MCQEasyJEE 2023Significant Figures & Error Analysis

JEE Physics 2023 Question with Solution

A physical quantity PP is given as P=a2b3cdP = \frac{a^2b^3}{c\sqrt{d}}. The percentage error in the measurement of aa, bb, cc, and dd are 1%1\%, 2%2\%, 3%3\%, and 4%4\% respectively. The percentage error in the measurement of PP is:

  • A

    14%14\%

  • B

    13%13\%

  • C

    16%16\%

  • D

    12%12\%

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: P=a2b3cdP = \frac{a^2b^3}{c\sqrt{d}} and the percentage errors are 1%1\% in aa, 2%2\% in bb, 3%3\% in cc, and 4%4\% in dd.

Find: The percentage error in PP.

For multiplication, division, powers, and roots, percentage errors add with their respective powers. Therefore,

ΔPP×100=2Δaa+3Δbb+Δcc+12Δdd\frac{\Delta P}{P} \times 100 = 2\frac{\Delta a}{a} + 3\frac{\Delta b}{b} + \frac{\Delta c}{c} + \frac{1}{2}\frac{\Delta d}{d}

Substitute the given values:

2Δaa=2×1=22\frac{\Delta a}{a} = 2 \times 1 = 2 3Δbb=3×2=63\frac{\Delta b}{b} = 3 \times 2 = 6 Δcc=3\frac{\Delta c}{c} = 3 12Δdd=12×4=2\frac{1}{2}\frac{\Delta d}{d} = \frac{1}{2} \times 4 = 2

Now add all contributions:

ΔPP×100=2+6+3+2=13%\frac{\Delta P}{P} \times 100 = 2 + 6 + 3 + 2 = 13\%

Therefore, the percentage error in PP is 13%13\%. The correct option is B.

Term-wise Error Contribution

Given: PP depends on a2a^2, b3b^3, c1c^{-1}, and d1/2d^{-1/2}.

Find: Total percentage error in PP.

The error contribution from each factor is obtained by multiplying the percentage error by the magnitude of its power:

  • From a2a^2: 2×1%=2%2 \times 1\% = 2\%
  • From b3b^3: 3×2%=6%3 \times 2\% = 6\%
  • From c1c^{-1}: 1×3%=3%1 \times 3\% = 3\%
  • From d1/2d^{-1/2}: 12×4%=2%\frac{1}{2} \times 4\% = 2\%

Adding them gives:

2%+6%+3%+2%=13%2\% + 6\% + 3\% + 2\% = 13\%

Hence, the required percentage error is 13%13\%, so the correct option is B.

Common mistakes

  • Using subtraction for quantities in the denominator is incorrect in error propagation. Percentage errors are added irrespective of whether the quantity is in the numerator or denominator. Add the magnitudes of the contributions instead.

  • Taking the error contribution of d\sqrt{d} as 4%4\% is incorrect. For a square root, the percentage error is multiplied by 12\frac{1}{2}, so the contribution is 2%2\%.

  • Ignoring the powers on aa and bb is incorrect. The terms a2a^2 and b3b^3 contribute 22 times and 33 times their percentage errors respectively. Always multiply by the absolute power.

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