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Electric bulb glows dimmer or brighter when connected to AC source with variable freq, and Capacitor?


An AC voltage source of variable angular frequency 'w' and fixed amplitude Vis connected in series with a capacitance C and an electric bulb of resistance R (inductance zero). When 'w' is increased:

a) the bulb glows dimmer

b) the bulb glows brighter

c) total impedance of circuit is unchanged

d) total impedance of circuit increases ??

What is its correct option? this ques came in IIT JEE 2010. 


15-Sep-2015 11:17 PM
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Answers (1)


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In the AC circuit, there's a Capacitor and a Resistor (bulb) in series.

Impedance of circuit (Z) = sqrt ( (Xc)+ (R) )

X= 1 / (wC)2  

Therefore, as w increases, Z decreases.

 Current in circuit is given as: I = V/ Z

When w is increased, the impedance of circuit decreases and current through bulb increases. Therefore, bulb glows brighter.

Correct answer is (b)



15-09-2015 23:25
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