Integrating Current-Voltage Relationship for a Capacitor

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SUMMARY

The discussion focuses on deriving the voltage-current relationship for a capacitor using the equation i = C(dv/dt). The integration of both sides leads to the equations v = (1/C)(∫ from -∞ to t of i dt) and v = (1/C)(∫ from t₀ to t of i dt) + v(t₀). The integration from negative infinity accounts for all historical current flow affecting the capacitor's voltage, while v(t₀) represents the initial voltage at time t₀, serving as an integration constant. Understanding these concepts is crucial for accurately modeling capacitor behavior in electrical circuits.

PREREQUISITES
  • Understanding of calculus, specifically integration techniques.
  • Familiarity with electrical engineering concepts, particularly capacitors.
  • Knowledge of current-voltage relationships in circuit theory.
  • Basic understanding of initial conditions in differential equations.
NEXT STEPS
  • Study the fundamentals of capacitor behavior in AC and DC circuits.
  • Learn about the Laplace transform and its application in circuit analysis.
  • Explore the concept of initial conditions in differential equations.
  • Investigate the role of integration constants in solving differential equations.
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Electrical engineering students, circuit designers, and anyone interested in understanding the mathematical modeling of capacitors in electrical circuits.

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Homework Statement



To find the voltage-current relationship of a capcitor, integrate both sides of
i = C(dv/dt)


The Attempt at a Solution



In the book they say, v = (1/C)(The Integral from -tve infitity to t) of i dt.

or

v = (1/C)(The Integral from tnot to t) of i dt + v(tnot)


I am trying to understand why they integrated from negative infinity. Why not start the integration at 0. I also do not undersstand in the second equation where the + v(tnot) came from.

Thanks in advance
 
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An ideal capacitor will store all the charge that's ever applied to it. What you call 0 time is arbitrary (it could be a certain date and time, or when you start taking data, or anything else)--but the capacitor voltage also reflects currents that flowed before that time, all the way back.

BTW, the subscript is "nought" which means zero, not "not".

V_t0 is the integration constant (look back at your calculus book for an indefinite integral). The capacitor might have started with a voltage on it before it was hooked into your circuit.
 

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