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Lecture09

# Lecture09 - 1 Physics 132-Winter 2008 Prof Jim Beatty-Ohio...

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Unformatted text preview: 1 January 25, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 1 Physics 132 Introductory Physics: Electricity and Magnetism Winter Quarter 2008 Lecture 9 January 25, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 2 The Electric Potential Just like we defined E as the force per unit charge, define the electric potential V(r) as the potential energy per unit charge. V = U q E = F q Δ U = − F • d s ∫ ⇒ Δ V = − E • d s ∫ E and F are vectors. V and U are scalars. V is measured in volts. Moving 1C through 1V potential difference does 1J of work. 1 V = 1 J C = 1 Nm C E x = − ∂ V ∂ x ; E y = − ∂ V ∂ y ; E z = − ∂ V ∂ z Alternate units for E: Volts/meter (same as Newtons/Coulomb) 2 January 25, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 3 Equipotential Surfaces and the Electric Field Equipotential surfaces are surfaces of constant V . Equipotentials are perpendicular to the field lines.-40V 0V-10V-20V -30V 40V 30V 20V 10V U=qV Positive charges move to lower V when allowed to move freely. Negative charges move to higher V. Both are moving to lower U! January 25, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 4 Breakdown in Air and the Wimshurst Machine Dry air is a poor conductor because there are few free charges. When E > 3 × 10 6 V/m, some electrons begin to be pulled from nitrogen molecules, creating free electrons and positive ions. These electrons accelerate and knock loose more electrons ⇒ breakdown See arcs, lightning-like discharges, etc. +- d At what potential can you make a 5 cm spark in dry air? E ≅ Δ V d = 3 × 10 6 V m Δ V = 0.05 m 3 × 10 6 V m = 1.5 × 10 5 V (There are tricks to lower the required voltage.) 3 January 25, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 5 Electric Potential of a Point Charge The reference point for the electric potential is often placed at infinity....
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Lecture09 - 1 Physics 132-Winter 2008 Prof Jim Beatty-Ohio...

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