capacitance

[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?

[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.

[UrbanXrisis]

How do I find resistance with that graph?

[Chen]

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