Question

correct answer is 40 ohm how?​

Answer 1

Answer:

Ans is 4) 40Ω

Explanation:

Resistance of a wire is given by the formula :-

$\boxed{R=\rho\frac{l}{A} }$

where $\rho :$ resistivity

          $l :$ length of wire

         $A :$ area of cross section of wire

Also, it is given that

$\frac{A_1}{A_2}=\frac{3}{1} \\\\\therefore Let \ A_1=3A\ and\ A_2=A$

Now,

$R_1=\rho\frac{l}{3A}\\\\ R_2=\rho\frac{l}{A}$

Since R₁ and R₂ are connected in series, $\boxed{R_n_e_t=R_1+R_2}$

$R_n_e_t=\rho\frac{l}{3A}+\rho\frac{l}{A}\\\\R_n_e_t=\rho\frac{4l}{3A}\\\\\boxed{R_n_e_t=4(\rho\frac{l}{3A})}----(1)$

Since R₁ = 10Ω;

$\boxed{30=\rho\frac{l}{A} }----(2)$

From (1) and (2), we get

$R_n_e_t=4(10)=40$

Hope this answer helped you :)