Find Capacitance from Graph of a Capacitor

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    Capacitor Graph
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Homework Help Overview

The discussion revolves around analyzing a graph of a capacitor that depicts potential versus time, specifically focusing on how to derive capacitance from this graph. The subject area includes concepts related to capacitors, exponential decay, and circuit analysis.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation, Assumption checking

Approaches and Questions Raised

  • Participants discuss the relationship between capacitance, resistance, and the graph's exponential curve. Questions are raised about how to determine resistance from the graph and what conditions are necessary for voltage to be present across a capacitor or resistor.

Discussion Status

The discussion is ongoing, with participants exploring different methods to extract values from the graph. Some guidance is provided regarding the formula for a discharging capacitor, and the limitations of determining resistance from the graph are noted. There is no explicit consensus on the questions raised, indicating a variety of interpretations and approaches being considered.

Contextual Notes

Participants mention the need for initial current to find resistance and highlight the potential for multiple combinations of resistance and capacitance yielding the same graph, suggesting a lack of sufficient information to resolve certain questions.

UrbanXrisis
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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?
 
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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:
How do I find resistance with that graph?
 
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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