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Unformatted text preview: PHY 184 Final Practice exam 2 Solutions 1. Coulomb’s law, superposition principle. The magnitude of the electric fiels is the same from all four charges: 7 2 2.25 10 / q E k N C r = = × . Three charges are positive, one negative. The two field components in the vertical direction add up, and the two fields in the horizontal direction cancel each other (point in opposite directions). Correct answer: F 2. Electric potential, simply add the potential due to the 4 charges. The magnitude of the potential is again the same from all four charges, but the sign is negative for tle last one. 5 9 10 V V = × . Correct answer: A 3. First the direction: Electric field lines are always perpendicular to equipotential surfaces. Therefore the field at the origin only has an X component and no Y component. Also, the potential is lower on the right then on the left. Therefore the field is in the +x direction. That already narrows it down to answers A or H. Next, estimate the field from the field lines: field line -20V passes through the origin, and field line +20V passes roughly through the -1cm mark. Therefore the field is approximately 40V/cm, or 4000V/m. Correct answer: A 4. Use Gauss’ law: 5 1 2 5.56 10 q q q Vm ε ε + Φ = = = × .Correct answer: A 5. Find the equivalent resistance: R3 is in series with R4. R34 is parallel to R2, R234 is in series with R1. From the equivalent resistance determine the current. Correct is in series with R1....
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