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Transcript
Electrostatics History • The word electricity comes from the Greek elektron which means “amber”. • The “amber effect” is what we call static electricity. History • Ben Franklin made the arbitrary choice of calling one of the demo situations positive and one negative. • He also argued that when a certain amount of charge is produced on one body, an equal amount of the opposite charge is produced on the other body… Charge Concepts • Opposite charges attract, like charges repel. • Law of Conservation of Charge: – The net amount of electric charge produced in any process is zero. thanks Ben!!! • Symbol: q, Q • Unit: C, Coulomb Elementary Particles Particle Charge, (C) Mass, (kg) electron -1.6x10-19 9.109x10-31 proton neutron +1.6x10-19 0 1.673x10-27 1.675x10-27 • If an object has a… + charge it has less electrons than normal - charge it has more electrons than normal Ions and Polarity • If an atom loses or gains valence electrons to become + or - , that atom is now called an ion. • If a molecule, such as H2O, has a net positive charge on one side and negative charge on the other it is said to be polar Why does… Chemistry work? Physics!!! The electrostatic forces between ions (within molecules) form bonds called ionic bonds…all bonds are ionic; others, like covalent, are to a much lesser degree so that you can ignore the ionic properties of that type of bond. Why does… Biology work? Physics!!! The intermolecular electrostatic forces between polar molecules make such things as the DNA double helix possible. Types of materials 1. 2. 3. 4. Conductor: a material that transfers charge easily (ex. Metals). Insulator: a material that does not transfer charge easily (ex. Nonmetals) Semiconductors: somewhere between 1 & 2 (ex. Silicon, carbon, germanium). Superconductors: some metals become perfect conductors below certain temperatures Ways to Charge • By Friction: two objects rubbed against each other may cause a transfer of charge from one to another (triboelectrification) • Result: each object has a net charge which is equal and opposite to the other • By Conduction: contact occurs between charged object and neutral object. • Result: two objects with same charge • By induction: no contact occurs between charged object and neutral object. • Result: two objects with opposite charge • Credit Card: You may use Visa, Master Card, or American Express • Result: Debt from high interest rates 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 Electrons are transferred from the rod to the ball, leaving them both negatively charged. CHARGING POSITIVELY 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. Conduction 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. Induction Polarization Conduction or Induction A B Lightning Becomes very “negative” Becomes very “positive” Lightning (1) Lightning (2) Lightning (3) Lightning (4) Lightning (5) Lightning (6) Lightning Video compliments of Tom A. Warner, ztresearch.com 640x480 pixels 7,200 images per second 0.15 seconds recording time Visual aspects: -stepped leaders -dart leaders -return strokes -continuing current Lightning Rod Simulator Lightning striking the Empire State Building Van de Graaff electrostatic generator: simulates lightning from cloud to ground Electric Forces and Electric Fields CHARLES COULOMB (1736-1806) MICHAEL FARADAY (1791-1867) Electric Force AKA: Coulomb’s Law Using a torsion balance, Coulomb found that: the electric force between two charges is proportional to the product of the two charges and inversely proportional to the square of the distance between the charges. Electric Force Electric Force q1q2 F E kc 2 r • • • • q charge, C r distance between charges, m FE Electric Force, N VECTOR kc coulomb constant, 8.99x109Nm2/C2 The Electrostatic Force EXAMPLE 1 - Find the force between these two charges 9.0 10 5 10 9 Fe 6 C 8 10 6 C 0.04 m 2 Fe 225 N The negative signs means force of attraction, but does not indicate left or right direction EXAMPLE 2 - Find the net force on the left charge 9.0 10 5 10 9 Fe Fe 360 N 6 C 5 10 6 C 0.025 m 2 (force of repulsion) Fnet Fleft Fright Fnet 360 N 225 N 135 N, to the left Electric Field The electric force is a field force, it applies force without touching (like the gravitational force) In the region around a charged object, an Electric Field is said to exist Electric Field Rules for Drawing Electric Field Lines 1. The lines must originate on a positive charge (or infinity) and end on a negative charge (or infinity). 2. The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge. 3. No two field lines can cross each other. 4. The line must be perpendicular to the surface of the charge Electric Field FE E q0 • • • • • • becomes q E kc 2 r E electric field strength, N/C VECTOR q0 + test charge, C q charge producing field, C r distance between charges, m FE Electric Force, N VECTOR kc coulomb constant, 8.99x109Nm2/C2 E-Field vs g-field E Field Fe E q0 g field Fg g m0 Electric Field Strength Field Theory Visualizes Force At A Distance DEFINITION OF GRAVITATIONAL FIELD DEFINITION OF ELECTRIC FIELD force g field mass g E field force charge Fe E q0 Fg m q0 is a small, positive test charge Electric field is a vector quantity E field points toward negative charges E field points away from positive charges SI unit of electric field click for applet newton N coulomb C Van der Graff Generator Conductors in Electrostatic Equilibrium • When no net motion of charge occurs within a conductor, the conductor is said to be in electrostatic equilibrium • There are four properties to consider when looking at conductors Property 1 • The electric field is zero everywhere inside the conducting material – Consider if this were not true • if there were an electric field inside the conductor, the free charge there would move and there would be a flow of charge • If there were a movement of charge, the conductor would not be in equilibrium Property 2 • Any excess charge on an isolated conductor resides entirely on its surface – If some excess of charge could be placed inside the conductor, the repulsive forces would push them as far apart as possible, causing them to migrate to the surface Property 3 • The electric field just outside a charged conductor is perpendicular to the conductor’s surface – Consider what would happen if this was not true • The component along the surface would cause the charge to move • It would not be in equilibrium Property 4 • On an irregularly shaped conductor, the charge accumulates at locations where the radius of curvature of the surface is smallest (that is, at sharp points) Conductors in Electrostatic Equilibrium 1. The electric field is zero everywhere inside a conductor. 2. Any excess charge on an isolated conductor resides entirely on the outside surface of the conductor. 3. The electric field just outside the charged conductor is perpendicular to the conductor’s surface. 4. On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature is the smallest, i.e. AT SHARP POINTS. Charge resides along the surface => Charges try to get as far away as possible E=0 Otherwise charges would be moved around (Not equilibrium) Perpendicular otherwise there would be a force acting on the charges along the surface Charge accumulates at smallest curvature Electric Potential Difference (a.k.a. Voltage, Potential Difference) The difference of potential between two points is defined as the work done to move a charge from a point of lower potential to a point of higher potential Potential Difference W V q Where: V is the difference in potential between two points W is the work done in moving a charge in Joules Units: Joules volts Coulombs q is the charge being moved in coulombs Electronvolts (eV) The Joule is a large unit of energy…much too large a unit to use when moving elementary charges around. The amount of work done to move a single elementary charge across a potential difference of one volt is called an electronvolt V W q 1volt W (1.6 x1019 C ) W 1.6 x10 19 J 1eV 1.6 x10 19 J