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Unformatted text preview: 1 Physics 9HD, Midterm 2 review DC Circuits Current dt dq I = Ohm’s Law, V=IR for resistor, , E J r r σ = where ρ is electrical resistivity of material, σ =electrical conductivity=1/ ρ , and J r is the current density, i.e., ∫ ⋅ = a d J I r r . v ne J r r = , where n=number of carriers/volume, e=charge of carriers, and v r is the average velocity of the carriers. Resistance of wire in terms of resistivity: A L R ρ = , length L, area A Battery E with internal resistance r develops effective voltage = ∆ V E - Ir Kirchhoff’s Laws: 1. Current Law: Sum of currents entering a node = sum of currents leaving that node. (Charge conservation) 2. Voltage Law: Sum of voltage drops around a closed loop is zero. (Energy conservation) Series Circuit has same current in each element Series resistances: n eq R R R R R + + + + = ... 3 2 1 Parallel Circuit has same voltage across each element Parallel resistances: n eq R R R R R 1 ... 1 1 1 1 3 2 1 + + + + = Power dissipated in circuit: V I P = Power dissipated in resistor: R V R I P 2 2 = = Charging capacitor with battery E, q=C E ) 1 ( RC t e − − ; time constant...
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