# MasteringPhysics.com problem (Electric Field)

In summary: Glad I could help. In summary, the net force exerted by the two point charges on the third charge is approximately -1.48 * 10^13 N. This is a large force due to the strong electric fields created by the two point charges.
Two point charges are located on the x axis: one charge, q_1 = -19.5 nC, is located at -1.660 m ; the second charge, q_2 = 38.0 nC, is at the origin (x=0).

What is the net force exerted by these two charges on a third charge q_3 = 48.5 nC placed between q_1 and q_2 at x_3 = -1.210 m?

Note that your answer may be positive or negative, depending on the direction of the force.

Use 8.85×10−12 C^2/(N \cdot m^2) for the permittivity of free space.

I have no idea how to do this. any help?
thank you

Use superposition.

You must show some work or thought on the problem to get help here. We do not do homework problems for people, we help people do their own homework problems.

So, give me some thought on this problem. Can you find the force from one of the charges? What about the force from the other one then? How do you go about adding the two quantities?

Hay, I'm stuck on the same problem but have some working. :P

e = 8.854 x 10^-12

Consider two point charges located on the x axis: one charge, q_1 = -11.5 nC, is located at x_1 = -1.665 m; the second charge, q_2 = 38.5 nC, is at the origin (x=0.0000). What is the net force exerted by these two charges on a third charge q_3 = 53.5 nC placed between q_1 and q_2 at x_3 = -1.215 m?

MasteringPhysics is saying its wrong. :(
http://files.trozza.com/Uni/2010/Physics%201a/Electromagnatism.jpg

I only have one attempt left and I'm not sure what I could be doing wrong.. Perhaps it should be positive?

Kind Regards,
Troz

Trozza said:
Hay, I'm stuck on the same problem but have some working. :P

e = 8.854 x 10^-12

Consider two point charges located on the x axis: one charge, q_1 = -11.5 nC, is located at x_1 = -1.665 m; the second charge, q_2 = 38.5 nC, is at the origin (x=0.0000). What is the net force exerted by these two charges on a third charge q_3 = 53.5 nC placed between q_1 and q_2 at x_3 = -1.215 m?

MasteringPhysics is saying its wrong. :(
http://files.trozza.com/Uni/2010/Physics%201a/Electromagnatism.jpg

I only have one attempt left and I'm not sure what I could be doing wrong.. Perhaps it should be positive?

Kind Regards,
Troz

You missed a negative on F2.

Thanks for the prompt reply.

So I should be getting something around (-)1.48 *10^13?

The 10^13 part seems very large and wrong to me. Its something like 14.7 Tera Newtons... Awfully large force..

Trozza said:
Thanks for the prompt reply.

So I should be getting something around (-)1.48 *10^13?

The 10^13 part seems very large and wrong to me. Its something like 14.7 Tera Newtons... Awfully large force..

Charge is given in nano-coloumbs (nC) not coulumbs.

zachzach said:
Charge is given in nano-coloumbs (nC) not coulumbs.

Thanks I missed it. :P So I re-did the calculations based on your help, and I think it produces a much more sensible answer. Does this look correct? The magnitude is at least looking better :P

http://files.trozza.com/Uni/2010/Physics%201a/ElectromagnatismUpdate.jpg

Trozza said:
Thanks I missed it. :P So I re-did the calculations based on your help, and I think it produces a much more sensible answer. Does this look correct? The magnitude is at least looking better :P

http://files.trozza.com/Uni/2010/Physics%201a/ElectromagnatismUpdate.jpg

Draw the electric field lines to see which way they point on the x-axis. They both add but both are in the negative x direction.

zachzach said:
Trozza said:
Draw the electric field lines to see which way they point on the x-axis. They both add but both are in the negative x direction.

Sweet, I think I have it now..

http://files.trozza.com/Uni/2010/Physics%201a/ElectromagnatismUpdate2.jpg
But minus... I didn't write that bit in before scanning.

Right.

Sweet, thanks Heaps Zach, just typed it into mastering physics and it said correct.

Sweet, no problem.

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## 3. What is an Electric Field?

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