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Homework Help: Graph of a capacitor

  1. Apr 14, 2004 #1
    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. jcsd
  3. Apr 14, 2004 #2
    Did you also learn what the formula for that graph is?

    For a discharging capacitor it is:

    [tex]V(t) = V_0e^{-\frac{t}{\tau }}[/tex]

    There are, if I remember correctly, about four ways to find [itex]\tau [/itex] 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 [itex]\tau [/itex], 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
  4. Apr 14, 2004 #3
    How do I find resistance with that graph?
  5. Apr 15, 2004 #4
    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.
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