Lecture15

# Lecture15 - 1 February 8 2008 Physics 132-Winter 2008 Prof...

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Unformatted text preview: 1 February 8, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 1 Physics 132 Introductory Physics: Electricity and Magnetism Winter Quarter 2008 Lecture 15 February 8, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 2 Resistance Electron-nucleus collisions impede current flow in conductors. Energy is needed to keep the current going against the resistance. +V I When current flows through a potential drop, work is done that allows the current to continue to flow. When the potential drop and the work needed are the same, we get a steady current and voltage. Resistance is the ratio of the voltage to the current. R = V I V = IR Ohm’s Law Schematic Symbol Units for R: Ohms( Ω ) 2 February 8, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 3 Resistance depends on… Resistance is a function of the type of material, the shape and size of the object, and where voltage is applied in relationship to the object’s shape. Ohm’s Law (V=IR) applies to many (but not all) materials. Different resistances! February 8, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 4 Resistivity Resistance R is the property of a particular object (material + shape + arrangement of V) Resistivity ρ is the property of a material. ρ = E J E = ρ J Units : ρ [ ] = Ω m Conductivity σ = 1 ρ J = σ E Copper: ρ = 1.7 × 10-7 Ω m Glass: ρ = ~10 12 Ω m Superconductors: ρ = 0 Ω m 3 February 8, 2008 Physics 132-Winter 2008 Prof. Jim Beatty-Ohio State 5 Going from Resistivity to Resistance What is the resistance between the ends of a conductor with a uniform cross section? L A +V 0V I Put a potential difference across the ends. Then V=EL....
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Lecture15 - 1 February 8 2008 Physics 132-Winter 2008 Prof...

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