A null point is obtained at 200 cm on potentiometer wire when cell in secondary circuit is shunted by $5 \Omega$. When a resistance of $15 \Omega$ is used for shunting, null point moves to 300 cm . The internal resistance of the cell is

A resistance of $200 \Omega$ and an inductor of $\frac{1}{2 \pi} \mathrm{H}$ are connected in series to a.c. voltage of 40 V and 100 Hz frequency. The phase angle between the voltage and current is

A wire of length L , diameter ' d ' density of material ' e ' is under tension ' T ', having fundamental frequency of vibration $\mathrm{n}_{\mathrm{A}}$. Another wire of length 2 L , tension 2 T , density 2 e and diameter 3 d has fundamental frequency of vibration $\mathrm{n}_{\mathrm{B}}$. The ratio $\mathrm{n}_{\mathrm{B}}: \mathrm{n}_{\mathrm{A}}$ is

' n ' small spherical drops of same size which are charged to ' $V$ ' volt each coalesce to form a single big drop. The potential of the big drop is