Question

q1------------------q2-----------q3
<--------- ---------><---- ----->
Calculate the Coulomb Force on q1 in the figure. Choose forces to right to be positive and to the left to be negative. Calculate the Coulomb force on q2. Calculate the Coulomb force on q3.
The part that I don't understand is how to set up the formula to separate the charges. I think this is the formula I should use: F=k (q1)(q2)/r^2

Answer 1

The formula you've written down is that for the Coulomb force.

The question I have about the problem set up is what is the distance between the charges?

Answer 2

yes that is for Coulomb Force.
The rest of the givens would probably be helpful.
Assume q1 = 1.13μC , q2 = 6.73μC and q3 = -2.84μC. r1 = 2.94 cm and r2 = 1.59 cm.
BTW: Physics is killing me

Answer 3

Ok. Well, calculate away then. The force acting on q1 is the force due to q2 and q3. Calculate them separately and then add them up.

Repeat for q2 with q1 and q3;
and for q3 with q1 and q2.

But also note that the force of q1 on q2 is the negative of the force of of q2 on q1. Hence you don't have to do nine calculations, just four, and then change sign where appropriate.

Answer 4

How would the formula be set up to different for q1 with respect to q2 and so on. That is where my confusion comes in. If I calculate:
F=9.0*10^9((1.13*10^-6) * (6.73*10^-6))/2.94^2 then how do I change it to calculate F for q2.

The magnitude of q is the absolute value, so it should always be positive. How would I change the signs?

Answer 5

No, in the formulae, the charges, q1, q2, q3 can have positive or negative sign.

Answer 6

and\[ F_{1 \ acting \ on 2} = F_{1,2} = - F_{2,1} \]
Similarly,
\[ F_{2,3} = - F_{3,2} \]
\[ F_{1,3} = - F_{3,1} \]

So you only have three calculations to make (not four, I was wrong above).

The sign convention of when a force is positive or negative is given in your question. Suppose you had two positive charges. If the force acts in the positive direction, i.e., to the right, then the sign is positive.

For example, between q1 and q2 is repulsive. Hence the force on q1 due to q2 is to the left, therefore that force has a negative sign. The force of q2 from q1 is to the right, therefore that force has a positive sign.

Answer 7

WOOHOO, finally got it.

Answer 8

Good