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Transcript
What Gives an Electric Charge? An imbalance of protons and electrons. Neutral objects have equal numbers of electrons and protons. Positively charged objects have more protons than electrons. Negatively charged objects have more electrons than protons. When you rub a rubber rod with rabbit fur electrons are transferred to the rubber rod and it becomes negatively charged. Conservations of charge When electrons are transferred from one substance to another the overall charge is conserved. The SI Unit for Charge is the Coulomb The charge of a proton is +1.6 x 10-19 Coulombs. The charge of a electron is -1.6 x 10-19 Coulombs. This is why an atom is neutral when there are equal numbers of protons and neutrons. Conductors Conductors have electrons that are more loosely held and therefore allow charges to flow more easily. Good conductors are: Metals Water and solutions with ions present. Wires are usually composed of copper wire. Insulators Insulators have more tightly held electrons and are not good conductors of electricity. Good Insulators are: Rubber Plastic Air Silk Good electrical insulators are usually also good thermal insulators. Copper wires are usually encased in rubber to prevent shock. Millikan In 1909 Robert Millikan a physicist at the University of Chicago ran an experiment that revealed that charges occur in discrete quantities. These quantities always occur in whole number intervals and are said to be “quantized”. These quantities are known as electrons which have a charge of -1.6 x 10-19 Coulombs. Electrons cannot be divided into fractions. Any object that is charges has a surplus or deficit of some whole number of electrons. # of electrons = total charge/charge of an electron. If something has a -1C of charge it contains 6.2 x 1018 electrons. Millikan’s Oil Drop Experiemnt Charging Objects Charging by Friction Electrical charges can be transferred from one object to another. Two substances can be rubbed together. Rub a balloon with wool. The wool will transfer electrons to the balloon and the balloon will become negatively charged. Static Cling Clothing in the dryer rub electrons off each other and they become oppositely charged and stick together. Conduction Objects can be charged by contact. Since electrons can move from one place to another they will move into another object to even out the overall charge of the two objects. Conservation of charge. - - - - - - - - - - - Induction Charges within a neutral conductor can move. Objects can gain an induced charged when a charges object is brought near a neutral object. Charges become polarized. This induced charge is temporary if the objects never touch. Once the charged object is removed the charges evenly spread out again. Grounding Grounding an object provides a conduit in which the excess electrons can leave the object. Objects that are charged by induction can gain a permanent charge by grounding. Objects that are charged by induction and then grounded end up with an opposite charge than the conductor. Objects charged by conduction end up with the same charge as the original conductor. Grounding Grounding Charging by Induction Electric Forces Electric Force Force between charges Repulsion between like charges and attraction between opposite charges. Coulomb’s Law Fe = k q1q2 d2 The electric force decreases with the square of the distance between the charges Look Familiar? Remember Fg = G m1m2 d2 Quantity of charge is like the mass of an object Let’s compare G and k G = 6.67 x 10-11 N m 2/kg k = 8.99 x 109 N m 2/C2 Which force is stronger? A pair of positively charged particles of 1 C each 1 meter apart would experience a repulsive force of 9.0 x 109N The weight of a battleship! Law of Superposition The resultant force on any given charge is the vector sum of all the forces acting on that charge. Applied to three or more charges with a certain distance to each other. Must consider whether or not the force is attractive or repulsive. Gravitational and Electric Fields Review of gravitation Gravitational field strength “g” gives the ratio of force to mass. Fg = mbodymearth Fg is the force of gravity (weight) d2 d is the distance from the center of the earth We can think of mearth as creating the field and mbody “experiencing the” force in response to the field. “g” is the field strength of the earth’s gravitational field (9.8N/kg at the surface). We can calculate “g” by using the equation: g = G mearth d 2 Electric Field Strength Electric field strength E gives the ratio of force to charge. Fe = q 2E like Fg = mg Fe = electrostatic force q2= charge experiencing the field E = field strength E = kq1 gives the electric field strength at any given d2 distance from the charge creating the field. q1 is the charge creating the field K is 8.99 x 109 N m/C2 d is the distance from the center of the charge to any given point in the field. Si unit for field strength is N/C Field Lines Associated with Electric Fields Stronger charges have more concentrated field lines. A positive charge creates a field that emanates outward from the charge. A negative charge creates a field that is directed towards the charge from the charge. Field lines are always perpendicular to the charge or charged object. Field lines never intersect. (this would indicate two different values of force at the same point)