Physics - Current Electricity Question with Solution | TestHub

The relation between current $I$ and drift velocity $v_d$ is $I=nAe{v}_d$, where $n$ is the number density of electrons, $A$ is the cross-sectional area, and $e$ is the charge of an electron. If $I$, $n$, and $e$ are constant, then $A{v}_d=\text{constant}$, which implies $v_d\propto 1/A$. Since $A=\pi r^2$, if the radius $r$ is doubled, the area $A$ becomes $4A$. Therefore, the drift velocity $v_d$ must decrease by a factor of 4. Both Assertion and Reason are true, and the Reason correctly explains the Assertion.