Electric Forces and Electric Fields CHARLES COULOMB (1736-1806) MICHAEL FARADAY (1791-1867)

Electrostatic Charges ATTRACTION AND REPULSION A New Fundamental Physics Quantity Electrostatic charge is a fundamental quantity like length, mass, and time. The symbol for charge is q. The SI unit for charge is called the coulomb (C). ATTRACTION AND REPULSION The charge of an electron (qe) is -1.6 x 10-19 C The charge of a proton (qp) is 1.6 x 10-19 C Common electrostatic charges are small: millicoulomb = mC = 10-3 C microcoulomb = C = 10-6 C nanocoulomb = nC = 10-9 C

The Electrostatic Force COULOMB’S LAW OF ELECTROSTATIC FORCE constant charges electrostatic force distance The constant of proportionality, k, is equal to 9.0 x 109 Nm2/C2. A negative force is attractive, and a positive force is repulsive.

The Electrostatic Force EXAMPLE 1 - Find the force between these two charges EXAMPLE 2 - Find the net force on the left charge

Coulomb’s Law The force between two charges gets stronger as the charges move closer together. The force also gets stronger if the amount of charge becomes larger.

Coulomb’s Law The force between charges is directly proportional to the magnitude, or amount, of each charge. Doubling one charge doubles the force. Doubling both charges quadruples the force.

Coulomb’s Law The force between charges is inversely proportional to the square of the distance between them. Doubling the distance reduces the force by a factor of 22 = (4), decreasing the force to one-fourth its original value (1/4).