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

V_Capacitor = V_Battery(1 - e^-t/2T)

V_Capacitor = V_Maximum * 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 V_Maximum to 0? Wouldn't I need to know V_Battery? 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 = V_o*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?

Homework Equations

V = V_o*e^-t/RC

The Attempt at a Solution

0.01*V_o = V_o*e^-t/RC
0.01 = e^-t/RC
100 = e^t/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!

 

[xcvxcvvc]

For the first problem, I think your teacher wants a symbolic answer such as ".5V"

Your 4.61 time constants is right.

 

[chrisfnet]

I'm not even sure how to work either of the first two problems? It seems like I should need to know more information...

 

[chrisfnet]

Anyone? I've been trying to work the first two still, and I just can't get past the fact that I think that I need more information.