Q.

When a potential difference is applied across a cylindrical wire, average drift speed of free electrons is ‘V’. Now the wire is stretched to two times its original length and same potential difference is applied across it. Then the new drift speed of free electrons will be ______

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answer is 3.

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Detailed Solution

Volume of wire remains constant.∴ a'l' = al ⇒ a' = al2l = a2R = Initial  resistance  = ρla∴ R' = ρ.l'a'  = ρ.2la2 = 4RNow  Vd = iane = ERane∴ vd'vd = RR'.aa' = 14.2  = 12∴  v'd = 12vd = v2
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