Homework #7
203-1-1721
Physics 2 for Students of Mechanical Engineering
Part A
5. In Fig. 31-25 below, the potential at point P is 100 V. What is the potential at point Q?
7. (a) In Fig. 31-27 below, what value must R have if the current in the circuit is to be 50 mA? Take 1
= 2.0 V, 2 = 3.0 V, and r1 = r2 = 3.0 . (b) What is the rate at which internal energy appears in the
resistor R?
10. Internal energy is to be generated in a 108-m resistor at the rate of 9.88 W by connecting it to a
battery whose emf is 1.50 V. (a) What is the internal resistance of the battery? (b) What potential
difference exists across the resistor?
11. What current in terms of  and R, does the ammeter A in Fig. 31-29 read? Assume that A has zero
resistance.
13. (a) Calculate the current through each source of emf in Fig. 31-31. (b) Calculate Vb-Va. Assume
that R1 = 1.20 , R2 = 2.30 , 1 = 2.00 V, 2 = 3.80 V, and 3 = 5.00 V.
19. A circuit containing five resistors connected to a 12-V battery is shown in Fig. 31-33. Find the
potential difference across the 5.0- resistor.
23. (a) In Fig. 31-34, find the equivalent resistance of the network shown. (b) Calculate the current in
each resistor. Put R1 = 112 , R2 = 42.0 , R3 = 61.6 , R4 = 75.0 , and  = 6.22 V.
31. A space heater, operating from a 120-V line in the U.S., has a hot resistance of 14.0 . (a) At what
rate is electrical energy transferred into internal energy? (b) At 5.22 cents/kW-h, what does it cost to
operate the device for 6 hours and 25 minutes?
35. A 100-Watt light bulb is plugged into a standard 120-V outlet. (a) How much does it cost per
month (31 days) to leave the light turned on? Assume that electric energy cost 6 cents/kW-h. (b)
What is the resistance of the bulb? (c) What is the current in the bulb? (d) Is the resistance different
when the bulb is turned off?
42. In an RC series circuit of Fig. 31-20 below  = 11.0 V, R = 1.42 M, and C = 1.80 F. (a)
Calculate the time constant. (b) Find the maximum charge that will appear on the capacitor during
the charging. (c) How long does it take for the charge to build up to 15.5 C?
Figure 31-20
44. A 15.2-k resistor and a capacitor are connected in series and a 13.0-V potential is suddenly
applied. The potential across the capacitor rises to 5.00 V in 1.28 s. (a) Calculate the time constant.
(b) Find the capacitance of the capacitor.
45. An RC circuit of Fig. 31-20 above is discharged by closing a switch at time t = 0. The initial
potential difference across the capacitor is 100 V. (a) If the potential difference has decreased to
1.06 V after 10.0 s, calculate the time constant of the circuit. (b) What will be the potential
difference at t = 17 s?
Part B
18. An initially uncharged capacitor C is fully charged by a constant emf  in series with a resistor R as
shown in Fig. 31-20 above. (a) Show that the final energy stored in the capacitor is half the energy
supplied by the emf. (b) By direct integration of i2R over the charging time, show that the internal
energy dissipated by the resistor is also have the energy supplied by the emf.
19. At what time after charging begins in question 18 above is the rate of energy dissipation in the
resistor equal to the rate of energy storage in the capacitor?