When should I consider the sign of charges when applying Coulomb's Law?

[panther1]

I am studying Physics Vol2 by halliday, resnick and krane. I was attempting the coloumb force exercises..If we put one charge negative and other positive in the formula..then the force comes out to be negative but the instructor manual for the book ignores the negative charge, and take it as positive, then the force comes out as positive...what I want to ask is when do I put negative charge with the force?

 

[norhh]

Same force acts on both the objects.If one object is positive and other is negative then then both come towards each other.Now take a direction as positive let it be right(positive x direction probable you can take reverse but i took like this).let + charge be on- x-axis and - charge on + right axis.say both are equal in magnitude.then they both attract towards origin.so now say where the force is positive and where negative now the + charge move towards +x axis and - charge towards - x axis.Hence this concludes that force on + charge is positive(it is according to our convention) and negative charge is negative.Basically you can get opposite it is all according to sign convention you can take it as your wish.So we need to ignore charge for calculating force.

 

[Chi Meson]

When determining any vector quantity, it is a good practice to determine the direction and magnitude separately.

As norhh correctly states, find the direction of the force depending on the signs of hte charges. Don't rely on your memory either; draw a sketch, clearly showing q1 and q2, and force arrows showing the direction of force on each.

Then apply Coulomb's Law formula to determine the magnitude of force, ignoring the sign of the charges.

Technically, if you do put in the charge signs, a negative result to Coulomb's Law is defined as an attractive force, while positive forces are repulsive. Either way they do not indicate the absolute direction, just "toward" or "away from" the other charge.