Answer to Question #100718 in Molecular Physics | Thermodynamics for Karan

Question

a narrow glass tube 80 cm long opens at both the ends is half immersed in Hg . now top is closed and taken out of Hg a 20cm long Hg column remain in tube . find atmospheric pressure.

Answer 1

When we have half the tube is immersed in the mercury i.e 40 cm, the air pressure (P) inside the tube should be equal to the mercury pressure in the other half.

$P=h{\rho}g=40{\rho}g$

Once the tube is removed with upper end closed, the air pressure inside will remain same. Now since the other end is open,

this air pressure + the pressure of mercury remaining = the atmospheric pressure at the other end.

$P$₀$=P+$$h$ ₁${\rho}g$

$P$₀$=40{\rho}g+20{\rho}g$

$P$₀$=60{\rho}g$

Hence, the atmospheric pressure will be equal to the pressure exerted by 60cm of mercury column.