# 8.3 Electric Force

#coulomb'slaw

#electricity

#force

#electrostaticconstant

written by peter apps

April 16, 2020

## Enduring Understanding 3.C

At the macroscopic level, forces can be categorized as either long-range (action-at-a-distance) forces or contact forces.

### Essential Knowledge 3.C.2

Electric force results from the interaction of one object that has an electric charge with another object that has an electric charge.

## Coulomb’s Law

As described in 8.2, charged objects attract or repel each other based on their charges. Coulomb’s Law lets us calculate the amount of force this attraction or repulsion exerts. In the equation above:

• F = Force in Newtons

• k = Electrostatic Constant (on your reference tables)

• q = charge on the objects in Coulombs

• r = distance between the objects in meters

This equation leads to the following conclusions:

1. More charge => More Force

2. More distance => Less Force (Inverse Square Law, see graph below) Just as a review, there are many similarities between this electrostatic force equation and Newton’s law of Universal Gravitation. Both equations are inverse square laws (meaning they both have an r^2 term in the denominator). The key differences between them are that k is significantly greater than G (about 10^20 times larger) and that Fe can be attractive or repulsive. The fact that Fe is so much greater than Fg means that it only takes a small amount of charge difference to have a noticeable electrostatic force, while to see a gravitational attraction takes very large amounts of mass.     Watch AP Physics live streams here for more help.