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Chapter 17: Electric Forces and Fields

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.

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Chapter 17: Electric Forces and Fields

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  1. Chapter 17: Electric Forces and Fields

  2. 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.

  3. 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

  4. 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

  5. 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

  6. Objectives • Calculate electric force using Coulomb’s law. • Compare electric force with gravitational force.

  7. 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).

  8. Objectives • Calculate electric field strength. • Draw and interpret electric field lines. • Identify the properties associated with a conductor in electrostatic equilibrium.

  9. 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

  10. 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?

  11. 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?

  12. 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.

  13. Chapter 18: Electric Energy and Capacitance

  14. Electric Potential Energy (EPE) • uniform field only! • displacement in direction of the field g E

  15. 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?

  16. 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?

  17. 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”

  18. Voltage Problems What voltage exists in a 3.5 x10-6 N/C electric field between two points that are 0.25 m apart?

  19. 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

  20. Chapter 19: Electric Current and Electric Power

  21. 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

  22. 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?

  23. 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

  24. Electric Power and Energy Consider the units of voltage:

  25. 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?

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