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

AI Thread Summary
To charge a 195 µF capacitor to 125 volts with a current of 3.71 mA, the charge (Q) is calculated using the formula Q = CV, resulting in 24.3 mC. The initial attempt to find the time (t) using the equation Q = 1 - e^(t/rc) was incorrect due to misunderstanding the nature of the current. Instead, the correct approach is to use Q = IT, leading to the formula t = Q/I. This calculation yields a charging time of approximately 6.55 seconds. The discussion highlights the importance of using the appropriate equations for constant current scenarios.
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
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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