Two rods of copper and iron with the same cross-sectional area are joined at $S$ and a steady current $I$ flows through the rods as shown in the figure. Choose the most appropriate representation of charges accumulated near the junction $S$.
KVPY 2018, Diffcult
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$(b)$ When a current flows through a junction or a contact between two metals of different conductivities a charge is accumulated in the vicinity of contact. The charge accumulation is directly proportional to the current. Charge accumulation occurs in a region whose thickness is nearly equals to the mean free path in the metal.
In case current flows from a metal of large conductivity to a metal of small conductivity, charge accumulation is of positive type and vise-versa. So, correct option is $(b)$.
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