ExamBro
ExamBro
JEE Mains · Physics · STD 12 - Current electricity
  • JEE Mains 2022
  • Easy
  • MCQ

A wire of resistance \(R_{1}\) is drawn out so that its length is increased by twice of its original length.The ratio of new resistance to original resistance is.

Pick an option to check your answer.
Verified Solution

Answer & Solution

Correct Answer

(A) \(9: 1\)

Step-by-step Solution

Detailed explanation

Explanation:
Let us decode the phrasing and compute the resistance ratio step-by-step using the principles of conservation of volume:

1. Decoding the Phrase "Increased By":
There is a vital difference between increased "to" and increased "by":
- "Increased to 2 times" implies the final length is \( 2L_1 \).
- "Increased by 2 times" implies the extension added to the wire is \( 2L_1 \).
Therefore, the final length \( L_2 \) is:
\( L_2 = L_1 + 2L_1 = 3L_1 \)

2. Conservation of Volume:
When a wire is stretched, its total volume \( V \) remains constant. Let \( A_1 \) be the initial cross-sectional area and \( A_2 \) be the final cross-sectional area:
\( V = L_1 A_1 = L_2 A_2 \)
\( L_1 A_1 = (3L_1) A_2 \implies A_2 = \frac{A_1}{3} \)

3. Resistance Calculation:
The baseline resistance formula is \( R = \rho \frac{L}{A} \), where \( \rho \) is the material resistivity. Writing the ratio of final resistance \( R_2 \) to initial resistance \( R_1 \) gives:
\( \frac{R_2}{R_1} = \frac{\rho \frac{L_2}{A_2}}{\rho \frac{L_1}{A_1}} = \left(\frac{L_2}{L_1}\right) \times \left(\frac{A_1}{A_2}\right) \)

Substituting our derived values into this expression:
\( \frac{R_2}{R_1} = \left(\frac{3L_1}{L_1}\right) \times \left(\frac{A_1}{A_1 / 3}\right) = 3 \times 3 = 9 \)
Expressing this as a standard fraction ratio yields 9 : 1.

Hence, Option A is the correct choice.
Same subject
Explore more questions on app