Electrostatics

Insulators and Conductors In insulators, electrons are bound in “orbit” to the nucleus in each atom. 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 In conductors electrons can move from atom to atom, thus electricity can “flow”. 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

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.

Charging by Induction Induction uses the influence of one charged object to “coerce” charge flow. Step 1. A charged rod is brought near an isolated conductor. The influence of the charge object polarizes the conductor but does not yet charge it. Step 2. The conductor is grounded to the Earth, allowing charge to flow out between it and the Earth.

Charging by Induction (cont.) Step 3. The ground is removed while the charge rod is still nearby the conductor. Step 4. The rod is removed and the conductor is now charge (opposite of rod). An object charged by induction has the opposite sign of the influencing body. Notice that the original charged object does not lose charge.

Charge Smallest charge unit: charge of 1 electron 1 electron = -1.602x10-19Coulomb