Question

a wire of 9 ohm resistance having 30 cm length is tripled on itself .what is its new resistance???

Answer 1

When a wire tripled itself its length is 1/3 rd and area becomes tripled and area is inversely proportional to resistance.
case 1 :
R = p l/a                                                                    Here p = constant
9 ohm = p 30/a
a = 30/9 m² 
now length becomes 1/3 rd and area tripled
Therefore a = 30/9 * 3 = 10 m² and l = 1/3 * 30 = 10 m
Now R = p l/a
R = 10/10 = 1 ohm

Answer 2

Answer:

Given,

R = 9Ω

I = 30cm

rho/A = R/l

rho/A = 9/30 = 3/10.....(1)

Now,

For new resistance,

R' = (rho × l/3) / 3A........(2)

Substituting the value of rho/A from equation 1 in equation 2 we get ,

R' = (3 × 30/3) / 10 ×3

R'= 1Ω