Assertion–Reason on conductor response Assertion (A): In a homogeneous conductor, the current density J remains constant in time as long as the applied electric field E is constant. Reason (R): Ohm’s law in point form states J = σE (with σ constant for the given state).

Introduction / Context:
Ohm’s law at the continuum level links local current density to the electric field through the material conductivity. This item examines whether a time-invariant field leads to a time-invariant current density in a simple conductor.

Given Data / Assumptions:

• Homogeneous, isotropic conductor with constant σ (no heating-induced changes considered).
• Uniform, time-independent electric field E.
• Steady state (no transients, displacement-current effects negligible in a bulk conductor).

Concept / Approach:
Ohm’s law in differential form is J = σE. If E is constant in time and σ is constant, J is necessarily constant in time and space (where E is uniform). Any change in σ (heating) or E would alter J, but those are excluded by the assumptions.

Step-by-Step Solution:
Assume constant E and σ.Apply J = σE → J is constant.Therefore A follows directly from R.

Verification / Alternative check:
Macroscopic circuit law I = V/R in DC is the integral analog; at fixed V and fixed R, I is constant in time, consistent with the point-form relation.

Why Other Options Are Wrong:
(b) denies the explanatory link; however, R directly implies A. (c) and (d) contradict Ohmic behavior. (e) is inconsistent with standard conduction theory.

Common Pitfalls:

• Overlooking temperature dependence of σ in real materials; the statement presumes σ remains constant.
• Confusing transient charging in dielectrics with conduction in metals.

Final Answer:
Both A and R are true and R is correct explanation of A