Ncert Exercises & Solutions

Consider a current carrying wire (current \(\text{I}\)) in the shape of a circle. Note that as the current progresses along the wire, the direction of \(\text{j}\) (current density) changes in an exact manner, while the current \(\text{I}\) remains unaffected. The agent that is essentially responsible for it is:

1.	source of emf.
2.	the electric field produced by charges accumulated on the surface of the wire.
3.	the charges just behind a given segment of wire which push them just the right way by repulsion.
4.	the charges ahead.

Hint: The current density depends on the electric field produced.

Explanation: Current per unit area (taken normal to the current), \(\frac{I}{{A}}\) is called current density and is denoted by \(\vec{{J}}.\) The SI units of the current density are \(\frac{\text{A}}{\text{m}^2}.\) The current density is also directed along the \(\vec E\) and is also a vector and the relationship is given by is \(\vec{{J}} = \sigma \vec E =\frac{\vec E}{\rho} ,\) where \(\sigma \) is the conductivity and \(\rho\) is the resistivity. The \(\vec{{J}}\) changes due to the electric field produced by charges accumulated on the surface of the wire.
Therefore, the \(\vec{{J}}\) changes due to the electric field produced by charges accumulated on the surface of the wire.
Hence, option (2) is the correct answer.

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