Question

This ‘average velocity’ is found to be only a few mm/s for currents in the range of a few amperes. How then is current established almost the instant a circuit is closed?

Hint:

Do not confuse drift velocity with signal speed. Drift velocity is only a few mm/s, whereas the electric field responsible for current propagation travels nearly with the speed of light.

Answer 1

Concept: Although the drift velocity of electrons is extremely small, the electric field produced when a circuit is closed propagates through the conductor nearly at the speed of light. Explanation: When a switch is closed, the electric field is established throughout the conductor almost immediately. The free electrons already present everywhere inside the conductor begin responding simultaneously to this electric field. Thus, electrons near the battery push neighbouring electrons, which in turn push other electrons, resulting in the flow of current throughout the circuit almost instantly. An analogy can be made with a long pipe completely filled with water. When water is pushed at one end, water starts emerging from the other end almost immediately, even though individual water molecules move slowly. Similarly, electrons drift slowly, but the signal responsible for current flow travels very rapidly. Result: Current is established almost instantaneously because the electric field propagates through the conductor nearly with the speed of electromagnetic waves, causing electrons everywhere in the circuit to start drifting simultaneously.