1200 2 The ability of a camera (or phone) to generate a bright flash of light depends on using a capacitor to store energy slowly and release it quickly. Consider the circuit shown at right which represents the flash circuit in a camera. S: 3000 µF 100 V 0.25 2
Initially, the switch S is set to connect the (uncharged) capacitor to the battery. How
much time is required for the capacitor to charge to 50% of fully charged?
How much energy is stored in the capacitor when it is fully charged?
Same picture: When a picture is taken, the switch S moves so that battery is no longer connected but the capacitor is now connected to the light bulb. The bulb acts like a resistor with a very small resistance.
Calculate the power dissipated in the bulb at the instant the capacitor starts to discharge
through the bulb. (Note: the power will diminish as the capacitor discharges; we want to know
the maximum power, which is at the very first moment of discharge, t ≈ 0 s)
Only 5% of the flash power is emitted as visible light. If we assume (for simplicity)
that the emitted visible photons all have a wavelength lo = 550 nm, calculate the initial rate of
visible photon emission—the number of visible photons emitted per second by the flash. (Note:
still consider t ≈ 0 s , and photons are emitted in air)
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