Q125 / 180 · NEET 2013 (All India) · Physics

A coil of self-inductance $L$ is connected in series with a bulb $B$ and an $A C$ source. Brightness of the bulb decreases when

A
frequency of the $\mathrm{AC}$ source is decreased
B
number of turns in the coil is reduced
C
a capacitance of reactance $X_C=X_L$ is included in the same circuit
Correct answer
D
an iron rod is inserted in the coil

Solution

As $Z=\sqrt{R^2+X_L^2}=\sqrt{R^2+(2 \pi f V L)^2}$ As $/=\frac{V}{Z}, P=/^2 R$ i.e., $V \uparrow, L \uparrow \Rightarrow Z \uparrow, / \downarrow$ and $P \downarrow$

Hence, the correct answer is option: 'D'.

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