Current Electricity Question 7
Question 7 - 25 January - Shift 2
The resistance of a wire is $5 \Omega$. It’s new resistance
in ohm if stretched to 5 times of it’s original length will be :
(1) 625
(2) 5
(3) 125
(4) 25
Show Answer
Answer: (3)
Solution:
Formula: Resistance of a conductor
$R _{\text{initial }}=\frac{\rho \ell}{A}=5 \Omega$
$A^{\prime}$
$\because$ Volume of wire is constant in stretching
$V_i=V_f$
$A_i \ell_i=A_f \ell_f$
$A \ell=A^{\prime}(5 \ell)$
$A^{\prime}=\frac{A}{5}$
$R_f=\frac{\rho \ell_f}{A_f}=\frac{\rho(5 \ell)}{(\frac{A}{5})}$
$=25(\frac{\rho \ell}{A})$
$=25 \times 5=125 \Omega$
