MCQEasyJEE 2023Young's Modulus, Bulk & Rigidity Modulus

JEE Physics 2023 Question with Solution

Under the same load, wire A having length 5.0m5.0 \, \text{m} and cross section 2.5×105m22.5 \times 10^{-5} \, \text{m}^2 stretches uniformly by the same amount as another wire B of length 6.0m6.0 \, \text{m} and a cross section of 3.0×105m23.0 \times 10^{-5} \, \text{m}^2. The ratio of the Young's modulus of wire A to that of wire B will be:

  • A

    1:41:4

  • B

    1:11:1

  • C

    1:101:10

  • D

    1:21:2

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: Same load acts on both wires and both wires have the same elongation. For wire A, LA=5.0mL_A = 5.0 \, \text{m} and AA=2.5×105m2A_A = 2.5 \times 10^{-5} \, \text{m}^2. For wire B, LB=6.0mL_B = 6.0 \, \text{m} and AB=3.0×105m2A_B = 3.0 \times 10^{-5} \, \text{m}^2.

Find: The ratio YAYB\dfrac{Y_A}{Y_B}.

Young's modulus is given by

Y=FLAΔLY = \frac{F L}{A \, \Delta L}

where FF is the load, LL is the length, AA is the cross-sectional area, and ΔL\Delta L is the elongation.

Since the load and elongation are the same for both wires,

YAYB=LAABLBAA\frac{Y_A}{Y_B} = \frac{L_A A_B}{L_B A_A}

Substituting the given values,

YAYB=5×3.0×1056.0×2.5×105=1\frac{Y_A}{Y_B} = \frac{5 \times 3.0 \times 10^{-5}}{6.0 \times 2.5 \times 10^{-5}} = 1

Therefore, the ratio of the Young's modulus of wire A to wire B is 1:11:1. The correct option is B.

Common mistakes

  • Using YAYB=LAAALBAB\dfrac{Y_A}{Y_B} = \dfrac{L_A A_A}{L_B A_B} is incorrect because the area terms get cross-multiplied when forming the ratio from Y=FLAΔLY = \dfrac{F L}{A \, \Delta L}. Write the full ratio first and then cancel the common terms carefully.

  • Assuming equal elongation means equal strain is incorrect because strain is ΔLL\dfrac{\Delta L}{L}, and the lengths of the two wires are different. Here only the elongation ΔL\Delta L is same, not the strain.

  • Ignoring that the same load acts on both wires is wrong because cancellation of FF is essential in the ratio. Keep track of which quantities are common before substituting values.

Practice more Young's Modulus, Bulk & Rigidity Modulus questions

Get unlimited AI-adaptive practice, mastery tracking, and an AI tutor that explains every step — free to start.

Related questions