Electric Force of Two Charges on Third Charge

[physgrl]

Homework Statement

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

Homework Equations

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

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/r_1-3²+kq2q3/r_2-3²
Ftotal=kq3((q1/r_1-3²)+(q2/r_2-3²))
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?

 

[gneill]

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

 

[physgrl]

It doesn't specify, so I assumed since the first one is along the x-axis the second one is two.

 

[gneill]

It doesn't specify, so I assumed since the first one is along the x-axis the second one is two.

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?

 

[physgrl]

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

 

[gneill]

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

That would be my guess, yes.