Key relationship
vd=µE; I=neAvd; σ=neµ.
Physics · Class 12
Drift velocity and mobility
A small average drift speed can produce appreciable current because a metal contains many mobile electrons. Electron drift is opposite to conventional current.
Open the interactive experiment →Drift velocity and mobility
எலக்ட்ரான்களின் சராசரி நகர்வு வேகம் சிறியது. துகள்களின் அடர்த்தி அதிகமாதலால் மின்னோட்டம் பெரியதாகும்.
Key notes, relationships and model limits
Remember
A small average drift speed can produce appreciable current because a metal contains many mobile electrons. Electron drift is opposite to conventional current.
Common trap
Electron drift is opposite to the electric field. Conventional current follows the motion of positive charge.
Model limits
Uniform conductor, one carrier species, constant mobility; drift speed readout is magnitude along conventional-current direction.
What you will learn
Use the controls and plotted relation to explain drift velocity and mobility.
Try the experiment
Try both scenarios, change one control, and predict the result before checking the graph and readouts.
Think of it this way
Change one input while keeping the others fixed, as you would in a controlled experiment.
Worked example
For n=8.5×10²⁸/m³, A=1 mm² and vd=0.44 mm/s, I≈5.99 A.
Why this matters
The model connects drift velocity and mobility with an observable reading. Check the equation and assumptions before applying it.
Before you start
SI units · algebra · graph reading
Check your understanding
If wire area doubles at fixed density and drift speed, current?
- Halves because carriers have more room.
- Doubles according to I=neAvd.
- Stays fixed because drift speed is fixed.
Read the explanation
Doubles according to I=neAvd.
Course connection: Class XII Chapter 3. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.