The rc charging circuit in a camera flash unit has a voltage source of 265 v and a capacitance of 136 μf hint (a) find its resistance r (in ohms) if the capacitor charges to 90.0% of its final value in 16.2 s. (b) find the average current (in a) delivered to the flash bulb if the capacitor discharges 90.0% of its full charge in 1.14 ms.

a)

51764.7 ohm

b)

28.5 A

Explanation:

a)

t = time taken to charge to 90.0% of its final value = 16.2 s

T = time constant

C = capacitance = 136 x 10⁻⁶ F

R = resistance of the resistor

Q₀ = Maximum charge stored

Q = Charge after time "t" = 0.90 Q₀

Using the equation

T = 7.04 s

Time constant is given as

T = RC

7.04 = R (136 x 10⁻⁶)

R = 51764.7 ohm

b)

V = Potential difference of voltage source = 265 volts

q = Amount of charge discharged = 0.90 Q₀ = 0.90 (CV) = (0.90) (136 x 10⁻⁶) (265) = 0.032436 C

t = time taken to discharge = 1.14 x 10⁻³ s

Current is given as

i = 28.5 A

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