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Transcript
Chapter 17: Electric Forces and Fields Objectives • Understand the basic properties of electric charge. • Differentiate between conductors and insulators. • Distinguish between charging by contact, charging by induction, and charging by polarization. Electric Charge • Ben Franklin: two kinds of charge, positive and negative • opposite charges attract; like charges repel • Law of Conservation of Charge: it can’t be destroyed, total is constant • charge (q) is measured in coulombs (C) • electrons (–), protons (+) • Robert Millikan (1909): fundamental charge = +/– 1.60 x 10-19 C Transfer of Electric Charge • charges move freely through conductors (typically metals, ionic solutions) • charges do not move freely in insulators (most other substances) Electric charge can be transferred 3 ways: • friction/contact • induction • polarization Objectives • Calculate electric force using Coulomb’s law. • Compare electric force with gravitational force. Coulomb’s Law m1 m2 FG G r2 q1 q 2 Fe k r2 Law of Universal Gravitation Coulomb’s Law k = 8.99 x 109 Nm2/C2 Which is Stronger, Fe or FG? • Compare the Fg and the Fe between the p+ and e- in a hydrogen atom (r = 53 pm). Objectives • Calculate electric field strength. • Draw and interpret electric field lines. • Identify the properties associated with a conductor in electrostatic equilibrium. Electric Fields • E-field lines show direction and strength of force (by line density) acting on a small charge • E-field: (+) → (–) applet • units are N/C Fe q 0 E Fe E q0 Electric Fields • The nucleus applies a force of 8.16 x 10-11 N on the electron in a hydrogen atom. What is the electric field strength at the position of the electron? • What is the acceleration of an electron in a 2.5 x 103 N/C electric field? What is the acceleration of a proton in the same field? Conductors in Electrostatic Equilibrium electrostatic equilibrium: no net motion of charge (a) The total electric field inside a conductor equals zero. (b) Excess charge resides on the surface. (c) E-field lines extend perpendicular to the surface. (d) Charge accumulates at points. Chapter 18: Electric Energy and Capacitance Objectives • Understand the concept of electric potential energy (EPE). • Calculate the DEPE when a charged particle is moved in a uniform electric field. Electric Potential Energy (EPE) • uniform field only! • displacement in direction of the field PE grav m g h PE electric q E d g E EPE Problems • What is the change in EPE if a proton is moved 2.5mm in the direction of a uniform 7.0 x1011 N/C electric field? • What is the change in EPE if an electron is moved in the same direction? Potential Difference (Voltage) • voltage (V) is EPE per charge PE grav gh • 1 volt = 1 J/C m • measured with a PE electric q E d voltmeter PE electric • voltage is like an E d q “electric pressure” DPE electric that pushes charges " voltage" E Dd q • batteries, outlets, (uniform field only) generators, etc. supply DV E Dd voltage PE grav m g h Voltage Problems What voltage exists in a 3.5 x10-6 N/C electric field between two points that are 0.25 m apart? Capacitors • Capacitors store EPE between two closely-spaced conductors (separated by an insulator). • Capacitance is measured in farads (F). 1 F = 1 C/V Q C DV PE electric 21 Q DV • A capacitor can discharge very quickly—makes a short burst of electrical current Chapter 19: Electric Current and Electric Power Electric Current Electric charges will flow between areas of different electric potential (voltage) • electric current (I): a flow of electric charge • 1 ampere (A) = 1 C/s • measured with an ammeter • although electrons typically flow, current is defined as direction of positive flow (+ → –) • drift speed of e– in Cu at 10 A is only 0.00025 m/s • 0.005 A is painful and 0.070 A can kill you Electric Resistance • resistance (R): resistance to electron flow • measured in Ohms (Ω) • V ↑, I ↑ • R ↑, I ↓ V I R A 2400-Ω resistor is attached to a 12-V power source. What is the current through the wire? AC/DC • alternating current: electric field reverses periodically, current alternates direction (60 hz in USA) • direct current: field is constant, current is constant • batteries produce DC • electric generators can make AC or DC Electric Power and Energy Consider the units of voltage: J V C J C V J C V C V s s s W A V Pelec I V E elec I V t P = IV = I2R. Electric power is transported at high V and low I to minimize “I2R loss” (high I causes too much friction and heat). Power Problems An electric oven operates on a 240 V circuit (not the regular 120 V). How much current flows through the element in the oven if the power usage is 3200 W? At $0.0599 / kW·hr, how much does it cost to watch a 2-hour movie on a 280-W big-screen television? Objectives • To understand the concepts of series and parallel circuits. • To calculate the total resistance and current flowing through a circuit containing series and/or parallel circuits. Series Circuit Series Circuit • Resistors (or loads) “in series” just combine to make a larger resistance. • RT = R1 + R2 + R3 + … • In a series circuit, if V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω, what is RT and current? • Holiday lights are often in series: if one bulb burns out, nothing works! Parallel Circuit Parallel Circuits • Resistors in parallel provide additional paths for current to flow, so resistance decreases. • 1/RT = 1/R1 + 1/R2 + 1/R3 + … • In a parallel circuit, if V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω, what is RT and IT flowing through the entire circuit? What is the current in each resistor? • Household circuits are wired in parallel. Voltage Drops • The current flowing through a resistor depends on the voltage drop “across” the resistor. • Series example: V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω • Parallel example: V = 12 V, R1 = 1 Ω, R2 = 2 Ω, and R3 = 3 Ω