Which one of the following elements is not an essential micronutrient for plant growth?

If the charge on a body is increased by $2 \ C$,the energy stored in it increases by $21 \%$. The original charge on the body in coulomb is

$A$ transformer works on the principle of

An electron revolves in a circle of radius $0.4 \text{ \AA}$ with a speed of $10^6 \text{ m/s}$ in a hydrogen atom. The magnetic field produced at the centre of the orbit due to the motion of the electron (in Tesla) is: $\left[\mu_0 = 4\pi \times 10^{-7} \text{ H/m}, e = 1.6 \times 10^{-19} \text{ C}\right]$

The value of the series $\cos 12^{\circ} + \cos 84^{\circ} + \cos 132^{\circ} + \cos 156^{\circ}$ is

$A$ liquid drop of diameter $D$ breaks into $27$ tiny drops. The change in energy is