The ratio of angular momentum of an electron in $\mathrm{n}^{\text {th }}$ orbit of hydrogen atom to the velocity of electron in $\mathrm{n}^{\text {th }}$ orbit is proportional to

An element $\overrightarrow{\Delta l}=\Delta \mathrm{xi}$ is placed at the origin and carries a current of 10 A . The magnitude of magnetic field on the Y axis at a distance of 0.5 m if $\Delta x=1 \mathrm{~cm}$ is $\left(\frac{\mu_0}{4 \pi}=10^{-7}\right.$ SI unit $)\left(\sin 90^{\circ}=1\right)$

In an n-p-n transistor amplifier, the collector current is 9 mA . If $90 \%$ of the electrons emitted reach the collector, then

The closed and open organ pipe have same length and when they are vibrating simultaneously in first overtone produce 3 beats. The length of open pipe is made $\left(\frac{1}{3}\right)^{\mathrm{rd}}$ and that of closed pipe is made 3 times the original, the number of beats produced will be (Neglect end correction)