When current is flowing in an ordinary metal wire, the magnitude of the average velocity of the electrons is closest toa) 10 m/sb) the speed of lightc) 10 microns/sd) 1 km/se) 1 m/s

c) 10 microns/s

Explanation:

The drift velocity (the velocity of the current) of the electrons in a wire is given by

where

I is the current

n is the electron number density (the number of electrons per unit volume)

A is the cross-sectional area of the wire

is the charge of one electron

Taking a current of

I = 1 A

in a wire of radius r = 1 mm (0.001 m), so with cross-sectional area

made of copper, whose electron density number is around

we find

which means that the closest estimate is

c) 10 microns/s

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