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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 point P and Q is vP and vQ.


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
We know from current -drift velocity relation

I=neAvd

As the cross-section are in series, so the current in them will be the same. And n & e are also same. So we get

vd1A

At one cross-section is more, hence the drift velocity is less.
Here,
AP<AQ

vP>vQ

Hence, (c) is correct option.
Why This Question?
If current is constant then as area of conductor increases then drift velocity decreases.

Current density can be represented by a similar set of lines known as streamlines. The spacing of the streamlines suggests the value of current density. Narrower streamlines mean more current density, spaced streamlines mean less current density

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