RC Circuits. AP Physics C Montwood High School R. Casao. RC Circuits. To date, we have studied steady-state direct current circuits in which the current is constant. In circuits containing a capacitor, the current varies over time.
AP Physics C
Montwood High School
Current has its maximum value of I max = E/R at t = 0 s and decays exponentially to 0 A as t infinity.
The charge on a capacitor is 0 C at t = 0 s and approaches a maximum value of Qmax = C·E as t infinity.
Charging Capacitor Graphs
current voltage charge
Ω · F = (V/A)(C/V) = C/(C/s) = s
Rise in Charge
In a time t of one time constant, the chargeq rises to63%of its maximum, while the currenti decays to 37%of its maximum value.
Discharging Capacitor Graphs
voltage charge current
U = 0.5QV = 0.5QmaxE = 0.5CE2
capacitor acts as a short circuit between points c and d.
12 V – 4 Ω·If– 8 Ω·If = 0; 12 V – 12 Ω·If; If = 1 A
V = 0.5 A·18 Ω = 9 V.
between the two
points is equal to
the voltage across
the capacitor: 9V – 1 V = 8 V.