Charging Time for RC Circuit with Capacitor and Resistor

[RougeSun]

Homework Statement

A 1234. pF capacitor and a 5.6 × 106 ohm resistor are connected in series to 78. volts EMF. Approximately how long does it take the capacitor to become almost fully charged?

Homework Equations

Vf=Vo(1-e^(-t/RC))

The Attempt at a Solution

I'm not sure if the equation I'm using is correct. I know there is one that has Q in it but couldn't find it. If there is Q in it how do I link it to volts? Through C=Q/Z?

 

[kuruman]

You can use the equation you have. Just think, what will the voltage across the capacitor be when it is fully charged?

 

[RougeSun]

You can use the equation you have. Just think, what will the voltage across the capacitor be when it is fully charged?

Ahh..okay. But wouldn't Vo be 0?

 

[kuruman]

Look at your expression. If V₀ were zero, then the voltage would be zero at all times and the capacitor would never charge. Here, V₀ does not represent the "voltage at time t = 0". It represents the constant emf provided by the battery. What happens to your expression if you let time t go to infinity?

 

[lee.perrin@gm]

Homework Statement

A 1234. pF capacitor and a 5.6 × 106 ohm resistor are connected in series to 78. volts EMF. Approximately how long does it take the capacitor to become almost fully charged?

Homework Equations

Vf=Vo(1-e^(-t/RC))

The Attempt at a Solution

I'm not sure if the equation I'm using is correct. I know there is one that has Q in it but couldn't find it. If there is Q in it how do I link it to volts? Through C=Q/Z?

The equation that I have is Vc(t)=Vs(1-e^-t/RC)

Where Vc is the voltage across the capacitor
Vs is the voltae of the supply

Hope this helps.

 

[lee.perrin@gm]

Now I have a question:

Im working with a signal generator RC circuit. And I am trying to work out t=RC to workout the voltage waveform across the capacitor with the above mentioned formula.

This is a very basic formula, but following a few of them in a textbook has thrown me.

eg.
100 k Ohm
0.001 mirco F
This is what is in the textbook
t=RC=(100k)(0.001mF)=100millisecond
Now to me this is should be
t=RC=(100000 Ohm)(0.000000001 F)=0.0001 sec

I seem to be missing a decimal place each time. Can someone help please?

 

[kuruman]

This is what is in the textbook
t=RC=(100k)(0.001mF)=100millisecond

This is correct as it stands.

1 mF (one millifarad) = 10^-3F

1 μF (one microfarad) = 10^-6F

Are you perhaps confusing the two?

 

[lee.perrin@gm]

The mF that I have in there is microfarad. I did not have the mu sign.

And as it is I don't get that value on the calculator.

I get 0.0001sec

 

[kuruman]

The mF that I have in there is microfarad. I did not have the mu sign.

And as it is I don't get that value on the calculator.

I get 0.0001sec

Then you are correct and

RC=(100k)(0.001mF)=100millisecond

is also correct. In your case the capacitance is in microfarads and in the other case the capacitance is in millifarads.

 

[lee.perrin@gm]

Thank you for your help.

What I have done was made a mistake where it should be mircosecond not millisecond.