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Combination of Errors

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Topics

  • Introduction
  • Errors in sum and in Difference
  • Errors in Products and in Division
  • Errors Due to the Power Quantities
  • Example 1
  • Example 2
Maharashtra State Board: Class 11

Introduction

Whenever you measure something in an experiment, your results may have errors caused by limitations in instruments or techniques. Understanding how these errors combine is essential for getting reliable results.

Maharashtra State Board: Class 11

Errors in Sum and Difference

If you measure quantities A ± ΔA and B ± ΔB: 

  • For Z = A + B or Z = A − B,
  • Absolute error: ΔZ = ΔA + ΔB.
  • Example: Measuring resistance using Ohm’s law—the errors from voltage and current add up in the final resistance. 
Maharashtra State Board: Class 11

Errors in Product and Division

For Z = AB or Z = \[\frac {A}{B}\]: 

  • Relative error: \[\frac {ΔZ}{Z}\] = \[\frac {ΔA}{A}\] + \[\frac {ΔB}{B}\]
  • Key Point: Errors add up as fractions for multiplication or division.
Maharashtra State Board: Class 11

Errors in Powers of Quantities

For Z = An: 

  • Relative error: \[\frac{\Delta Z}{Z}=n\left(\frac{\Delta A}{A}\right)\]

For Z = ApBqCr:

  • \[\frac{\Delta Z}{Z}=p
    \begin{pmatrix}
    \frac{\Delta A}{A}
    \end{pmatrix}+q
    \begin{pmatrix}
    \frac{\Delta B}{B}
    \end{pmatrix}+r
    \begin{pmatrix}
    \frac{\Delta C}{C}
    \end{pmatrix}\]
  • Key Point: The term with the highest power contributes most to error. 

Maharashtra State Board: Class 11

Example 1

Volume Measurement 

  • Mass: m = 5 ± 0.15 kg
  • Density: ρ = 5 ± 0.2 kg/m3
  • Volume = \[\frac{m}{\rho}\]
  • Relative errors: 0.15/5 = 0.03, 0.2/5 = 0.04
  • Max percent error: (0.03 + 0.04) × 100 = 7%
Maharashtra State Board: Class 11

Example 2

Acceleration due to Gravity 

  • Length: l = 100 ± 0.1 cm 

  • Time: T = 2 ± 0.01 s 

  • Formula: g = \[\frac{4\pi^2l}{T^2}\]

  • Max percent error: \[\frac{\Delta l}{l}+2\frac{\Delta T}{T}\] = 0.001 + 0.01 = 0.011 or 1.1% 

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