A cool easy problem 3

Electricity and Magnetism Level 2

A wire of resistance 10 ohm is stretched so as to increase its length by 20% .its resistance then would be

12 ohm 14.4 ohm 10 ohm 10.2 ohm

A Former Brilliant Member by A Former Brilliant Member

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3 solutions

let length of wire is 100 cm
new length 120 cm
therefore
length is increased 6/5 times
so resistance will be increased by
6/5 × 6/5 × 10 =14.4 ohm

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please check

roland casuga - 6 years, 5 months ago

how sure are you that the length is 100 cm? can we not just do this in this same way: R=pl/A, where p is the resistivity of the material and l is the initial length and A is the area; let R1= 10 ohms (given) so when the wire is stretch the new length would be l2=1.2l1 we have l1=10A/p from the original equation and given then the required resistance would be R2=pl2/A the resistivity and area would be still the same because the same material was used the substituting the given R2=10(1..2)=12 ohms

roland casuga - 6 years, 5 months ago

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It is a standard assumption that we initially take any quantity as 100 if it is not mentioned..

Parag Zode - 6 years, 4 months ago
Animesh Jha
Feb 8, 2015

let the length of wire be 'l' new length = '6l/5', new crossection='5l/6' . New resistance = 10*36/25=14.4

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Anwar Arif
Jan 1, 2015

V1=V2 so,A1L1=A2L2 A1/A2=L1/L2 so,R2=R1 (L2/L1)^2 R2=10 (6/5)^2=14.4

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