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

The Nature of Electric Charge Discovery of charge The Greeks first noticed electric charged by rubbing amber with fur, then picking up bits of matter. The Greek word for amber is elektron. Benjamin Franklin arbitrarily called the two kinds of charge positive and negative. In most cases, only the negative charge is mobile. 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. Charge is quantized, meaning it comes in discrete amounts (like money). total charge = integer x fundamental unit of charge click for animation

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 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

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. click for animation click for animation 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 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 an 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 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)

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

Electric Field Strength Field Theory Visualizes Force At A Distance DEFINITION OF GRAVITATIONAL FIELD DEFINITION OF ELECTRIC FIELD q0 is a small, positive test charge Electric field is a vector quantity SI unit of electric field click for applet

Electric Field Lines Single Point Charges POSITIVE CHARGE NEGATIVE CHARGE Density of field lines indicates electric field strength Inverse square law obeyed click for applet Definition of E Field for single point charge constant charge electric field distance

Electric Field Lines Electric fields for multiple point charges click for applet click for applet click for applet POSITIVE AND NEGATIVE POINT CHARGES TWO POSITIVE POINT CHARGES OPPOSITE MAGNETIC POLES ALIKE MAGNETIC POLES

Electric Potential Energy Electric Potential Energy versus Gravitational Potential Energy FALLING MASS VS. FALLING CHARGE STORING POTENTIAL ENERGY Electric potential energy (EPE) is stored when a charge is moved within an electric field. POTENTIAL ENERGY GAIN OR LOSS positive (+) charge negative (–) toward E loses PE gains PE opposite E EPE can be converted to kinetic energy, heat, light, sound etc. EPE is a scalar quantity measured in joules (J).

EPE for Constant Electric Field CONSTANT GRAVITATIONAL FIELD CONSTANT ELECTRIC FIELD electric potential energy charge distance E field

PE for Two Point Charges Potential Energy is force times distance (like WORK!) constant charges electric potential energy distance Potential energy is positive for like charges Potential energy is negative for opposite charges Potential energy is zero at infinite distance

Potential Difference (Voltage) Electric potential is average energy per charge. Potential difference is the difference in energy values of the charge in two separate locations in the electric field. Potential difference is often called voltage. Voltage is only dangerous when a lot of energy is transferred. source voltage (V) common dry cell 1.5 car battery 12 household (US) 120 comb through hair 500 utility pole 4,400 transmission line 120,000 Van de Graaff 400,000 lightning 1,000,000,000 SI Units click for web page Voltage, like energy, is a scalar. A volt (v) is the unit for voltage named in honor of Alessandro Volta, inventor of the first battery.

Potential Difference (Voltage) A SEVERAL THOUSAND VOLT POWERLINE CAN ILLUMINATE A FLUORESCENT LIGHT A PARACHUTE ACCIDENT LANDED THIS MAN ON A 138,000 THOUSAND VOLT LINE, BUT HE SUFFERED ONLY MINOR BURNS

Potential Difference (Voltage) Batteries are sources of potential difference. Inside a battery, a chemical reaction occurs that creates a separation of charge: + at one end, - at the other. This separation is maintained as long as the terminals of the batteries are not connected. When the terminals are connected by a conductor, the electrons, attracted to the positive terminal, flow through the conductor.

Summary of Electrostatic Equations Electrostatic Force Potential Energy for constant E field force between two charges Electric Field for two charges Potential Difference definition definition for point charge for point charge for constant E field