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Q. A sinker of weight $W_0$ has an apparent weight $W_1$ when weighed in a liquid at a temperature $t_1$ and $W_2$ when weight in the same liquid at temperature $t_2$. The coefficient of cubical expansion of the material of sinker is $\beta$. What is the coefficient of volume expansion of the liquid?

Thermal Properties of Matter

Solution:

$W_0-W_1=V \times d_{\ell} \times g$......(i)
$W_0-W_2=V^{\prime} \times d^{\prime \prime} \times g$......(ii)
Also, $V^{\prime}=V(1+\beta \Delta T)$.......(iii)
and $d_{\ell}=d^{\prime}{ }_{\ell}\left(1+\gamma_{\ell} \Delta T\right)$.......(iv)
From (ii), (iii) and (iv)
$W_0-W_2=\frac{V(1+\beta \Delta T) \times d_{\ell}}{1+\gamma_{\ell} \Delta T} \times g$.......(v)
Dividing (i) and (v), we get
$\frac{W_0-W_1}{W_0-W_2}=\frac{V d_{\ell} g\left(1+\gamma_{\ell} \Delta T\right)}{V(1+\beta \Delta T) d_{\ell} g}$
$\Rightarrow \frac{W_0-W_1}{W_0-W_2}=\frac{1+\gamma_{\ell} \Delta T}{1+\beta \Delta T} $
$\Rightarrow \frac{W_0-W_1}{W_0-W_2}=\frac{1+\gamma_{\ell}\left(t_2-t_1\right)}{1+\beta\left(t_2-t_1\right)}$
$\Rightarrow\left(W_0-W_1\right)\left[1+\beta\left(t_2-t_1\right)=\left(W_0-W_2\right)\left[1+\gamma_{\ell}\left(t_2-t_1\right)\right]\right.$
$\Rightarrow \gamma_{\ell}=\frac{W_2-W_1}{\left(W_0-W_2\right)\left(t_2-t_1\right)}+\frac{\beta\left(W_0-W_1\right)}{\left(W_0-W_2\right)}$