1. Aug 7, 2012

### Cetullah

My friends, it is probably an easy one, but I dont know it and I have an exam tomorrow :X

1. The problem statement, all variables and given/known data
How much time does it take for a capacitor to reach %80 of its max. potential difference?

2. Relevant equations

Thats the part I m needing.

3. The attempt at a solution

2. Aug 7, 2012

### 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.

3. Aug 7, 2012

### Cetullah

Hmm... I have translated the question from my test completely, and there isnt any other information given. So, does it mean that the question is incomplete?

4. Aug 7, 2012

### cepheid

Staff Emeritus
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?

5. Aug 7, 2012

### tiny-tim

Hi Cetullah!
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?​

6. Aug 7, 2012

### Cetullah

Hi tiny-tim

Yeah, thats 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 "Vc=V*(1-e^(-t/RC)), right?

7. Aug 7, 2012

### cepheid

Staff Emeritus
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.

8. Aug 7, 2012

i agree

9. Aug 7, 2012

### 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"