The charges on two spheres are \(+7~\mu C\) and \(-5~\mu C\) respectively. They experience a force \(F.\) If each of them is given an additional charge of \(-2~\mu C,\) then the new force of attraction will be:
1. \(F\)
2. \(\frac{F}{2}\)
3. \(\frac{F}{\sqrt3}\)
4. \(2F\)
A rod made up of metal is 1.2 m long and 0.8 cm in diameter. Its resistance is 3.5 × 10-3 \(\Omega.\) Another disc made of the same metal is 2.0 cm in diameter and 1.25 mm thick. What is the resistance between the round faces of the disc?
1. 1.35 × 10-8 \(\Omega\)
2. 2.70 × 10-7 \(\Omega\)
3. 5.82 × 10-7 \(\Omega\)
4. 8.10 × 10-5 \(\Omega\)
A circular current-carrying coil has a radius \(R.\) The distance from the centre of the coil, on the axis, where \(B\) will be \(\frac18\) of its value at the centre of the coil is:
1. \(\frac{R}{\sqrt3}\)
2. \(\sqrt3R\)
3. \(2\sqrt3R\)
4. \(\frac{2R}{\sqrt3}\)
A 7Li target is bombarded with a proton beam current 10-4 A for one hour to produce 7Be of activity 1.8 × 108 disintegrations per second. Assuming that one 7Be radioactive nuclei is produced by bombarding 1000 protons, its half-life is:
1. 0.87 × 107 s
2. 0.2 × 107 s
3. 0.67 × 108 s
4. 0.87 × 106 s