# RC Circuit - Post-Lab Questions

chrisfnet

## Homework Statement

What is the voltage across a capacitor after a time of two constants when charging from zero voltage. When discharging from a fully charged condition?

## Homework Equations

VCapacitor = VBattery(1 - e-t/2T)

VCapacitor = VMaximum * e-t/2T

## The Attempt at a Solution

I'm confused at how to start. I can do the algebra...

Charging from zero voltage - so would I set VMaximum to 0? Wouldn't I need to know VBattery? That wouldn't make sense to me. If I can get an idea of how to approach this problem, I should be able to solve it no problem. Thanks!

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## Homework Statement

With V = Vo*e-t/RC, it mathematically takes an infinite time for a capacitor in an RC circuit to discharge. Practically, how many time constants does it take for a capacitor to discharge to less than 1% of its initial voltage?

V = Vo*e-t/RC

## The Attempt at a Solution

0.01*Vo = Vo*e-t/RC
0.01 = e-t/RC
100 = et/RC
ln(100) = t/RC
2*ln(10)*RC = t

So, I'm saying that it takes approximately 4.61 time constants.

Would this be correct? Thanks!