# Graph of a capacitor

1. Apr 14, 2004

### UrbanXrisis

My class did a graph of a capacitor. Potential v. time. It showed an exponential curve. How do I find the capacitance by using that graph?

C=t/R but how do I find the resistance?

What must be present to produce a voltage across a capacitor or a resistor?

2. Apr 14, 2004

### Chen

Did you also learn what the formula for that graph is?

For a discharging capacitor it is:

$$V(t) = V_0e^{-\frac{t}{\tau }}$$

There are, if I remember correctly, about four ways to find $\tau$ from that graph, but the easiest method is to pick any point on the graph, and subtitute the values of t and V(t) in the equation above, and you can find $\tau$, from which you can find C providing you know R.

"What must be present to produce a voltage across a capacitor or a resistor?" -- I don't understand your question.

Last edited: Apr 14, 2004
3. Apr 14, 2004

### UrbanXrisis

How do I find resistance with that graph?

4. Apr 15, 2004

### Chen

You can't. R = 5ohm and C = 2&mu;F will give you the same graph as R = 2ohm and C = 5&mu;F. You might be able to find R if you know the inital current in the circuit, I0, which is equal to Vf/R.