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Question

# Two wires each of radius of cross-section r but of different materials are connected together end to end (in series). If the densities of charge carriers in the two wires are in the ratio 1:4, the drift velocity of electrons in the two wires will be in the ratio

A
1:2
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B
2:1
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C
4:1
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D
1:4
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Solution

## The correct option is C 4:1If the wires are connected end to end (in series) then current in them will be same. We know formula of current, I=neAvd where, n→ number of free electrons per unit volume e=1.6×10−19 C A→ Area of cross-section vd→ Drift velocity As current and cross-section is same then n1eAvd1=n2eAvd2 ⇒vd1vd2=n2n1=41 Hence, option (c) is correct. Why this question ? If two wires are connected in series, then current in them will be same Key Formula: If electrons are moving with velocity vd,A is area of cross-section and n is the number of free electrons per unit volume then, I=neAvd

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