A 200-V bulb takes a current of 0.2 A. The length of its filament is 22 cm and the specific resistance of the material of the filament is 7 * 10^-5 ohm-cm. Find the radius of the filament. (pi=22/7)

We know

V=IR

V= 220 V

I= 0.2 A

So R = 220/0.2

R= 1100 ohm

Also R= ρL/A

Where

 L= Length = 22cm=0.22m

ρ= 7 x 10-5 ohm-m

so A= ρL/R

A= 7x10-5 x 0.22/1100

A=  1.4x10-8 m2

A= πr2 = 1.4x10-8

r2 = 0.14x10-8 /3.14

r2= 0.44 x 10-8

r=0.667 x10-4 m

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