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

[boneill3]

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?

 

[CEL]

In this problem the current is constant and not exponential. So you have simply
Q = IT

 

[boneill3]

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