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A copper wire has a square cross-section, $2.0\, mm$ on a side. It carries a current of $8\, A$ and the density of free electrons is $8 \times {10^{28}}\,{m^{ - 3}}$. The drift speed of electrons is equal to
In a conductor, if the number of conduction electrons per unit volume is $8.5 \times 10^{28}\, m^{-3}$ and mean free time is $25\,fs$ (femto second), its approximate resistivity is $\left( {{m_e} = 9.1 \times {{10}^{ - 31}}\,kg} \right)$
In a Wheatstone bridge (see figure) Resistance $P$ and $Q$ are approximately equal. When $R = 400\,\Omega ,$ the bridge is balanced. On interchanging $P$ and $Q,$ the value of $R,$ for balance is $405\,\Omega .$ The value of $X$ is close to ................. $ohm$