# Electromagnetism - Need Help for Test

I know I'm suppose to use the equation F = (kq1q2)/r^2

I tried doing [.1/sq. root (2)] m for two of the forces and just .1 m for one. I plug all the numbers into the equation and add up the 3 but it doesn't work.

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dynamicsolo
Homework Helper
View attachment 121483

I know I'm suppose to use the equation F = (kq1q2)/r^2

I tried doing [.1/sq. root (2)] m for two of the forces and just .1 m for one. I plug all the numbers into the equation and add up the 3 but it doesn't work.
Don't forget that the forces you found have to be added as vectors. (You did realize that the forces from the charges on the y-axis cancel.) What angles do the other forces from the two charges above and below the x-axis make to the x-axis? How do you add up such vectors?

BTW, since all of those charges are on a semicircle, they are all at the same distance from the charge at the center, so you would use the same distance, 0.1 m., for all of them.

Well you would do c^2 = a^2 + b^2. Since it is a 45 degree angle, you would do .1/sq. root of 2 for two of the vectors. You would just use .1 m for the third vector. I've plugged in the number and added the 3 together and it doesn't work.

Also, can you help me on the 2nd question?

dynamicsolo
Homework Helper
Well you would do c^2 = a^2 + b^2. Since it is a 45 degree angle, you would do .1/sq. root of 2 for two of the vectors. You would just use .1 m for the third vector. I've plugged in the number and added the 3 together and it doesn't work.
Wait a minute -- let's sort this out first. The distance of the other two charges from the center is not 0.1/sqrt(2) ; all of the charges are on the same circle. So the magnitude of all of the forces is the same

kQq/(0.1^2) .

What are the x-components (since I see there is a note about that pencilled onto the diagram) for each of the charges? The sum of all the x-components gives you the x-components of the total force.

What happens to the sum of the y-components?

Also, can you help me on the 2nd question?
What they are asking for here is the function of the electric field strength on the x-axis for all values of x. You have Coulomb's Law,

F = k(q_1)(q_2)/(r^2) ,

to work with. What can you say about the way the fields of the two individual charges point anywhere along the x-axis? That will tell you how to add up the terms that Coulomb's Law will give you for each charge.

Now, there are three regions to think about along the x-axis. They've placed q_1 at the origin (x = 0) and q_2 a distance d to the right (x = d). So you need to look at the intervals

x < d , 0 < x < d , and x > d.

First off, which way do the fields from each charge point in each of those regions?