# Coulomb's Law formula help

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1. Oct 18, 2014

### Mary O'Donovzn

• OP warned about not using the homework template
How do you calculate compute the the magnitude of the total force of three charges and also the angle it makes with the x-axis? Knowing the magnitude and also the 2d co ordinates of the charges.

I have deliberately not given any specific values because this is not homework its a question from old exam paper and I want to know how to do it because I have a gut feeling it will be on the mid term and I honestly don't know what to do so if somebody could give me a step by step that would be great!

The only thing I know for definite I use the below formula but that's the only a clue I got from my tutor but I don't know how to use it I don't even know where to start I have no examples or anything.

K0 * Q1.Q2 / d^2 * r hat

Thanks :)

2. Oct 18, 2014

### Staff: Mentor

I assume you need to find the total force on one of the charge due to the others? Just find the force exerted by each charge separately. (Coulomb's law will tell you that.) Then add up the force vectors to find the total.

FYI: Even though it's not strictly homework, problems like this (or textbook, exam, coursework, homework type problems in general) do belong in the HW section.

3. Oct 18, 2014

### Simon Bridge

Do you know how to work out the magnitude and direction of the force on one charge due to only one other charge?

4. Oct 19, 2014

### Hawzel Abel

it is easy to understanding，but to me it is difficult to display in English。

5. Oct 19, 2014

### Hawzel Abel

Do you want to calculate the force of one charge by the other two charge, the magnitude of the two forces are directly related to the vector sum.

Last edited by a moderator: Oct 19, 2014
6. Oct 19, 2014

### Staff: Mentor

Have you studied how to add vectors?

7. Oct 19, 2014

### Brian_H

As I see it, you must need to study about Vector. Vector is very important in Physics as well as Mathmatics

8. Oct 20, 2014

### Mary O'Donovzn

One I find the force F12, F13 and F23 do I add them all up?
I don't know how to work out the magnitude of a unit vector which seems to be called R hat? Like its something to do with cosi + sinj
Do Vector components come into this at any point and if so how are they calculated?

9. Oct 20, 2014

### Staff: Mentor

That depends on exactly what you are asked to find. Can you post the complete question, word for word as it was given?

The magnitude of a unit vector is 1. (That's why it's called a unit vector.) Its purpose is to indicate direction.

You'll undoubtedly need to add vectors together. Finding and adding their components is one way to do it. Read up on vectors here: Basic Vector Operations

10. Oct 20, 2014

### Mary O'Donovzn

The question I posted is the full question like I just took out the values which were in brackets. Is it unsolvable?

So with regards to the unit vector do I just but the distance it is over one and get r hat? I dont use cos θ or sinθ?

okay thank you I will look at the link

11. Oct 20, 2014

### Staff: Mentor

It's still a bit unclear to me.

How many charges are there? The total force of the three charges on what? Is there a diagram to go along with it?