Question

a flash unit for a camera has a capacitance of 1200μF.

what resistance is needed in this RC circuit if the flash isto charge to 90% of its full charge in 20s? R=

Answer 1

I think the answer should be as follow:

Given, C = 1200μF, t = 20s, R = ?

Using this equation, q(t) = Cε (1- e^{^-t/RC}), set upthe charge to be 90% (0.90)

                        0.90Cε = Cε(1- e^{^-t/RC}), now you can ignore the Cε since they're on each side

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

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

                        e^{^-t/RC}=0.10

                       ln e^{^-t/RC}= ln 0.10

                       - t/RC = ln 0.10

       -20/((R)(1200μF)) =-2.3 , now solve for R

                  

                   R = 7246Ω

I hope that's what you're looking for.