Question

What is the guage pressure of the gas in the closed tube in the figure? Take the atmospheric pressure to be 76cm hg

Answer 1

Gauge pressure is our only system pressure where we taking the atmospheric pressure is 0

It is the difference between the mercury level in both limbs of manometer.

At left limb above the horizontal line, P_gas is the pressure of gas

At right limb above the horizontal line, P_mercury is the pressure due to mercury

At equilibrium pressure at both limbs above the horizontal line are equal

Thus Gauge pressure due to gas = P_mercury is the pressure due to mercury

Lets assume the height of mercury above the horizontal line is 20cm

Then Thus Gauge pressure due to gas in closed vessel = 20cm Hg