Q.Define electric current. Write the relation between current and the charge flowing through a cross-section of a conductor, and state the SI unit of current.

Electric current is defined as the net rate of flow of electric charge across a chosen cross-section of a conductor. For a steady flow, if a net charge $q$ crosses the section in time $t$, the current is $I=q/t$; more generally, for a current that varies with time, it is the instantaneous limit $I=\lim_{\Delta t\to0}\Delta q/\Delta t=dq/dt$. Its SI unit is the ampere (A), equal to one coulomb of charge crossing per second.

A current can, in principle, be carried by positive charges, negative charges, or both together (as in an electrolyte, where both ion species contribute). In a metal specifically, only the free (conduction) electrons are mobile, so the current is entirely due to electron flow -- yet, by a long-standing sign convention fixed before the electron was discovered, the direction of current in a circuit is always drawn as the direction positive charge would move, even though the actual electrons are moving in the opposite direction. Everyday currents span an enormous range, from a lightning strike (~10,000 A) down to the milliampere-to-nanoampere currents typical of semiconductor devices.