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21-2 Application of Electric Fields

21-2 Application of Electric Fields. Energy and Electric Potential. As you lift an object off the ground, you are increasing its potential energy Same is for electric potential Electric potential ( Δ V)

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21-2 Application of Electric Fields

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  1. 21-2 Application of Electric Fields

  2. Energy and Electric Potential • As you lift an object off the ground, you are increasing its potential energy • Same is for electric potential • Electric potential (ΔV) • Work done moving a test charge in an electric field by dividing the magnitude of the test charge • ΔV = W / q (Work is the Potential energy need to remove the charge over some distance = joules) • Measured in joules /coulomb (J/C = Volt (V))

  3. Electric Potential Energy • Work is required to push a charged particle against the electric field of a charged body. • EPE is the energy a charge particle possesses because of its location in an electric field. • If the particle is released it will accelerate away turning the EPE into kinetic energy.

  4. Problem • If you apply 150 J of work to move a positive charge of 3.5 x 10-6 C from a negative plate, what is the electric potential difference? • Known • Work on q = 150 J • q=3.5 x 10-6 C • Unkown • ΔV • ΔV = W / q = 150 J / 3.5 x 10-6 C • =

  5. Electric Potential • Electric Potential • Smaller when two unlike charges are closer together • Larger when two like charges are

  6. Electric Potential in a Uniform Field • Uniform electric force and field made by placing 2 large conducting plates parallel to each other • Direction is from + plate to –plate • Potential difference, ΔV, between 2 points a distance (d) apart, in a uniform field (E) • ΔV = Ed

  7. Problem • 2 Parallel plates are given opposite charges. A voltmeter measures the EPD to be 60.0 V. The plates are 3.0 cm apart. What is the magnitude of the electric field between them? • Known • ΔV = 60.0 V • D = 0.030 m • Unkown • E = ???

  8. E = V / d • = 60.0 V / 0.030 m • = 2.0 x 103 N/C

  9. Storing Charges • Storing energy in an electric field • Leyden Jar • Developed by Dutch physicist Pieter Van Musschenbroek • Used by Ben Franklin to store charges from lightning • Version is still used today: Capacitor

  10. Storing Charges: Capacitor • Ratio of charge stored to electric potential difference: called Capacitance, (C) • Capacitor designed to store electric charges and energy • Made of two conductors separated by an insulator • Capacitance = charge / electric potential difference • C = q / ΔV • Measured in Coulomb per volt (C/V) or 1 Farad (F)

  11. Problem • A sphere has an eletric potential difference between it and Earth of 60.0 V when it has been charged to 3.0 x 10-6 C. What is the capacitance? • Known • V = 60.0 V • q = 3.0 x 10-6 • Unknown • C = ???

  12. C = q / ΔV • = 3.0 x 10-6 / 60.0 V • = 0.00000005 F • = 0.05 µF

  13. Capacitors • Examples: crank/shake flashlight, computer keyboards, flashes in cameras, electronics.

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