Question

Consider a current-carrying wire(current I) in the shape of a circle. Note that as the current progresses along the wire, the direction of $J$(current density) changes in an exact manner, while the current I remains unaffected. The agent that is essentially responsible for it is?

• A. Source of emf
• B. Electric field produced by charges accumulated on the surface of wire
• C. The charges just behind a given segment of wire which push them just right way by repulsion
• D. The charges ahead

Answer 1

Answer: (B)

Electric field produced by charges accumulated on the surface of wire

Current per unit area (taken normal to the current),$\frac{I}{A}$ is called current density and is denoted by $\overrightarrow{J}$

The $S.I$ unit of the current density are $J/m^2$. The current density is also directed along $E$ and which is also a vector quantity and the relationship is given by, $\overrightarrow{J}=\sigma \overrightarrow{E}=\frac{\overrightarrow{E}}{\rho }$

where, $\sigma =$ conductivity and $\rho$ = resistivity or specific resistance of the substance.

The $\overrightarrow{J}$ changes due to the electric field produced by charges accumulated on the surface of wire.