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

The Nature of Electric 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. Charge is conserved, meaning it cannot be created or destroyed, only transferred from one location to another. Benjamin Franklin arbitrarily called the two kinds of charge positive and negative. In most cases, only the negative charge is mobile. In all atoms, electrons (q e ) have negative charge and protons (q p ) have positive charge. Discovery of charge Like charges repel, and unlike charges attract. Properties of 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. 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. Insulators Conductors Wood, plastic, glass, air, and cloth are good insulators. Metals (copper, gold, and aluminum) are good conductors. CHARGED INSULATOR CHARGED CONDUCTOR

Polarization In a conductor, “free” electrons can move around the surface of the material, leaving one side positive and the other side negative. Polarization is the separation of charge 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 Materials have different affinities for electrons. A triboelectric series rates this relative affinity. 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. plastic foodwrap that sticks to a container sweater pulled over your head that sparks laundry from the dryer that clings Common examples of charging by friction: A material will give up electrons to another material below it on a triboelectric series. small shocks from a doorknob after walking on carpet with rubber-soled shoes 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. 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. CHARGING NEGATIVELYCHARGING POSITIVELY click for animation Notice that the original charged object loses some charge.

Charging by Induction 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. Induction uses the influence of one charged object to “coerce” charge flow.

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). click for animation Charging by Induction (cont.) 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

Electric Forces and Electric Fields CHARLES COULOMB ( ) MICHAEL FARADAY ( )

Electrostatic Charges A New Fundamental Physics Quantity The charge of an electron (q e ) is -1.6 x C The SI unit for charge is called the coulomb (C). Common electrostatic charges are small: millicoulomb = mC = C microcoulomb =  C = C nanocoulomb = nC = C Electrostatic charge is a fundamental quantity like length, mass, and time. The symbol for charge is q. ATTRACTION AND REPULSION The charge of an proton (q p ) is 1.6 x C

The Electrostatic Force The constant of proportionality, k, is equal to 9.0 x 10 9 Nm 2 /C 2. COULOMB’S LAW OF ELECTROSTATIC FORCE constant distance charges electrostatic force The force depends inversely on the square of distance between charges (another “inverse square law”)! A torsion balance measures the force between small charges. The electrostatic force depends directly on the magnitude of the charges. TORSION BALANCE Charles Coulomb’s Torsion Balance A negative force is attractive, and a positive force is repulsive. 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

The Electrostatic Force EXAMPLE 3 - Find the net force on the upper left charge EXAMPLE 4 (Honors only) - Find the net force on the lower left charge 425 N, 58˚ 307 N, -63.4˚

Electric Field Strength DEFINITION OF GRAVITATIONAL FIELD DEFINITION OF ELECTRIC FIELD Field Theory Visualizes Force At A Distance SI unit of electric fieldElectric field is a vector quantity q 0 is a small, positive test charge click for applet

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

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

EXAMPLE 1 EXAMPLE 2 Electric Fields Find the force on an proton placed 2 meters from the 5 millicoulomb charge in the problem above. OR Find the electric field strength at 2 meters from the 5 millicoulomb charge. E

Electric Potential Energy Potential energy can be converted to kinetic energy, heat, light, sound etc. Electric potential energy (PE) is stored when a positive charge is moved against an electric field. Electric Potential Energy versus Gravitational Potential Energy FALLING MASS VS. FALLING CHARGESTORING POTENTIAL ENERGY Potential energy is a scalar quantity measured in joules (J). positive (+) charge negative (–) charge toward Eloses PEgains PE opposite Egains PEloses PE POTENTIAL ENERGY GAIN OR LOSS

PE for Constant Electric Field charge E field distance CONSTANT GRAVITATIONAL FIELD CONSTANT ELECTRIC FIELD electric potential energy Example How much potential energy is converted when an electron is accelerated through 0.25 m in a cathode ray tube (TV set) with an electric field strength of 2 x 10 5 N/C?

PE for Two Point Charges (Honors only) Potential energy is zero at infinite distance Potential energy is positive for like charges Potential energy is negative for opposite charges Potential Energy is force times distance charges distance electric potential energy constant Example How much electrostatic potential energy in a hydrogen atom, which consists of one electron at a distance of 5.3 x meters from the nucleus (proton).

Potential Difference (Voltage) A volt (v) is the unit for voltage named in honor of Alessandro Volta, inventor of the first battery. SI Units sourcevoltage (V) common dry cell1.5 car battery12 household (US)120 comb through hair500 utility pole4,400 transmission line120,000 Van de Graaff400,000 lightning1,000,000,000 A good analogy: potential is to temperature, as potential energy is to heat. Electric potential is average energy per charge. Potential difference is often called voltage. Energy is a relative quantity (absolute energy doesn’t exist), so the change in electric potential, called potential difference, is meaningful. Voltage is only dangerous when a lot of energy is transferred. click for web page Voltage, like energy, is a scalar.

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

Potential Difference for Constant Electric Field voltage E field distance Potential energy is often stored in a capacitor. Most capacitors have constant electric fields. Capacitors are made by putting an insulator in between two conductors. Example Calculate the magnitude of the electric field set up in a 2-millimeter wide capacitor connected to a 9-volt battery.

Consider a test charge to measure potential Potential Difference for Point Charge (Honors only) charge distance potential difference constant Example -4 nC 10 nC6 nC 0.3 m 0.4 m find ∆V here

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