How Long Does It Take to Charge a 195 µF Capacitor to 125 Volts?

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SUMMARY

The discussion centers on calculating the time required to charge a 195 µF capacitor to 125 volts using a constant current of 3.71 mA. The initial attempt involved using the formula Q = CV, yielding a charge of 24.3 mC, and applying the equation Q = 1 - e^(t/RC), which led to an incorrect time of 162 milliseconds. The correct approach involves using the formula Q = IT, resulting in a charging time of 6.55 seconds. This highlights the importance of recognizing the nature of current in capacitor charging scenarios.

PREREQUISITES
  • Understanding of capacitor charging equations, specifically Q = CV and Q = IT.
  • Basic knowledge of Ohm's Law, particularly R = V/I.
  • Familiarity with exponential functions and their application in electrical circuits.
  • Concept of time constants in RC circuits.
NEXT STEPS
  • Study the relationship between current, voltage, and charge in capacitors.
  • Learn about the time constant in RC circuits and its implications on charging and discharging.
  • Explore practical applications of capacitors in flash units and other electronic devices.
  • Investigate the effects of varying current on capacitor charging times.
USEFUL FOR

Students in electrical engineering, electronics enthusiasts, and anyone involved in circuit design or analysis, particularly those working with capacitors in timing applications.

boneill3
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Homework Statement



In a camera flash unit, a capacitor is charged up over a period of several seconds (from a battery). The capacitor is discharged very rapidly -- in about 1 millisecond -- into a gas tube producing a very brief, bright flash of light. How long will it take to charge a 195 µF capacitor to a potential difference of 125 volts with a current of 3.71 mA? Answer in seconds.

I was given a hint to solve for Q first.

Homework Equations



Q= CV

Q= 1-e^(t/rc)

The Attempt at a Solution




FIrst I calculated Q= CV to be 195 µF * 125v = 24.3 mC

than I used ohms law to R= V/I = 33.693Kohms


Than I put those values into Q= 1-e^(t/rc)
and solved for t which gave me 162 msec

which was incorrect so I'm note sure where I am going wrong

any pointers?
 
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In this problem the current is constant and not exponential. So you have simply
Q = IT
 
Thanks mate your a ledgend

FIrst I calculated Q= CV to be 195 µF * 125v = 24.3 mC

Q = IT solve for t = Q/I t = 6.55 seconds
 

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