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Electrostatics Atomic Particles and Properties Interaction of Charge Conductors and Insulators Methods of Charging Coulomb‟s Law Electric Fields Electric Potential Energy Voltage (Potential Difference) Capacitors... Think about it... Try to picture an atom... Now, draw a picture of a model of an atom that you‟ve seen in the past. If you haven‟t seen one, make one up! The Bohr model of the atom Atomic Particles All have mass and charge SI units? Fundamental unit of charge: “e” = 1.60 x 10 -19 Coulombs chargeis quantized; it comes in set amounts! Electrons negatively charged (-e) mass = 9.11 x 10-31 kg “orbit” the nucleus in a probability sphere (cloud) held in place by electrostatic forces held more weakly as “orbits” increase Protons charged (+e) positively mass = 1.67 x 10 -27 kg ~2,000 times the mass of an electron alwaysoccur in the same number as electrons unless it‟s an ion Neutrons neutral electrically mass ≈ 1.67 x 10 -27 kg the same as a proton! may or may not occur in the same number as protons this leads to isotopes Quarks and Leptons Of course, at this stage of our scientific understanding, we know that even protons, neutrons, and electrons are not elementary. They are made up of quarks and leptons Charge is Quantized! That is...electric charge will appear in packets or multiples of “e”. Hence, q = N·e N is the # of charges, e is the charge of an electron q is measured in Coulombs Ex: How many protons would there be in one Coulomb of positive charge? q 1.0C N 6.25 1018 E 1.6 10 C 19 That number of basketballs would fill the Earth 11,000 times over!!! Think and Explain... „Think about what happens when a balloon is rubbed against your hair and then placed next to the wall... „Can you explain why this happens? Electric charge An electrical property of matter; forces exist between two charged objects. Just like “mass” is an intrinsic property of matter and forces exist between two masses. Likecharges repel; opposite charges attract. Separation of Charge How are electrostatic forces and gravitational forces alike? How are they different? Demo: comb and paper bits… Charges can be “separated” by rubbing two things together literally this shows how friction is an electrostatic force when charges move objects become electrified Conservation of Charge Law of Conservation of Charge: During any process, the net electric charge of an isolated system remains the same. „Electrons flow from regions of negative charge to regions of positive charge. „Read the Physics of electronic ink on page 524 Conductors and Insulators Not only can charge exist on an object, it can move through an object Materials that readily allow the movement of charges are called conductors metals, water outer shell electrons (valence) that can move „freely‟ through the substance flows from e- surplus (-) to e- deficiency (+) Materials that inhibit the movement of charges are called insulators glass, ceramic, rubber, plastic Back to the Balloon Question… As the balloon is rubbed against your head electrons are pulled off of your hair. This causes the balloon to be charged negatively. When the negative balloon is placed near the neutral wall the electrons in the wall rearrange. The electrons in the wall are repelled giving the wall a local positive charge. Thus the negative balloon is attracted to the positive wall. Detecting Charge Charge is detected with an electroscope The Electroscope Ifa charged object comes in contact with the electroscope, the leaves of the electroscope push away from each other. Why??? But you can‟t tell what kind of charge it is with an electroscope. Why? Methods of Charging Friction (rubbing) electronsget „rubbed off‟ leaving extra/not enough electrons (net charge) excess --> negative charge -- deficiency --> positive charge carpets, static cling, the Van de Graff generator hard rubber & fur = negative charge glass & silk = positive charge Methods of Charging Conduction chargingby contact Charges flow from a charged conductor to a neutral conductor Induction no contact is necessary! Just move close to a neutral object and charges rearrange. Why??? A neutral object coming close to a charged object can have its charges moved without taking on electrons. The object is still neutral while it has a + and – region. (polar molecules in chemistry) Coulomb’s Law The electrostatic force between two charged particles is directed along a line joining their centers. If the charges are similar the force is repulsive, otherwise it is attractive. k = 8.99 x 109 q1q2 F k 2 N·m2/C2 F is in Newton's d How is this similar to the gravitational force? Coulomb’s Law... double the distance, force drops by 1/4. double the charges, force increases by a factor of four. Coulomb’s Law Example Ex: How do the forces of gravity and electrostatic repulsion compare between two 100.0 kg metal spheres separated by 10.0 cm and given a charge of 1.00 mC? What does this say about levitation??? Coulomb’s Law Example Ex: A gold atom normally contains 79 protons. If all of the electrons are removed except for one, what force would it undergo at a distance of 6 x 10-10 m from the nucleus? Is this a very large force? Example: Adding Forces 20cm 30cm q1=-6µC q2=+2µC q3=-4µC Determine the net electrostatic force on q2 in the figure above. Could charges q1 and q3 be positioned so that the net force is zero? Ex: Adding Forces in 2D Determine the net electrostatic force on q2 q1=-6µC in the figure above. Could charges q1 and q3 be positioned so that 20cm the net force is zero? 30cm q2=+2µC q3=-4µC Speed of a Orbiting electron satellite + planet nucleus In a hydrogen atom one electron (-e) orbits around a proton (+e) at a radius of 0.0529 nm. Use UCM techniques to calculate the orbital speed (can you calculate gamma for this speed and determine if there are relativistic effects?) Electric Fields All charged bodies will affect other charges when placed near each other. Explain… We say that a charged body sets up an ELECTRIC FIELD around it (think of how a massive body sets up a gravitational field around it.) When a charge is placed in an electric field, it experiences a Coulomb attraction/repulsion It will accelerate in the direction of the net force. What would you have to do in order to prevent the acceleration? The Electric Field Just like gravitational fields exist wherever there is mass, electric fields exist wherever there is charge. SI units: N/C The way we determine field strength is to place a (positive) point charge in the field and measure the applied force. Electricfields are vectors: Ex 8-11 and fig. 18.17 E F q0 Electric Field Lines Conventions: begin on positive and end on negative charges directed away from positive charges and toward negative charges (tells which way a positive test charge would go) the density of lines indicates the strength of the field An Electric Dipole (two positive charges) Where is the field the strongest? The weakest? Can you draw the field for a dipole made up of opposing charges? A more complex field… Neat Fact: When electric charge is placed on a conductor. All the charge resides on the surface. This is behind the idea of electric shielding. Think and Explain... What would happen to you if you were struck by lightning in your car??? Think and Explain... What must you do to lift a stone off the ground? What happens to the stone’s potential energy? What must you do to move a positive test charge closer to another positive charge? What happens to the charge’s energy? Remember... Tolift a stone you must do work. The amount of work you do is equal to the stone‟s change in G.P.E. W = ∆G.P.E. = mgh - mgh0 When raised in a gravitational field a stone has potential energy. That energy depends on how high you raise the stone above some reference level. Apply... How would the G.P.E. of the stone compare to that of a large boulder raised to the same height? This dependence on mass could be overcome by dividing by mass. Let’s call this new value the gravitational potential (G). G = G.P.E./m (units?) Now we can determine the gravitational potential at any point above the earth for any sized object. Electric Potential and Work Suppose we move a charge, +q, from A to B in the field below. The work done to move q is equal to the charge‟s change in electric potential energy (E.P.E) W = ∆E.P.E. = EPEB-EPEA When the positive charge is moved from A to B does it gain or lose EPE? How would the work done change for a larger charge? Electric Potential The Electric Potential (or potential difference) is also called Voltage. It is found by dividing the Work done to move a charge by the magnitude of the charge moved. V = W/q = ∆EPE/q Voltage is measured in Volts (J/C) Voltage is the “push” that causes electrons to move through wires Field Lines and Equipotentials Equipotential surfaces are places in an electric field where the potential difference is the same They are perpendicular to the electric field lines. In the diagram at right, the field lines are blue and the equipot. Lines are red. Go Figure... How much work must be done to move a 300 µC charge through a potential difference of 6.0 V? 1.8 x 10-3 J If GPE is zero at ground level, where is EPE zero??? An infinite distance away! Point Charges... The E.P.E. due a point charge is: qq0 E.P.E. k r The potential of a point charge is: Vk q r Go Figure... •Ex: A 2.00 µC point charge is located at the center of an equilateral triangle with sides of 50.0cm •What is the potential at each vertex? •62,000V •What is the EPE of a 1.00 µC charge placed at a vertex? •6.2x10-2 J •How much work must be done to move the test charge from one vertex to the next? •None! Why??? Charge Distribution and Shape „All charge resides on the surface of a conductor (shielding) „Charge spreads out evenly on a perfect conducting sphere „Charge is more confined to original area on a non conductor „There is no potential diff. between any points on a conductor „The surface of a conductor is an equipotential surface Charge Distribution and Shape „Charge concentrates at points or sharp edges on a conducting surface „There can be no potential difference between any two points on a conductor „If the field intensity at a sharp edge is great enough, gas around surface will ionize and electric discharge will occur „Spark plugs, lightning, lightning rods St. Elmo‟s Fire St. Elmo's fire is a spark discharge from soaring buildings. It is generated by a high voltage between the ground and the air. St. Elmo‟s fire has been observed among others from steeples, masts, mountain tops and barbed wire fences. It is very rare. When you see St. Elmo‟s fire near yourself, there is a high danger of being struck by lightning. The high voltage can also show up by your hair standing on end. Although this may look "funny", you must immediately leave your position as a flash of lightning is imminent. Go Figure... Ex. How much work must be done to place a 10.0 µC charge at each of the corners of a square that is 50.0 cm on each side? [Hint: place them one at a time…] ~18J Capacitors 1 Devices that store electric energy are called capacitors. The most common type is made up of two parallel plates separated by some medium (called a dielectric). The plates are usually rolled into a cylindrical shape Energy comes from the work it took to separate the charge! Capacitors 2 The charge “stored” on a capacitor is given by the q CV equations: “C” is the capacitance of the capacitor, a constant that depends on its makeup. What are the units of C? Capacitors 3 The capacitance, C, of a capacitor can be found using: 0A C d κ is the dielectric constant, ε0 is the permittivity of free space, A is the area of the capacitor, and d is the separation between the two sides Capactors 4 Ex:The electric field strength between the plates of a capacitor can be found using: E = V/d where V is the voltage across the plates and d is the plate separation. Capacitor Examples Ex: What voltage is required to store 7.2 x 10-5 C of charge on the plates of a 6.0 mF capacitor? Ex: #40 on page 583 Capacitor Examples Ex: How much charge can be stored by a parallel plate capacitor made up of 3.0 cm x 4.0 cm sides, separated by a distance of 0.025 mm when a voltage of 12 V is placed across it and… a.) no dielectric is used. b.) ruby micra is used as a dielectric. c.) What would the electric field strength be between the plates of the capacitor? Did you know??? Photo Flashes use capacitors Computer keyboards use capacitors (p 573) Capacitors are used to prevent power surges in brown outs. The greater the dielectric constant the more charge can be stored (Why?). Oil and water can be used as dielectrics, air too! I’m shocked… Can this be the end, already?!?
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