Question

The drift velocity of free electrons in a conducting wire is the order of 10 − 4 m / s . Yet, the bulb glows very quickly after the switch is put ON because

• A. The electrons transfer their energy very quickly through collisions
• B. Drift velocity is independent of electric field
• C. Electric field is set up in the wire very quickly, producing a current through each cross section almost instantaneously
• D. The random speed of electrons is very high

Answer 1

Answer: (C)

Electric field is set up in the wire very quickly, producing a current through each cross section almost instantaneously.

Answer 2

When the switch is put ON, a potential difference is applied across the conducting wire connecting the power source to the bulb. This potential difference establishes an electric field throughout the wire. The electric field propagates along the wire at a speed close to the speed of light in the material.

As soon as the electric field is established, it exerts a force on the free electrons, causing them to drift. Since the electric field is established very quickly along the entire length of the wire, the free electrons throughout the circuit begin to drift almost simultaneously.

This simultaneous drift of electrons constitutes an electric current that flows through the entire circuit, including the filament of the bulb, almost instantaneously. The flow of current through the filament causes it to heat up rapidly and glow.

The drift velocity (order of $10^{-4}$ m/s) is the average speed of the directed motion of individual electrons. It is very low. However, the quick glowing of the bulb is not dependent on how quickly a single electron travels from the switch to the bulb. Instead, it depends on how quickly the electric field is established and how quickly the collective drift motion of the vast number of free electrons already present in the wire begins throughout the circuit.

Therefore, the bulb glows quickly because the electric field is set up very quickly throughout the wire, causing a current to flow almost instantaneously in every part of the circuit.