Current formula in conductors

Steps:

• Recall the standard formula for electric current in a metal wire: I = n q A v, where A is cross-sectional area.
• The given expression I = n q v implies A is incorporated or omitted, but n remains the density of charge carriers.
• In metals, charge carriers are free electrons, so n denotes their concentration.
• Eliminate options: atoms (A) don't carry current; total electrons (D) include bound ones; "per unit" in C is vague without specifying volume.

Why B is correct:

• In the Drude model, n is defined as the number of free electrons per unit volume, directly from I = n q v A.

Why the others are wrong:

• A: Atoms are fixed in the lattice and do not contribute to current flow.
• C: "Per unit of the metal" lacks specificity (e.g., volume, length), making it imprecise.
• D: Total electrons include bound electrons in atoms, not just mobile free electrons.

Final answer: B