The $K_{\alpha}$ $X$-ray emission line of tungsten occurs at $\lambda = 0.021 \, nm$. The energy difference between $K$ and $L$ levels in this atom is about:

$A$ straight wire of length $20 \ cm$ carrying a current of $\frac{3}{\pi^2} \ A$ is bent in the form of a circle. The magnetic field at the centre of the circle is

Given below are two statements regarding silent heart attack.
Statement $I$: This type of heart attack shows symptoms like chest pain and shortness of breath.
Statement $II$: In this type of heart attack,usually person shows very mild symptoms and thus it is ignored.
In the light of above two statements,choose the correct answer from options given below.

Water is flowing with a velocity of $2\,m/s$ in a horizontal pipe where the cross-sectional area is $2 \times 10^{-2}\, m^2$ at a pressure of $4 \times 10^4\, Pa$. The pressure at a cross-section of area $0.01\, m^2$ in $Pa$ will be:

When $Mg$ burns in air,it produces

In which of the following,ortho/para substitution by an electrophile is very facile?