3 (28.76). Large capacitors were once commonly used as camera flash units. They

Question

3 (28.76). Large capacitors were once commonly used as camera flash units. They are potentially dangerous if someone handles the capacitor while it is still charged. Suppose a 120 ?F capacitor from a camera flash unit retains a voltage of 200 V when an unwary photographer removes it from the camera If the photographer accidentally touches the two terminals with his hands, and the resistance of his body between his hands is 1.8 k2, for how long will the current across his chest exceed the danger level of 50 mA?

Explanation / Answer

Time Constant

T=RC =(1800)(120*10-6)=0.216 s

Maximum Current

I=V/R =200/1800 =111.111 mA

In a RC circuit current as a function of time for charging is given by

I=Imaxe-t/T

50 =111.11e-t/0.216

Ln(50/111.111) =-t/0.216

t=0.7985*0.216 =0.172 s

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