The relation between current density j, drift velocity v_d, number density n and charge e for a metallic conductor is
A{'text': 'j = n e / v_d', 'label': 'A'}
B{'text': 'j = e v_d / n', 'label': 'B'}
C{'text': 'j = n v_d / e', 'label': 'C'}
D{'text': 'j = n e v_d', 'label': 'D'}
Answer & Solution
Correct answer: D. {'text': 'j = n e v_d', 'label': 'D'}
1. The number of free electrons crossing area A in time Δt is n A v_d Δt.
2. Each electron carries charge e, so total charge is n e A v_d Δt.
3. Current I = charge / time = n e A v_d, giving current density j = I / A = n e v_d.
4. So current density is the product of number density, charge and drift velocity.
_Source: NCERT Class 12 Physics, Ch 3 "Current Electricity", §3.5_
Related questions
For a battery of emf ε and internal resistance r driving current I through external resistA galvanometer of resistance G is converted into a voltmeter reading full-scale voltage V A galvanometer of resistance G shows full-scale deflection at current I_g. To convert it iA wire has a resistance R. It is stretched uniformly so that its length becomes 2L. The neKirchhoff current law at a junction is a statement ofTwo cells of emf ε and internal resistance r each are connected in parallel. The equivalenA potentiometer of length L is used to compare emfs. If the balance lengths for cells of eIn a Wheatstone bridge, arms have resistances P, Q, R, S in cyclic order with a galvanomet