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Question

An electron beam has an aperture 1.0 mm2. A total of 6.0×1016 electrons go through any perpendicular cross-section per second. Find (a) the current and (b) the current density in the beam.

A
6.3×103 A & 6.3×103 A/m2
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B
3.2×103 A & 3.2×103 A/m2
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C
9.6×103 A & 9.6×103 A/m2
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D
9.6×103 A & 9.6×103 A/m2
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Solution

The correct option is D 9.6×103 A & 9.6×103 A/m2
(a). The total charge crossing a perpendicular cross-section in one second is

q=ne=6.0×1016×1.6×1019 C

q=9.6×103 C

So, current will be

I=qt=9.6×103 C1 s=9.6×103 A

As the charge is negative, the current is opposite to be direction of motion of the beam.

(b). The current density is defined as

J=IA=9.6×103 A(1.0 mm)2=9.6×103A1.0×106m2

J=9.6×103 A/m2

Hence, option (a) is correct.

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