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Transcript
Unit 7: Electrostatics and Electric Fields Electrostatics A. Background • The study of an objects electric charge, field, and its potential –Electronsnegative charges that move/transfer between objects B. The structure of Matter • Atoms make up ___________ • Protons and Neutrons are in the _______________ • Electrons are in the electron cloud and have the freedom to ________________ C. Quantities of Charge • Electric charge, q, is a _____________________ • q is used to measure both the charges of ____________ and ___________ • Units for q is a coulomb (C ) • On ref table page 1 _________________ is the charge of a proton or electron +1.60 x 10-19 C is the charge of _____________(positive charge) -1.60 x 10-19 C is the charge of _____________(negative charge) • Other quantities to know: – 1.60 x 10-17 C is the charge of 100 protons – 2.4 x 10-19 C is the charge of 1.5 protons; this is impossible because you cant split a proton in half! • 1 coulomb (C ) = ____________________________________ D. Charged Objects • Objects can gain or lose _____________ (negative charges) ________: – Lose electrons- becomes more __________ – Gain electrons- becomes more __________ • Same charges _________ each other • opposite charges _________ each other E. Transfer of Charge • When objects rub against each other electrons are transferred from one object to the other – Think about the balloon/sweater/wall example from PHeT • The flow of electrons goes from the _________________________ object to the more positive object – This flow of electrons causes the overall charges after the objects come into contact to be ________________ Practice Question: Oil droplets may gain electrical charges as they are projected through a nozzle. Which quantity of charge is not possible on an oil droplet? 1) 8.0 × 10-19 C 2) 4.8 × 10-19 C 3) 3.2 × 10-19 C 4) 2.6 × 10-19 C Practice Question: When a neutral metal sphere is charged by contact with a positively charged glass rod, the sphere 1. loses electrons 2. gains electrons 3. loses protons 4. gains protons F. Law of Conservation of Charge • Charge is transferred from the more negative object to the more positive object until they are both ________________________ +6 μC +2 μC G. Coulombs Law • This law shows the relationship between charges, the distances between them, and the force they exert on each other • F= • Based on the equation for Coulombs Law: – What is the relationship between force and charge? – What is the relationship between force and distance? • In Coulombs law, the charges of the particles determine if they will be attracted or repelled. – Make sure you keep track of the charges: • Protons are positive • Electrons are negative – If there are 2 like charges, there is a _________________. The 2 particles are repelled away from each other – If there are 2 opposite charges there is an _________________________. The 2 particles are attracted towards each other Example: • A point charge A of +3x10-7 coulomb is placed 2.0x10-2 meter from a second point charge B of +4x10-7 coulomb. Calculate the magnitude of the electrostatic force that charge A exerts on charge B. Example: • Calculate the electrostatic force that a small sphere, A, with a net charge of +2microcoulombs exerts on another small sphere, B, with a charge of -3microcoulombs when the distance between their centers is 10 meters. H. Conduction vs. Induction Conduction- direct contact between 2 objects with a transfer of charge Charge on the rod will equal the new charge of the object Diagrams: I. Induction Induction- Transfer of charge without direct contact Objects need to be grounded Diagrams: Lightning Induction Electric Fields A. Background Electric fields- the regions around charged particles where a force is exerted Electric Field LinesImaginary lines along which a positive charge would move Starts at positive points and ends at negative points The closer to a point charge, the closer the force 1. Individual point charges 2. Fields between a positive and a negative charge 3. Fields between 2 negative charges 4. Fields between 2 positive charges B. Parallel Plates Field lines will always be parallel when talking about parallel plates If the plates are close together: The electrostatic force, and the electrostatic field strength will be the same at any point A B A and B have the same force and field strength C. Electric Field Strength The strength of an electric field can be determined using the formula: E= Fe/Q measured in N/C E= electric field strength Fe= electrostatic force Q= charge of particle moving in the field Calculate the magnitude of the electric field strength at a point in a field where an electron experiences a force with a magnitude of 1.0x10^-15 newtons What is the magnitude of the electrostatic force experienced by one elementary charge at a point in an electric field where the magnitude of the electric field strength is 3.0x10^3 newtons/coulomb D. Potential Differences Potential difference (voltage)- the work done to move a particle/charge V=W/q V= voltage (potential difference in volts, J/C or eV/C) W= work (in Joules, J) Q= charge (in coulombs, C) Voltage is created when an electron is repelled through the electric field to move away from the negative particle Electronvolt (eV)- the amount of work, or gain in potential energy 1eV = 1.6x10^-19J Moving a charge of 3.2x10^-19C between points A and B in an electric field requires 4.8x10^-18J of energy. Calculate the potential difference between these points. A helium ion with a charge of +2e is accelerated by a potential difference of 5x10^3 volts. What is the kinetic energy acquired by the ion? Hint- PE and KE can be the same as work E. Current The amount of charge that flows through a wire ∆𝒒 I= 𝒕 I= current (C/s or A) Q= charge (C ) t= time (s) Ammeter- measures current, and is used to read amps Symbol for an ammeter: A 1. Conditions for an electric current: There must be a potential difference, meaning a charge from either a cell or a battery Currents move from positive to negative potentials Symbols for potential differences: Cell Battery Voltmeter V RB p124 #29, 30, 31 29. A total of 20 coulombs of charge pass a given point in a conductor in 4 seconds. Calculate the current in the conductor. 30. A wire carries a current of 2 amperes. How many electrons pass a given point in this wire in 1 second? 31. Which condition must exit between 2 points in a conductor in order to maintain a flow of charge?