PHYS 211 Exam 2, Fall 2010

PHYSICS 211 Quiz II TIME: 60 minutes December 14, 2010 DO NOT OPEN THIS EXAM BEFORE YOU ARE TOLD TO BEGIN NAME________________________________________________ ID Number ________________________ Useful information 0=8.85x10-12 C2/N.m2. Grading Problem 1 Problem 2 Problem 3 TOTAL Score:________ Check if solution is continued on the back. 1 Problems 1.(40%) Consider a battery with a potential difference V connected to a cylindrical conductor with length l and a cross-section A. (a) (7%) Determine the velocity of one electron as a function of time assuming that it makes no collisions. (b) (5%) When collisions are taken into account, briefly describe the necessary modifications that are to be undertaken for the velocity and time? (c) (10%) Determine the relation between the current density j and the electric field E Score:________ Check if solution is continued on the back. 2 (d) (10%) Determine Ohm’s law, the relation between the current I and the potential difference V (e) (6%) Comment on the relationship obtained and the proportionality constants between j and E and I and V. (f) (7%) What happens if there were no collisions? Score:________ Check if solution is continued on the back. 3 2. (20%) (a) (10%) Demonstrate that if three resistors are connected in parallel, then the expression of the equivalent capacitor is 1/Req = 1/R1 + 1/R2 + 1/R3 (b) (10%) Demonstrate that if three resistors are connected in series, then the expression of the equivalent capacitor is Req = R1 + R2 + R3 Score:________ Check if solution is continued on the back. 4 3. (40%) Using Kirchhoff’s rules, (a)(20%) find the current in each resistor in the Figure above. (b)(10%) Find the potential difference between points c and f. Which point is at the higher potential? Score:________ Check if solution is continued on the back. 5 (c) (10%) Re-determine the currents in the circuit long after (fully charged) including a capacitor between R2 and E2 Score:________ Check if solution is continued on the back. 6 SCRATCH PAPER Nothing on this page will be graded Score:________ Check if solution is continued on the back. 7