Alternating Current - Result Question 18
18. A coil of self-inductance $L$ is connected in series with a bulb $B$ and an $AC$ source. Brightness of the bulb decreases when
[2013]
(a) number of turns in the coil is reduced
(b) a capacitance of reactance $X_C=X_L$ is included in the same circuit
(c) an iron rod is inserted in the coil
(d) frequency of the $AC$ source is decreased
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Answer:
Correct Answer: 18. (c)
Solution:
- (c) By inserting iron rod in the coil,
$L \uparrow Z \uparrow I \downarrow$ so brightness $\downarrow$
Inductive reactance, $X_L=\omega L$. On inserting iron rod in the coil, $L$ increases. As a result Impedance $Z=\sqrt{X_L^{2}+R^{2}}=\sqrt{(\omega L)^{2}+R^{2}}$ also increases. So, Current, $I=\frac{V}{Z}$ decreases.
