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The current density j flowing through a conductor of cross-sectional area A carrying current I (with current normal to the area) is
A$j = IA$
B$j = I/A$
C$j = A/I$
D$j = I/A^2$
Answer & Solution
Correct answer: B. $j = I/A$
1. Current density is current per unit area normal to the flow.
2. By definition $j = I/A$, with SI unit $\text{A m}^{-2}$.
3. Rearranging gives $I = jA$, consistent with the text.
4. $IA$ and $A/I$ invert the definition; $I/A^2$ has wrong units.
_Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.4_
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