cheat sheet for the final

# cheat sheet for the final - Final Exam Current and the...

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Final Exam Current and the Motion of Charges -the rate of flow of electrical charge through a cross-sectional area I = ΔQ/Δt (1A = 1 C/s) I = qnAv d , n = # of free charges = (ρN avog. )/M molar , v d = drift velocity Resistance and Ohm’s Law potential drop V = V a – V b = EΔL Resistance : ratio of potential drop to the current R = V/I (1Ω = 1V/A) R = ρL/A -resistance of a conductivity wire Energy in Electrical Circuits P = IV = I 2 R = V 2 /R (1 watt = 1 J/s) -power dissipated (potential energy loss per unit time) in the conducting segment -electromotive force (emf) supplies energy to a circuit (volts), has the same equations as V -charge through a source of emf (battery), PE increased by ΔQЄ P = IЄ -rate at which energy is supplied by the source V a,+ – V b,- = Є – Ir, r = internal resistance of battery I = Є/(R + r) W = QЄ, Q = total charge the battery can deliver -total energy stored in a battery Combinations of Resistors Series: R eq = R 1 + R 2 + R 3 +… V = IR 1 + IR 2 = I(R 1 + R 2 ) Parallel: R eq = (1/R 1 + 1/R 2 + 1/R 3 +…) -1 V = I 1 R 1 = I 2 R 2 -the current divides into each branch proportionally Kirchhoff’s Rules 1. when any closed loop surface is traversed, the sum of the changes in potential must = 0 2. at any junction (branch point), the sum of the currents into the junction must equal the sum of the currents out of the junction Single Loop Circuits: ex.) –IR 1 – Є 2 – Ir 2 – IR 2 + Є 1 – Ir 1 = 0 Multi-Loop Circuits: -the ΔV must still sum to 0 for each loop, do the inner loop then th outer loop RC Circuits -contain a resistor and a capacitor Discharging a capacitor: no battery, switch is closed and a current flows, current = the rate of decrease of

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## This note was uploaded on 03/27/2008 for the course ENG 77 taught by Professor Cohen during the Spring '08 term at Tufts.

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cheat sheet for the final - Final Exam Current and the...

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