Drift Speed and Carrier Density
For one carrier species in a uniform conductor, current magnitude equals number density times carrier charge magnitude, cross-sectional area, and drift-speed magnitude.
Why this shows up in the exam
Estimating electron drift speed · Finding carrier density · Comparing wires carrying the same current
Learn the idea
A small average drift of many carriers produces an ordinary macroscopic current. Electrons move randomly even without a battery, but their random velocities cancel. An electric field adds a tiny directed drift whose collective charge transport is measurable.
🧠 Memory hook: Many carriers times tiny drift makes the current.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I = n |q| A v_d — magnitude for uniform n, A, and average drift speed
- v_d = I/(n |q| A) — drift speed inferred from a steady current
How to approach it
- 1Write I = n|q|Av_d
- 2Convert mm squared to m squared
- 3State direction separately from magnitude
Common slip-ups that cost marks
- •Using electron random thermal speed as drift speed
- •Forgetting cross-sectional area conversion
- •Assigning electron drift the conventional-current direction
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A cell of emf 6 V and internal resistance 1 ohm is connected to a 2 ohm resistor. Find the circuit current.
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