Electrostatics - (CAPS) Grade 11

Coulomb’s Law

From grade 10 you should be familiar with positive and negative charges. Now, in grade 11, we learn how charged particles interact with one another. 

Like charges repel, and unlike charges are attracted to one another. The Electrostatic force is the force which attracts/repels them.

Coulomb’s Law says that this force is proportional to both charges, and inversely proportional to the square of the distance between them:

Electrostatic Force 

We add a proportionality constant, 𝑘 = 9 × 109𝑁.𝑚2.𝐶−2, so that we can define the Electrostatic force as:

Using this formula will give us the magnitude of the force, but to get the direction we must see if the charges are like (repelling) or unlike (attracting). It won’t make sense if you get a negative answer for your magnitude, so double check your calculation!

Electric Field

The direction of the electric field at a point represents the direction of the force a positive charge would experience (and the opposite direction for a negative charge).

If we want to determine the magnitude of the force a particle, with charge q, will experience in an electric field we would use the following formula:

Drawing Electric Fields

When drawing the field lines, remember that the lines represent the force a particle charged with +1C would experience!