lec18_021907_-_Currents_and_Resistors_III_-v2 - Req, series...

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Unformatted text preview: Req, series = i Ri Kirchhoff's Rules Kirchhoff' I and R III 1 = Req, parallel i 1 Ri Question If you increase the charge on a parallel-plate capacitor from parallel3 C to 12 C and decrease the plate separation from 4 mm to 1 mm, the energy stored in the capacitor changes by a mm, factor of ... A. 1/4 B. 12 C. 4 D. 1/12 E. 64 Junction Rule: Derives from Conservation of Charge At any junction point in a circuit where the current can divide, the sum of the currents into the junction must equal the sum of the currents out of the junction! Loop Rule: Derives from Conservation of Energy When any closed-circuit loop is traversed, the sum of the closedchanges in potential must equal zero! C= o A d U= 2 1 Q2 1 Q 2d = 2 C 2 o A Sign Conventions Cross a battery from the negative terminal to the positive terminal: terminal: V is positive! Cross a resistor in the direction of positive current flow: V is negative! Cross from the negative plate of the capacitor to the positive plate of plate a capacitor: V is positive! And vice versa for all of the above! 1 U1 (3) 4 U 2 (12 ) 1 2 U 2 = 4 U1 Monday, 19 February 2007 Monday, 19 February 2007 2 Question A parallel-plate capacitor with a dielectric in the space in parallelbetween the plates has a capacitance of 8 F. Removing the dielectric and doubling the separation between the plates results in the capacitance becoming 2 F. What is the dielectric constant? Assume that the space between the plates is a vacuum once the dielectric is removed. A. 0.5 B. 2.0 C. 4.0 D. 0.25 E. 8.0 Monday, 19 February 2007 Question A fuse in an electric circuit is a wire that is designed to melt, melt, and thereby open the circuit, if the current exceeds a predetermined value. Suppose that the material composing the fuse melts once the current density rises to 500 A/cm2. What diameter of cylindrical wire should be used to limit the current to 0.2 A? A? I Ci = Cf = o A d o A 2d J= = 8F A. 0.02 cm B. 2 10 -5 cm = 2F =2 C. 0.04 cm D. 5 cm E. None of the above 3 Monday, 19 February 2007 A I 0.2 A A= = = 0.0004 cm 2 J 500 A cm 2 A = r 2 = 0.0004 cm 2 o A d = 4 F d = 2r = 0.04 cm 4 1 Question If the current in a resistor is doubled while the value of the resistance is cut in half, the power dissipated in that resistor is changed by a factor of ... A. 4 B. 2 C. D. 8 Question Light bulbs that have been on for a few seconds can be treated as resistors that glow more brightly when the power dissipated by their ohmic filaments is increase. Rank the identical light bulbs shown below according to brightness, from most to least bright. A. A > D > B > C B. A > B > D > C C. B = C > D > A D. A > D > B = C E. None of the above 5 Monday, 19 February 2007 6 Pi = I 2 R Pf = (2 I ) 2 R = 2I 2 R 2 A B D C Pf = 2 Pi E. There is no increase or decrease in the power dissipated Monday, 19 February 2007 2 ...
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This note was uploaded on 04/08/2008 for the course PHGN 200 taught by Professor Japguy during the Fall '07 term at Mines.

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