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Question

A wire has a non-uniform cross-section as shown in figure. A steady current flows through it. The drift speed of electrons at points P and Q is vP and vQ, respectively. Then


A
data insufficient
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B
vP=vQ
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C
vP>vQ
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D
vP<vQ
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Solution

The correct option is C vP>vQ
Explanation

The current flowing is same at every cross section throughout the conductor. If any charge enters through a cross section in any part of conductor an equal amount of charge must leave out through another cross section . This is because there is no accumulation or production of charge in any part of conductor , they only continuously flow through every cross section.

We know, current I=neAvd

vd=IneA

n charge Carrier density ( free
electron density )

A Area of cross-section

vd Drift velocity

Here n,e & I are constant.

So, vd1A

Since, AQ>AP

1AQ<1AP

Or (vd)Q<(vd)p(v)Q<(v)p

Hence, option (c) is correct.

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