Question

A narrow uniform glass tube $80cm$ long and open at both ends is half immersed in mercury. Then, the top of the tube is closed and it is taken out of mercury. A column of mercury $22cm$ long then remains in the tube. What is the atmospheric pressure?

• A. $60cm$ of $Hg$
• B. $70.9cm$ of $Hg$
• C. $76cm$ of $Hg$
• D. $none$

Answer 1

Answer: (A)

$60cm$ of $Hg$

$\text{Given:-}$ Length of the class tube $=80cm$

$\text{Solution:-}$

let the cross section area of the tube $Ac{m}^2$

When half tube is immersed in mercury .

Volume of air

$V_1=A\times 40c{m}^2$

Pressure of enclosed air

$P_1=P$

When tube taken out of mercury

$P_2=(p-20)cm$

$V_2=A\times 60c{m}^2$

As temperature is constant

$P_2{V}_2=P_1{V}_2$

$(P-20)(A\times 60)=\left(P\right)(a\times 40)$

$=60cmofHgcolumn.$

$\text{Hence the correct option is A}$