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Physics 121 Practice Problem Solutions 08B RC Circuits

Physics 121 Practice Problem Solutions 08B RC Circuits. RC Circuits Charging a Capacitor Discharging a Capacitor Discharging Solution of the RC Circuit Differential Equation The Time Constant Examples Charging Solution of the RC Circuit Differential Equation Features of the Solution

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Physics 121 Practice Problem Solutions 08B RC Circuits

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  1. Physics 121 Practice Problem Solutions 08B RC Circuits • RC Circuits • Charging a Capacitor • Discharging a Capacitor • Discharging Solution of the RC Circuit Differential Equation • The Time Constant • Examples • Charging Solution of the RC Circuit Differential Equation • Features of the Solution • Examples Contents: 121P08 – 44P46P, 50P, 51P, 52P, 53P, 55P

  2. PROB 121P08- 44

  3. PROBLEM 121P08 - 46P*: In an RC series circuit, = 12.0 V, R = 1.40 MW, and C = 1.80 mF. (a) Calculate the time constant. (b) Find the maximum charge that will appear on the capacitor during charging. (c) How long does it take for the charge to build up to 16.0 mC?

  4. PROBLEM 121P08 - 50: An initially uncharged capacitorC is fully charged by a device of constant emf connected in series with a resistorR. (a) Show that the final energy stored in the capacitor is half the energy supplied by the emf device. (b) By direct integration of I 2R over the charging time, show that the thermal energy dissipated by the resistor is also half the energy supplied by the emf device.

  5. PROBLEM 121P08 - 51P*: A capacitor with an initial potential difference of 100 V is discharged through a resistor when a switch between them is closed at t = 0. At t = 10.0 s, the potential difference across the capacitor is 1.00 V. (a) What is the time constant of the circuit? (b) What is the potential difference across the capacitor at t = 17.0 s?

  6. PROBLEM 121P08 - 52P: The figureshows the circuit of a flashing lamp, like those attached to barrels at highway construction sites. The fluorescent lamp L (of negligible capacitance) is connected in parallel across the capacitorC of an RC circuit. There is a current through the lamp only when the potential difference across it reaches the breakdown voltage VL; in this event, the capacitor discharges completely through the lamp and the lamp flashes briefly. Suppose that two flashes per second are needed. For a lamp with breakdown voltage VL = 72.0 V, wired to a 95.0 V ideal battery and a 0.150 mF capacitor, what should be the resistanceR?

  7. PROBLEM 121P08 - 53P*: A 1.0 mF capacitor with an initial stored energy of 0.50 J is discharged through a 1.0 MW resistor. (a) What is the initial charge on the capacitor? (b) What is the current through the resistor when the discharge starts? (c) Determine VC, the potential difference across the capacitor, and VR, the potential difference across the resistor, as functions of time. (d) Express the production rate of thermal energy in the resistor as a function of time.

  8. PROBLEM 121P08 - 55: In the circuit of the figure = 1.2 kV, C = 6.5 mF, R1 = R2 = R3 = 0.73 MW. With C completely uncharged, switch S is suddenly closed (at t = 0). (a) Determine the current through each resistor at t = 0 and as t -> . (b) Draw qualitatively a graph of the potential differenceV2 across R2 from t = 0 to t = . (c) What are the numerical values of V2 at t = 0 and as t -> ? (d) What is the physical meaning of “t -> ” in this case?

  9. PROB 121PO8-55, page 2

  10. PROB 121PO8-55, page 3

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