A potentiometer has an uncertainty of $\pm 0.0001 \mathrm{V}$. If it is used to measure current through a standard resistance of $\text{0.1}\pm \text{0.001}\text{ \%}\mathrm{ \Omega }$ (uncertainty). The voltage across the resistance is measured to be $0.2514 \mathrm{V}$. What is the percentage uncertainty in the measurement of current?

A thin metallic spherical shell carries a charge $Q$ on it. A point charge $q$ is placed at the centre of the shell and another charge $q_1$ is placed outside it, as shown in figure. All the three charges are positive.

The force on the charge at the centre is 

A copper rod of length $20 \mathrm{cm}$ and cross-sectional area $2 {\mathrm{mm}}^2$ is joined with a similar aluminium rod as shown below


The resistance of pair of rods is  ${\rho }_{Al}=2.6\times {10}^{-8}\mathrm{\Omega }-m$  and ${\rho }_{Cu}=1.7\times {10}^{-8}\mathrm{\Omega }-m$