Physics180 3D.pdf

# A true b false questions 5 and 6 consider two

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A True B False Questions 5 and 6: Consider two identical 6 nF parallel - plate air capacitors. Capacitor 1 is fully charged by a 10 V battery, and Capacitor 2 is also fully charged by a 10 V battery. The electrical potential energy stored in each of these charged capacitors is 300 nJ. 5. True or False? A dielectric, with dielectric constant equal to 3, is placed between the plates of Capacitor 1. If the battery that charged this capacitor was disconnected before the dielectric was inserted, then the voltage between the plates of Capacitor 1 will increase to a value greater than 10 V . A True B False 6. True or False? A dielectric, with dielectric constant equal to 3, is placed between the plates of Capacitor 2. If the battery that charged this capacitor remains connected while the dielectric is inserted, then the electrical potential energy stored in Capacitor 2 will decrease to a value less than 300 nJ. A True B False Physics 180B — Fall 2017 — TEST 3: FORM D Solutions — page 1 of 4 True: resistors in series share the same current. If I = 1 A and V = 1 V, then (1 A) ◊ (1 V) = IV , which equals P (power, measured in watts). So, (1 amp) ◊ (1 volt) = 1 watt. R eq for a parallel combination is always less than the lowest resistance in the combination . The only choice that is less than 15 X is 8.4 X , so this must be the answer. No calculation needed! (See Important Note 3 in the solution to Sheet 8, Problem 5(c).) The resistivity of Wire 1 is the same as the resistivity of Wire 2, because both wires are made of the same material (namely, gold). The resistance of Wire 1 is half the resistance of Wire 2, but their resistivities are the same . (See Sheet 8, Problem 2.) See Sheet 5, Problem 5(f)(i). Since C increases (because of the dielectric), and Q stays the same (because the battery was disconnected), the value of V must decrease (because Q = CV ). See Sheet 5, Problem 5(f)(ii). Since C increases (because of the dielectric), and V stays the same (because the battery remains connected), the value of 2 1 CV 2 (which equals PE elec ) will increase .

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7. True or False? Two resistors 1 X and 2 X are connected in parallel. If the current in the 1 X resistor is 4 A, then the current in the 2 X resistor is 2 A. A True B False Questions 8, 9, 10, and 11: The circuit to the right contains a switch S (shown open) and three resistors with resistances R 1 ohms, R 2 ohms, and R 3 ohms, as shown. It is known that R 3 is greater than R 2 , and R 2 is greater than R 1 . 8. True or False? If the switch is open, then the current in the R 1 X resistor will be the same as the current in the R 3 X resistor. A True B False 9. True or False? If the switch is closed, then the voltage drop across the R 1 X resistor will be less than the voltage drop across the R 2 X resistor.
• Fall '17
• Steven Leduc

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