# RC Circuit charging question

1. Dec 2, 2015

### Gabe805

1. The problem statement, all variables and given/known data
.
Switch http://edugen.wileyplus.com/edugen/courses/crs7165/halliday9781118230725/c27/math/math152.gif in Fig. 27-63 is closed at time http://edugen.wileyplus.com/edugen/courses/crs7165/halliday9781118230725/c27/math/math164.gif, to begin charging an initially uncharged capacitor of capacitance 15 microfarads through a resistor of resistance 20 ohms. At what time is the potential across the capacitor equal to that across the resistor?
http://edugen.wileyplus.com/edugen/courses/crs7165/halliday9781118230725/c27/image_n/w1548-nn.png
Figure 27-63
Problems 57 and 96

2. Relevant equations
v=vmax(1-e^-(t/RC))
V=iR
i=imax(e^-(t/RC))

3. The attempt at a solution
The answer winds up being .208 ms. so I am just looking for an answer. I want to know how to solve this. I tried using ohms law to figure out what the voltage is across R but of course that was a dead end because the emf is not given. next I tried iR=vmax(1-e^-t/RC). Again a dead end. Also I know that when the capacitor is fully charged the current is zero, hence the voltage across R will also be zero but I couldnt figure out how to implement that knowledge into a solution. Any help would be greatly appreciated.

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Last edited: Dec 2, 2015
2. Dec 2, 2015

### SteamKing

Staff Emeritus