Electrostatics GIRL SAFELY CHARGED TO SEVERAL HUNDRED THOUSAND VOLTS GIRL IN GREAT DANGER AT SEVERAL THOUSAND VOLTS

The Nature of Electric Charge Properties of charge Like charges repel, and unlike charges attract. Charge is conserved, meaning it cannot be created or destroyed, only transferred from one location to another. In all atoms, electrons (qe) have negative charge and protons (qp) have positive charge. click for animation

Insulators and Conductors When charge is placed on an insulator, it stays in one region and does not distribute. Wood, plastic, glass, air, and cloth are good insulators. Conductors CHARGED INSULATOR When charge is placed on a conductor, it redistributes to the outer surface. Metals (copper, gold, and aluminum) are good conductors. CHARGED CONDUCTOR

Polarization Polarization is the separation of charge In a conductor, “free” electrons can move around the surface of the material, leaving one side positive and the other side negative. In an insulator, the electrons “realign” themselves within the atom (or molecule), leaving one side of the atom positive and the other side of the atom negative. Polarization is not necessarily a charge imbalance!

Charging by Friction POSITIVE Rabbit's fur Glass Mica Nylon Wool Cat's fur Silk Paper Cotton Wood Lucite Wax Amber Polystyrene Polyethylene Rubber ballon Sulfur Celluloid Hard Rubber Vinylite Saran Wrap NEGATIVE When insulators are rubbed together, one gives up electrons and becomes positively charged, while the other gains electrons and becomes negatively charged. Materials have different affinities for electrons. A triboelectric series rates this relative affinity. A material will give up electrons to another material below it on a triboelectric series. Common examples of charging by friction: • small shocks from a doorknob after walking on carpet with rubber-soled shoes • plastic foodwrap that sticks to a container • sweater pulled over your head that sparks • laundry from the dryer that clings • balloon rubbed with hair sticks that to a wall click for applet

Charging by Conduction When a charged conductor makes contact with a neutral conductor there is a transfer of charge. CHARGING NEGATIVELY CHARGING POSITIVELY Electrons are transferred from the rod to the ball, leaving them both negatively charged. Electrons are transferred from the ball to the rod, leaving them both positively charged. Remember, only electrons are free to move in solids. Notice that the original charged object loses some charge. click for animation

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

Electrostatic Charges ATTRACTION AND REPULSION A New Fundamental Physics Quantity 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 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 Charles Coulomb’s Torsion Balance A torsion balance measures the force between small charges. The electrostatic force depends directly on the magnitude of the charges. The force depends inversely on the square of distance between charges (another “inverse square law”)! TORSION BALANCE COULOMB’S LAW OF ELECTROSTATIC FORCE The constant of proportionality, k, is equal to 9.0 x 109 Nm2/C2. constant charges A negative force is attractive, and a positive force is repulsive. electrostatic force distance The sign (+ or –) is different from a vector direction (left or right)