Question

The RC charging circuit in a camera flash unit has a voltage source 275 V and a capacitance of 125 F.

(a) Find its resistance R if the capacitor charges up to 90.0% of it's final value in 15.0 s.

(b)Find the average current delivered to the flash bulb if the capacitor discharges 90.0% of its full charge in 1.00 ms

Answer #1

Q = Qmax *( 1- e^{-t/RC} )

Q = 0.90* Qmax , so

0.90* Qmax = Qmax *( 1- e^{-t/RC} )

0.90 = 1- e^{-t/RC}

e^{-t/RC} = 1-0.90 = 0.10

-t/RC = ln(0.1)

*****check the data is C = 125 F or 125 μF , I took 125 F
as in question *****

R = -t/ ( ln(0.1) *C) = -15 / [ ln(0.1) *125]

**R = 0.0521153 ohm
answer**

(b)

I = ΔQ/Δt = (Qf- Qi ) / Δt =

full charge = C*V = 125*275 = 34375 C

90 % = 0.90*34375 =30937.5

i = 30937.5/ (1*10^-3 ) =**3.09375e7 A**

**i thnik your data may be wrong plz let me know in that
case**

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