Note that the current in an RL circuit looks much like that in an RC circuit. In fact, we can define a time
constant for an RL circuit, just as we did for the RC circuit, as the time for the current to lose 63% of its
value (or to reach 37% of its maximum value when increasing). The time constant for an RL circuit is L/R
.
LC Circuits
In a circuit consisting of just a capacitor and an inductor, both the capacitor and inductor try to store
energy. They take turns storing the energy in the circuit—the capacitor charges, then discharges, then
charges again ...
In fact, the charge on the capacitor oscillates from maximum to minimum sinusoidally with period
. You may have to write the solution to a second-order differential equation, just like you did forthe mass on a spring in Chapter 17 .
Practice Problems
Multiple Choice:
1 . A 100 Ω, 120 Ω, and 150 Ω resistor are connected to a 9-V battery in the circuit shown above. Which
of the three resistors dissipates the most power?
(A) the 100 Ω resistor
(B) the 120 Ω resistor
(C) the 150 Ω resistor
(D) both the 120 Ω and 150 Ω
(E) all dissipate the same power
2 . A 1.0-F capacitor is connected to a 12-V power supply until it is fully charged. The capacitor is then
disconnected from the power supply, and used to power a toy car. The average drag force on this car is
2 N. About how far will the car go?
(A) 36 m
(B) 72 m
(C) 144 m
(D) 24 m
(E) 12 m