Question:

A straight conductor of uniform cross-section carries a current I. Let S be the specific charge of an electron. The momentum of all the free electrons per unit length of the conductor due to their drift velocity only, is?

(IS)2

I.S

√IS

Specific charge is charge per unit mass, S = e/m
Momentum of electrons per unit length = mvl = (e/S)vl = (e/S)(I/t)l = eI/tS = IS