How Long Does It Take for a Capacitor to Charge to 80% of Its Maximum Voltage?

[Cetullah]

My friends, it is probably an easy one, but I don't know it and I have an exam tomorrow :X

Homework Statement

How much time does it take for a capacitor to reach %80 of its max. potential difference?

Homework Equations

Thats the part I m needing.

The Attempt at a Solution

Thank you for your help!

 

[lewando]

It depends on the charging circuit. If an ideal capacitor is attached to an ideal voltage source through a switch, when the switch is closed, the capacitor will charge instantaneously. Non-ideal devices will have some resistance which will affect the charging time.

 

[Cetullah]

Hmm... I have translated the question from my test completely, and there isn't any other information given. So, does it mean that the question is incomplete?

 

[cepheid]

Hmm... I have translated the question from my test completely, and there isn't any other information given. So, does it mean that the question is incomplete?

As mentioned before, in real life, even if there are no components in the circuit but the capacitor, the circuit will always have some non-zero resistance. So, if you assume that the capacitor is charging through a resistor, you can then answer the question.

How long does it take for the capacitor to reach 80% of full charge in an *RC* circuit?

 

[tiny-tim]

Hi Cetullah!

How much time does it take for a capacitor to reach %80 of its max. potential difference?

That's like asking "How long is a piece of string?"

You have to know something about the capacitor (and the circuit it's in) to answer that!

Are you sure a "time constant" isn't given?​

 

[Cetullah]

Hi tiny-tim

Yeah, that's a weird question, but that's what exactly writes on the paper.

Our teacher is a weird man anyways.

But I guess, if sufficient data were given, we would use the equation "V_c=V*(1-e^(-t/RC)), right?

 

[cepheid]

Hi tiny-tim

Yeah, that's a weird question, but that's what exactly writes on the paper.

Our teacher is a weird man anyways.

But I guess, if sufficient data were given, we would use the equation "V_c=V*(1-e^(-t/RC)), right?

That equation seems about right. The question I asked you in my previous post still stands. There is not reason why you cannot express an answer in terms of the time constant.

 

[tiny-tim]

i agree

 

[Cetullah]

Oh cepheid, so you advice me to substitute the "t" with something like "kRC"?

Edit: Did so, and found one of the answers on paper "1.6"