Electrostatic Force Coulomb’s Law

Charges Two charges of the same type repel one another ++ The two charges will experience a FORCE pushing them apart or pulling them together Two charges of the opposite type attract one another + -

How much force? The amount of force that two charged objects experience depends on three factors q1q1 q2q2 r 1. The charge on the 1 st object (q 1 ) 2. The charge on the 2 nd object (q 2 ) 3. The distance between them (r) We use Coulomb’s Law to calculate this force The constant for Coulomb’s Law is “k” “k” is the ELECTROSTATIC CONSTANT 8.99 x 10 9 N∙m 2 /C 2

Coulomb vs. Newton Look carefully at the Coulomb’s Law equation This equation looks very much like Newton’s Law of Universal Gravitation! Both have an INVERSE SQUARE relationship between FORCE and DISTANCE F r Gravitational force concerns MASS Coulomb force is about CHARGE

Charge could be measured in electrons, but they are too small to count. Instead we measure charge in Coulombs, which is 6 billion billion electrons (6.25 x electrons/Coulomb), equivalent to about 1 meter of lightning Find the number of electrons in 5 microCoulombs. What is the charge on object with 1 million electrons rubbed onto it? What is the charge on 1 electron?

Which is stronger? Consider two electrons 1μm apart –What is the gravitational force between them? Depends on mass –What is the electrostatic force between them? Depends on charge 5.54 x N 2.30 x N Electrostatic force is MUCH stronger thangravitational force for small, charged objects like electrons and protons!!!

Example #1 What is the electrostatic force between these two objects? r = 2.0 m q 1 = -2C q 2 = +2C A negative answer shows that the force is ATTRACTIVE

Example #2 What is the electrostatic force between these two objects? r = 5 nm q 1 = -5μC q 2 = -2μC A positive answer shows that the force is REPULSIVE

Charles Augustin de Coulomb Born in 1736, died in 1806 Created a device that helped him develop his theories on charges and electric force and field. Torsion Balance reated a device that helped him develop his theories on charges and electric force and field. Torsion Balance

Invented by Coulomb to measure very weak force Contributed in the creation of Coulomb’s Law by measuring Coulomb’s Constant This same device was also used by Henry Cavendish to find the gravitational constant. Coulomb ’ s sketch of his invention

How It Works When an unknown force is applied to the metallic object or plates in the container, the object spins. The greater the force is, the bigger the angle.

What is Coulomb’s Law The magnitudes of the electrostatic force between two point electric charges are directly proportional to the product of the magnitudes of each charge and inversely proportional to the square of the distance between the charges. The strongest formula in the universe

Practice Find the electrical repulsion between two balloons that are 2 meters apart each carrying a charge of -2 x Coulombs