acit0r
1taneOuls CUrTent
Differentiating Equation 27.22 with respectto time gives the inst:
as afunction of time:
4
tt號 一Fe ptasafurc 主二
where Q;/有CC 三方 is the initial current. Figure 27.15b 5
in the resistor that we guessedin order to-apply Kirch
Equations 27.15 and 27.21. Equation 27.23 shows that the ci
ing capacitor is negative, indicating that the current 1s
Figure 27.15c. Both the charge on the capacitor and the ct
tially at a rate characterized by the time'constant7了三用C.
人 i 一
GOuicK ouiz 27.5 Considerthe circuitin Figure 27.17 and assume the battery has
after the switch is closed, what is the currentin the Figure 27.17
no internal resistance. () 」
battery (a) 0 (b) &/2A(G 28/Z04 8/元(e) impossible to determine (ii) After a 有
eyerylong time, what is the Currentin the battery? Choose from the same choices. theswitch is
CC (3
前:銜1加信介2放 IntermittentWindshield Wipers
nobiles are equipped with windshield wipers that can operate intermittently during alight rainfall. How does
ofsuch wipers depend on the charging and discharging ofacapacitor7
民
The wipers are part ofan 及C circuit whose time constant can be varied by selecting different values of及through amultipo-
Sition sWitch. As the voltage across the capacitor increases, the capacitor reaches a point at which it discharges and Higgers
the wipers. The circuit then begins another charging cycle. The time interval between the individual sweeps of the wiperS 1
determinedjby the value ofthe time constant.
