Electronic flash units for cameras contain a capacitor for storing the energy us

Question

Electronic flash units for cameras contain a capacitor for storing the energy used to produce the flash. In one such unit, the flash lasts for a time interval of 1.50×103 s with an average light power output of 2.40×105 W .

Part A

If the conversion of electrical energy to light has an efficiency of 92.0 % (the rest of the energy goes to thermal energy), how much energy must be stored in the capacitor for one flash?

Part B

The capacitor has a potential difference between its plates of 130 V when the stored energy equals the value calculated in part A. What is the capacitance?

Electronic flash units for cameras contain a capacitor for storing the energy used to produce the flash. In one such unit, the flash lasts for a time interval of 1.50×103 s with an average light power output of 2.40×105 W .

Part A

If the conversion of electrical energy to light has an efficiency of 92.0 % (the rest of the energy goes to thermal energy), how much energy must be stored in the capacitor for one flash?

Part B

The capacitor has a potential difference between its plates of 130 V when the stored energy equals the value calculated in part A. What is the capacitance?

Explanation / Answer

a) total energy of the light is

U light = p light*time

92% of the total energy released from the capacitor

so u light = 0.92 uc

uc = u light/0.92 = p*t/0.92

uc = 2.4*10^5*1.5*10^-3/0.92

uc = 391.3 J

b) uc = 1/2cv^2

c = 2uc/v^2 = 2*391.3/130^2

c = 46.3 mF

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