LAW OF ELECTRIC CHARGES WHAT IS AN ELECTRIC CHARGE? LAW OF ELECTRIC CHARGES Opposite charges attract Similar charges repel each other Charged objects attract neutral objects PLASMA One of four states of matter solid, liquid, gas, plasma Like gas, no definite shape or volume Ionized gas which is electrically conductive and responds to magnetic fields Under a magnetic field it can form filaments, beams and double layers Has unbound positive and negative charges Most common form of matter in the universe In stars and intergalactic regions (also in Neon signs) CHARGING BY FRICTION Charges (electrons) are transferred when two objects are rubbed together Based on electronegativity (the ability to attract electrons) which depends on atomic radii and # of protons CHARGING BY CONTACT Charged when objects touch Electrons repel each other. If there are an excess of electrons, they will push each other off the materials (assuming they are allowed to flow) CHARGING BY INDUCTION Charge is created through a repulsion between charged objects LAW OF CONSERVATION OF CHARGE What do you think this means? (How many law of conservation of _________ do you know of?) The total charge (the difference between the amounts of negative and positive charges) within an isolated system is conserved CHARGE q = ne q Net Charge (C - Coulombs) n Number of excess charges (electrons or protons) e = x C/charge If we have protons, charge is positive If we have electrons, charge is negative COULOMBS LAW WHAT ARE THE SIMILARITIES AND DIFFERENCES BETWEEN ELECTRIC AND GRAVITATIONAL FIELDS? EXAMPLE The electrostatic force between two small charged objects is 7.0x10 -6 N. A) Calculate the force if the distance between charges is tripled. B) Calculate the force if the charge on one object is halved while the other is increased by four times. C) Calculate the force if both a and b occur EXAMPLE Charge sphere X and Y are in a fixed position and have charges as shown below: Calculate the net force on Z. q X = 5.5x10 -7 C q Y = -3.4x10 -8 Cq Z = 8.3x10 -7 C d= 40.0cm d= 15.0cm Ans: Fnet = 2.3 x N [right] EXAMPLE Calculate the net force on sphere # 1. q 1 = 4.0C d= 50.0cm q 2 = -3.0C q 4 = +5.0C q 3 = -3.0C Fnet = 0.25 N x 10 N [S 45 o E ]