Electric charges of $+10\,\mu C$,$+5\,\mu C$,$-3\,\mu C$,and $+8\,\mu C$ are placed at the corners of a square of side $\sqrt{2}\,m$. The potential at the centre of the square is:

The half-life of $Ra^{226}$ is $1620$ years. Calculate the number of atoms that decay in one second in $1 \ g$ of radium (Avogadro number $= 6.023 \times 10^{23}$).

$A$ six-faced unbiased die is thrown twice and the sum of the numbers appearing on the upper face is observed to be $7$. The probability that the number $3$ has appeared at least once is:

Two nuclei have their mass numbers in the ratio of $1 : 3.$ The ratio of their nuclear densities would be

In the following structure,the double bonds are marked as $I, II, III$ and $IV$. Geometrical isomerism is not possible at site$(s)$: