)/((3/100)^2)
Then for the magnitude: sqrt((160.977)^2+(90.54)^2))
Thank you in advance because I really cannot figure out what I'm doing wrong.

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2. Aug 28, 2009

Dear GreyGus;
The solution is just as simple as two steps:
1- Calculate the distance between the two charges using Pythagoras theory (i.e. r = sqrt((the x component of distance)^2 + (the y component of distance)^2)).
2- Calculate the electeric force exerted on Q1 by Q2 using coloumb's law
(F= kQ1Q2/r^2). and hen proceed.

3. Aug 28, 2009

### GreyGus

Thank you very much. I appreciate it.

4. Aug 28, 2009

### GreyGus

ok so I did
x=.04 m
y=.03m
3.10 uC=.000003 C
5.20 uC=.0000052 C
r=sqrt((.04^2)+(.03^2))=.05
F=(9*10^29)(.000003)(.000005)/(.05^2)=5.803e21 N but that's wrong. What am I doing wrong now?

5. Aug 29, 2009

The error would be with the proportionality constant k, it's (8.9*10^9) not (8.9*10^29).
I hope this makes sense

6. Aug 29, 2009

### GreyGus

Oh I see, Thank you very much for your help.

7. Aug 29, 2009

### GreyGus

8. Aug 30, 2009

### GreyGus

How would you find the x-component of force q1 to q3? I have tried:
r=.1
f=k(3.1*10^-6)(5.2*10^-6)/(.1^2)
tantheta=(.06/.08)=36.9
x-component=11.61 N. But that's not right. What did I do wrong on this one?

9. Aug 30, 2009

### Staff: Mentor

Make sure you're using the right charge.

10. Aug 30, 2009

### GreyGus

Oh my bad. Ok so I got the answer, but I want to know why is the answer negative?

11. Aug 30, 2009

### Staff: Mentor

The x-component of the force on q3 should not be negative.

12. Sep 1, 2009