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## Chapter 17: Electric Forces and Fields

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**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**• protons have positive (+) charge • electrons have negative (–) charge • opposite charges attract; like charges repel • charge is always conserved (cannot be destroyed) • charge (q) is measured in coulombs (C) Charles Augustin Coulomb 1736 - 1806**Electric Charge is Quantized**1909: Robert Millikan qE Charge on drop occurs in multiples of 1.60 x 10-19 C (±1e, ±2e, ±3e…). + mg**Transfer of Electric Charge**• charges move freely through conductors (typically metals) • charges do not move freely in insulators (most other solids) Electric charge can be transferred 3 ways: • contact • induction • polarization**Objectives**• Calculate electric force using Coulomb’s law. • Compare electric force with gravitational force.**Coulomb’s Law**Law of Universal Gravitation Coulomb’s Law k = 8.99 x 109 Nm2/C2 Compare the Fg and the Fe between the p+and e- in an H 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**Field lines show direction and strength of force (represented by the line density) • gravitational field (g) • electric field (E): (+) → (–) units are N/C**Electric Fields**The nucleus applies a force of 8.16 x 10-11N on the electron in a hydrogen atom. What is the electric field strength at the position of the electron?**Electric Field due to a Point Charge**(a) What is the E-field strength at a distance of 0.45 m from a 8.2mC charge? (b) What force would be felt by a proton at this location? (c) What is the initial acceleration of the proton?**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.**Electric Potential Energy (EPE)**• uniform field only! • displacement in direction of the field 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?**EPE for a Pair of Charges**• Two alpha particles (two helium nuclei, 2 p+ each) have an EPE of 6.32 x10-19 J. What is the distance between the two particles?**Potential Difference (Voltage)**(uniform field only) • voltage (V) is EPE per charge • 1 volt = 1 J/C • measured with a voltmeterormultimeter • voltage is like “electric pressure”**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 that are separated by an insulator. • Capacitance is measured in farads (F). 1 F = 1 C/V**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 ↓ 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:**Power Problems**If voltage = 120 V, what is the current through a 1200-W hairdryer? At $0.05 / kW·hr, how much does it cost to operate a 240-W television for 24 hrs?