Explain effect of multiplication or division of error on final result.
Suppose two physical quantities $\mathrm{A}$ and $\mathrm{B}$ have measured values $\mathrm{A} \pm \Delta \mathrm{A}, \mathrm{B} \pm \Delta \mathrm{B}$ respectively. Where $\triangle \mathrm{A}$ and $\triangle \mathrm{B}$ are their absolute errors.
For a product: Suppose the product of $\mathrm{A}$ and $\mathrm{B}$ is $\mathrm{Z}$ and the absolute error in $\mathrm{Z}$ is $\Delta Z$.
$\therefore \mathrm{Z}=\mathrm{AB}$
$\therefore \mathrm{Z} \pm \Delta \mathrm{Z}=(\mathrm{A} \pm \Delta \mathrm{A})(\mathrm{B} \pm \Delta \mathrm{B})$
$\therefore \mathrm{Z} \pm \Delta \mathrm{Z}=\mathrm{AB} \pm \mathrm{A} \Delta \mathrm{B} \pm \mathrm{B} \Delta \mathrm{A} \pm(\Delta \mathrm{A} \Delta \mathrm{B})$
Dividing the left side by $Z$ and right side by $A B$,
$1 \pm \frac{\Delta Z}{Z}=1 \pm \frac{\Delta A}{A} \pm \frac{\Delta B}{B} \pm\left(\frac{\Delta A}{A}\right)\left(\frac{\Delta B}{B}\right)$
$\frac{\Delta \mathrm{A}}{\mathrm{A}}$ and $\frac{\Delta \mathrm{B}}{\mathrm{B}}$ are very small hence their product will be very small and can be neglected. Neglecting negative sign for maximum error,
$\frac{\Delta Z}{Z}=\frac{\Delta A}{A}+\frac{\Delta B}{B}$
For a Quotient : Suppose the quotient of $A$ and $B$ is $Z$ and the absolute error of $Z$ is $\Delta Z$.
$\therefore \mathrm{Z}=\frac{\mathrm{A}}{\mathrm{B}}$
$\therefore \mathrm{Z} \pm \Delta \mathrm{Z}=\frac{\mathrm{A} \pm \Delta \mathrm{A}}{\mathrm{B} \pm \Delta \mathrm{B}}$
$Z\left(1 \pm \frac{\Delta Z}{Z}\right)=\frac{A\left(1 \pm \frac{\Delta A}{A}\right)}{B\left(1 \pm \frac{\Delta B}{B}\right)}$
Now, dividing the left side by $Z$ and right side by $\frac{A}{B}$,
$1 \pm \frac{\Delta Z}{Z}=\frac{1 \pm \frac{\Delta A}{A}}{1 \pm \frac{\Delta B}{B}}$
$\therefore 1 \pm \frac{\Delta Z}{Z}=\left(1 \pm \frac{\Delta A}{A}\right)\left(1 \pm \frac{\Delta B}{B}\right)^{-1}$
A student performs an experiment to determine the Young's modulus of a wire, exactly $2 \mathrm{~m}$ long, by Searle's method. In a particular reading, the student measures the extension in the length of the wire to be $0.8 \mathrm{~mm}$ with an uncertainty of $\pm 0.05 \mathrm{~mm}$ at a load of exactly $1.0 \mathrm{~kg}$. The student also measures the diameter of the wire to be $0.4 \mathrm{~mm}$ with an uncertainty of $\pm 0.01 \mathrm{~mm}$. Take $g=9.8 \mathrm{~m} / \mathrm{s}^2$ (exact). The Young's modulus obtained from the reading is
$Assertion$ : When percentage errors in the measurement of mass and velocity are $1\%$ and $2\%$ respectively, the percentage error in $K.E.$ is $5\%$.
$Reason$ : $\frac{{\Delta E}}{E} = \frac{{\Delta m}}{m} + \frac{{2\Delta v}}{v}$
A student determined Young's Modulus of elasticity using the formula $Y=\frac{M g L^{3}}{4 b d^{3} \delta} .$ The value of $g$ is taken to be $9.8 \,{m} / {s}^{2}$, without any significant error, his observation are as following.
Physical Quantity | Least count of the Equipment used for measurement | Observed value |
Mass $({M})$ | $1\; {g}$ | $2\; {kg}$ |
Length of bar $(L)$ | $1\; {mm}$ | $1 \;{m}$ |
Breadth of bar $(b)$ | $0.1\; {mm}$ | $4\; {cm}$ |
Thickness of bar $(d)$ | $0.01\; {mm}$ | $0.4 \;{cm}$ |
Depression $(\delta)$ | $0.01\; {mm}$ | $5 \;{mm}$ |
Then the fractional error in the measurement of ${Y}$ is
What is error in measurement, done by any instrument ?
A metal wire has mass $(0.4 \pm 0.002)\,g$, radius $(0.3 \pm 0.001)\,mm$ and length $(5 \pm 0.02) \,cm$. The maximum possible percentage error in the measurement of density will nearly be $.......\%$