(a) larger than speed of light
(b) almost equal to speed of light
(c) equal to speed of light
(d) very small compared to speed of light
Correct Answer: (d) very
small compared to speed of light
Explanation
Frequent collisions with lattice
atoms keep drift velocity very low.
Notes for
More Information
Concept: Drift Velocity
- Drift velocity is the average velocity gained
by free electrons when an electric field is
applied to a conductor.
- Inside
a conductor, free electrons are always in random motion due
to thermal energy.
- This
random motion exists even when no voltage is applied.
- When
the conductor is connected to a battery, an electric
field is created inside the conductor.
- Under
the influence of this electric field, electrons start drifting
slowly in a definite direction.
- This
slow, directed movement is superimposed on their random motion.
- The net
average velocity of electrons in the direction of the electric
field is called drift velocity.
Drift Velocity Formula
- The
drift velocity of electrons is given by:
Where:
= drift velocity (m/s)
= current flowing through the
conductor (A)
= number of free electrons per unit
volume
= cross-sectional area of the
conductor (m²)
= charge of an electron (1.6 ×
10⁻¹⁹ C)
Explanation
- Drift
velocity represents the flow velocity attained by
electrons due to an applied electric field.
- Its
value depends on:
- The current flowing in the conductor
- Electron density
- Cross-sectional area of the conductor
- Since
electrons collide frequently with atoms, their drift velocity is very
small.
- In a
typical copper conductor, the drift velocity is of the order
of:
- 10⁻⁴ m/s
- That is, a few millimeters per second (mm/s)
Drift Velocity vs Speed of Light
- The speed
of light in vacuum is approximately:
- 3 × 10⁸ m/s
- Compared
to this, the drift velocity of electrons is extremely small.
- Therefore, drift
velocity ≪ speed of light.
- This
shows that electrons move very slowly, even though electrical
energy propagates very fast through the conductor.
Key Takeaway
- Drift
velocity is the average directed motion of electrons due
to an electric field.
- It
is very small in magnitude and should not be confused
with the speed of electrical signal propagation.
