Practice portal › Current Electricity › Electric Current and Drift Velocity

What will be the time taken by an electron to move with drift velocity from one end to the other end of a copper conductor 3 m long and carrying a current of 3 A? Area of cross-section = 2×10⁻⁶ m² and the number density of free electrons in copper is 8.5×10²⁸ m⁻³.

Asked in RS Academy GUJCET booklet · Drift velocity, mobility and current density

• (1) 2.72×10³ s
• (2) 2.72×10⁴ s
• (3) 2.72 s
• (4) 2.72×10⁻⁴ s

Step-by-step solution

Why the other options are wrong

• (1) 2.72×10³ s loses a power of ten when tidying 8.5×1.6×2 = 27.2: the product is 27.2×10³ = 2.72×10⁴, not 2.72×10³.
• (3) No single slip gives 2.72 s; it keeps the digits and drops the power of ten. Sanity check: drift speeds are around 10⁻⁴ m s⁻¹, so crossing 3 m takes around 10⁴ s.
• (4) No single slip gives 2.72×10⁻⁴ s; the exponent has the wrong sign. That time would need a drift speed of about 10⁴ m s⁻¹, while real drift speeds are around 10⁻⁴ m s⁻¹.

Reading this solution needs no sign-in. You only sign in to practise against the clock and keep your progress.