Electric current from drifting charge carriers

Steps:

• Use the formula for current: I = n e v A, where A is the cross-sectional area perpendicular to carrier drift.
• From the diagram, carriers flow perpendicular to faces PQR/PQS, with cross-section A equal to dimension y.
• Substitute n (density), e (charge), v (drift speed), and A = y into the formula.
• This yields I = n e v y.

Why B is correct:

• It directly applies I = n e v A with A = y, the diagram's cross-sectional dimension perpendicular to flow.

Why the others are wrong:

• A: Uses x² for area, but diagram specifies y as the relevant dimension.
• C: Replaces v with v², violating the linear drift speed in the current formula.
• D: env y duplicates B but uses inconsistent notation; B matches standard form.

Final answer: B