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Question

A current passes through a wire of nonuniform cross-section. Which of the following quantities is independent of the cross-section?

A
The charge crossing in a given time interval.
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B
Drift speed
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C
Current density
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D
None of these
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Solution

The correct option is A The charge crossing in a given time interval.
Since,
Charge entering at one end of a conductor = charge leaving at the other end,

So current does not change with change in cross-section and conductor remains uncharged when current flows through it.

As the cross-sections are in series, so the current in them will be the same.

From current and drift velocity relation,

I=neAvd

Since, in this case current and n are constant

vd1/A.

At one cross-section is more, hence the drift velocity is less.

And current density J is given by

J=IA

J1A (I=constant)

So, current density depends upon area of cross-section.

Hence, option (a) is correct

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