Electrostatics Getting a Charge Out of Physics

What is Electrostatics? The study of stationary electric charges and electric fields - all ordinary matter contains both positive and negative charges. - most matter is electrically neutral because it contains the same number of positive and negative charges

Conservation of Charge Electrical charge is conserved Objects can become charged by gaining or losing electrons – not through the creation of charges The net charge is also called excess charge because a charged object has an excess of either positive or negative charges. - positive charge comes from the loss of electrons - negative charge comes from gaining electrons A tiny imbalance in either positive or negative charge on an object is the cause of static electricity.

Magnitude of Charge The unit of electric charge is the coulomb ( C ). - one coulomb is equal to the charge of 6.02 x 1018 electrons or protons The magnitude of the charge on the electrons and protons is the same 1.602 x 10-19 C this magnitude of charge is referred to as elementary charge – represented by ( e )

Quantization of Charge The quantity of charge found on all charged objects will always be a multiple of e q = n e Charge can be 2e, 3e, 4e etc… it can’t be 0.5e or 1.4 e an object with a charge of -4e has gained 4 electrons and has a negative charge an object with a charge of 3e has lost three electrons and has a positive charge n = the number of electrons moving q = net charge

Electric Force Electric forces are created between all electric charges. Because there are two kinds of charge (positive and negative) the electrical force between charges can attract or repel.

Coulomb’s Law Coulomb’s law relates the force between two single charges separated by a distance. Constant 9 x109 N.m2/C2 Force (N) F = k q1 q2 d2 Charges (C) Distance (m)

Basic Implication of 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.

The force between two charges is directed along the line connecting their centers. Electric forces always occur in pairs according to Newton’s third law, like all forces.

The Specifics of 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 an Inverse Square 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). This relationship is called an inverse square law because force and distance follow an inverse square relationship.

Coulomb’s Law vs. Newton’s Universal Law of Gravitation Newton’s Law F = k q1 q2 d2 F = G M m d2 k = 9 x109 N.m2/C2 G = 6.67 x10-11 N.m2/kg2 Electric force is much stronger than gravitational force Both are inverse square laws – the magnitude of both forces vary as the inverse of the distance between charges or masses Gravitational force is always an attractive force, electrical force is both attractive and repulsive