Physics 7B
WS C1 (rev. 2.0)
Page 1
C1. RC Circuits
Questions for discussion
1
. Why does the current vary with time in an RC circuit
1
?
2
. For the simplest type of RC circuit (consisting only of a battery
ε
, a resistance R, and a
capacitance C), the
time constant
is
τ
= RC. What is the significance of this time constant? In other
words, what does the time constant for an RC circuit tell you?
3.
Consider again the simple type of RC circuit discussed in question 2.
a)
The capacitor is initially uncharged and a switch closes and completes the circuit at time t=0.
What, if any, is the capacitor’s effect on the circuit at the instant the switch is thrown? That is, does
it act like a battery, an open switch, or a closed switch?
b)
After a very long time (a few time constants, in practice), what, if any, is the capacitor’s effect on
the circuit? That is, does it act like a battery, an open switch, or a closed switch?
c)
Now consider the case when the capacitor is initially charged to some charge q and determine
the behavior of the capacitor at the instant the switch is thrown and a very long time after the
switch is thrown.
1
Some problems in RC circuits require solutions to differential equations. For the results you will need, and
derivations of those results, see “Differential Equations for Circuit Problems” in the Supplementary Material at
the end of the workbook, in the section “RC and LR Equations.”
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WS C1 (rev. 2.0)
Page 2
Problems
1
. In the RC circuit shown below, the capacitor is
initially uncharged, and the switch is initially
open.
a)
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 Fall '10
 yildiz
 Physics, Capacitance, Current, RC Circuits, Resistance, RC circuit, switch, RC time constant, capacito r

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