Current as total charge flow rate

Steps:

• Total number of charge carriers in the block: N = nV.
• Total charge carried: Q = Nq = nVq.
• All carriers pass through the block in time t.
• Current I = total charge per unit time = Q/t = nqV/t.

Why B is correct:

• Matches the definition of current I = ΔQ/Δt, where ΔQ = nqV is the total charge and Δt = t.

Why the others are wrong:

• A: nqbc equals nq times cross-sectional area (missing flow rate V/t).
• C: V/t is volume flow rate (ignores charge q and density n).
• D: nqV is total charge (not divided by time t for rate).

Final answer: B