# Homework Help: Electric Force

1. Jan 25, 2012

### physgrl

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

1. Charge q1= 1.00*10^-9C is located +0.01 m from the origin along the x-axis. Charge q2= 2.00*10^-9C is located +0.03 m from the origin. What is the electric force exerted by these two charges on a third charge, q3= 1.5*10^-9C, located at the origin?

a. 1.65*10^-4N

*b. 1.4*10^-4N

c. 4.4*10^-4N

d. 6.1*10^-4N

2. Relevant equations

F=kq1q2/r^2
Ftotal=F1+F2...Fn

3. The attempt at a solution

both charges q1 and q2 exert a repelling force in the same direction with different magnitudes on charge q3

Ftotal=Fq1+Fq2
Ftotal=kq1q3/r1-32+kq2q3/r2-32
Ftotal=kq3((q1/r1-32)+(q2/r2-32))
Ftotal=9x109*1.5x10-9(1.0x10-9/(.01)2)+(2.0x10-9/(.03)2))
Ftotal=1.65x10^-4

the answer key says it is supposed to be B but I get A...what am I doing wrong?

2. Jan 25, 2012

### Staff: Mentor

Are you sure that all three charges are located along the x-axis?

3. Jan 25, 2012

### physgrl

It doesnt specify, so I assumed since the first one is along the x axis the second one is two.

4. Jan 25, 2012

### Staff: Mentor

Suppose, just for the sake of curiosity, that the second charge was located at y = +0.03m on the y-axis. What would the net force magnitude be on q3 at the origin?

5. Jan 25, 2012

### physgrl

1.38x10^-4 so it was supposed to say it was at the y axis

6. Jan 25, 2012

### Staff: Mentor

That would be my guess, yes.