Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
**Assertion (A):** A choke coil is simply a coil having a large inductance but a small resistance. Choke coils are used with fluorescent mercury-tube fittings. If household electric power is directly connected to a mercury tube, the tube will be damaged.
**Reason (R):** By using the choke coil, the voltage across the tube is reduced by a factor $\left(R/\sqrt{R^2 + \omega^2L^2}\right)$, where $\omega$ is the frequency of the supply across resistor $R$ and inductor $L$. If the choke coil were not used, the voltage across the resistor would be the same as the applied voltage.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer: (B) Both (A) and (R) are true and (R) is the correct explanation of (A)
**(A)** A choke has a large inductance and very small resistance, so it limits the current while wasting almost no power. Connected straight to the mains, the tube would draw too large a current and be damaged. True.
**(R)** With the choke in series, the current is $V/\sqrt{R^2 + \omega^2L^2}$ and the voltage across the tube (resistance $R$) is
$$V_R = V\cdot\frac{R}{\sqrt{R^2 + \omega^2L^2}}$$
Without the choke, the full supply voltage would appear across the tube. True, and this reduction is exactly why the tube is protected, so (R) explains (A).
Solution by Sreeraj P, M.Sc Physics