Conceptual Questions (2 points each) Circle the Best Answer

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Aron Lynch
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1 Physics 42 Exam 2 F13 Name: Lab Conceptual Questions (2 points each) Circle the Best Answer 1. Circle the one with the greatest equivalent capacitance. 2. Circle the the capacitor with the greatest voltage across it. 3. The wires are all made of the same material. Circle with wire with the greatest resistance. 4. The two segments of the wire in the figure have equal diameters but different conductivities such that 2 =2 1 How do the current densities and Electric fields in the wire segments compare? a. J 1 J 2, E 1 E 2 b. 2J 1 J 2, 2E 1 E 2 c. J 1 2J 2, E 1 2E 2 d. J 1 J 2, E 1 2E 2 5. Two conductors are of equal lengths and made of the same metal. Which conductor dissipates the greater amount of power? A) P1 P2 B) P1 P2 6. Circle the bulb that burns brighter. 7. Here is a Logger Pro Plot of an RC circuit during charging and discharging. Which segment represents which? Circle. Cap Charging A B C D Cap Discharging A B C D Resistor Charging A B C D Resistor Discharging A B C D 8. What is the voltage drop across the unspecified circuit element in the direction shown by the loop? a) +12V b) -12V c) +8V d) +2V e) -2V 9. A parallel plate capacitor of capacitance C 0 has plates of area A with separation d between them. When it is connected to a battery of voltage V 0, it has charge of magnitude Q 0 on its plates. While it is connected to the battery, the space between the plates is filled with a material of dielectric constant 3. After the dielectric is added, the magnitude of the charge on the plates and the new capacitance are a. Q 0, C 0 b. Q 0, C 0 c. Q 0, C 0 d. 3Q 0, C 0 e. 3Q 0, 3C A dipole is placed near to a point charge Q. The net force on this dipole is zero because the dipole is neutral. a) True b) False

2 Short Problems. (10 pts each) Show work! Box final answers. Circle best answer. Assume 3 sig figs. 1. A 4.0 Ω resistor is connected across the terminals of a 10.0 V battery. If 0.50 A of current flows, what is the internal resistance of the battery? A) 16 Ω B) 20 Ω C) -5.0 Ω D) 24.0 Ω 2. The current density in a 8.5 μm thick 75 μm wide gold long film is 750,000 A/ m 2. The current flows along the length of the film. What is the current in the film? A) 4.2 ma B) 54 μa C) 480 μa D) A 3. A silver wire with resistivity Ω m has a 4.0 A/mm 2 current density. What is the magnitude of the electric field inside the wire? Show how the units work out. A) 64 pv/m B) 64 µv/m C) V/m D) 2.5 V/m E) 0.25 G V/m

3 4. At t = 0 the switch S is closed with the capacitor uncharged. If C = 30 F, = 50 V, and R = 10 k, what is the potential difference across the capacitor when I = 2.0 ma? a. 6 V b. 15 V c. 25 V d. 30 V e. 45 V 5. What is the total energy stored in the group of capacitors shown if the charge on the 30- F capacitor is 0.90 mc? Hint: Use the table to find the answer. Use at least 2 sig figs. a. 29 mj b. 61 mj c. 21 mj d. 66 mj e. 32 mj C ( F) V (V) Q (mc) U (mj)

4 Long Problems. (20 points each) Show your work! BOX your answers. NEATNESS COUNTS! 1. A capacitor consists of a solid sphere and a concentric hollow sphere with radii a and b, respectively. Starting with Gauss s Law derive an expression for the capacitance. If the two spheres are separated by a 2.00 μm gap, and the inner sphere has a radius of 7.00 mm, find the value of the capacitance. Be complete and neat. Show how the units work out.

5 2. For the circuit shown find the following. Assume the voltage supplies are ideal & 3 significant figures for all values. a) Find the current through each resistor. Show ALL your work applying Kirchhoff s Rules. Redraw the circuit labeling the currents and the directions of the currents you select. Show an equation for each unknown, etc. Show your work! b) Find the voltage across and power dissipated by each resistor. Put your results in the table. c) Show in your diagram where you would have to put the minimum number of ammeters to measure every different current. Use a symbol ammeter (A with a circle in it). Make it clear. d) Show that energy is conserved for the circuit. R ( ) I (A) V (V) P (W) Total:

6 Extra Credit. (10 points) Show all your work. Explain what you are doing. NO PATIAL CREDIT. A 1.0 m long piece of coaxial cable has a wire with a radius of 1.1 mm and a concentric conductor with inner radius 1.3 mm. If the voltage drop is 6000 V, and the line charge density, λ, is 8.8 μc/m, find the value of the dielectric constant. Derive it all from scratch, explaining what you are doing. Make it neat and complete.

MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.

MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question. 1) If the voltage at a point in space is zero, then the electric field must be A) zero. B) positive.