Coulomb’s Law states that the force of charged object One on charged object Two, F12, equal to the force of charged object Two on charged object One, F21, (this is Newton’s Third Law), the magnitude of the force is given by

F is the force, r is the distance between the particles.

qb and qa are the-charges of particlea and particle b

and finally k is a constant and k=8.99x109 (Nm2/C2).

to calculate the magnitude of the force you should ignore the signs of the charges q_{a} and q_{b} .

on each particleThe direction of the-force vector is toward the-other if the one charge is positive,and the other neggativee (´´opposites attract´´); or directly away from theother if both of thecharges are positive or both negative (´´like charges repel each other´´).

Notethat the-force falls off quadratically with-distances, similarly to-the behavior ofthe gravitational-force.

if charge is in the presence of several b,c.... the force that a feels is the sum of the force vectors due to the remaining charges. like oter vector addition problems., the total force is computed by adding components, although it can also be found by a general formula which inthe case of three charges reads.

for the force on a due to b and c, with rab the separation between a and b.

Definition of a Coulomb

The Standard International Unit used to measure electric charge is the Coulomb (abbreviation C). The actual definition of this unit cannot be explained until later in the course when we study magnetism.

The charge of a proton is equal to e = 1.602x10-19 C. The charge of an electron is the negative of that.

Analogy between gravitational forces and electric force

The gravitational forces and electricity force have similar form:

first difference is that gravitational forces are always attractive, while the electric force can be either repulsive or attractive.

Another difference is that the electric force is generally much stronger:

k = 8.99x109 Nm2/C2 G = 6.67x10-11 Nm2/kg2

On the other hand, the gravitational force-on a large body such-as the entire earth can be much larger-than the electrical-force because-the earth contains nearly equal numbers of electrons and protons-and hence its-total electric charge-is small.

Electric Potential Energy

You include the signs of the charges when you use this formula (unlike the case of calculating the a force magnitude ), so if the two charges have the same sign and negative otherwise the potential energy is positive .

For a set of charges, qa,qb,qc,... the potential energy is the sum of the potential energies-of the pairs. For instance, if there are 3 charges, the net-potential energy is;

Potential-energy is not a vector quantity, so that the contributions from-various charges add up algebratically in this easy way.