Question

To construct a barometer, a tube of length 1 m is filled completely with mercury and is inverted in a mercury cup. The barometer reading on a particular day is 76 cm. Suppose a 1 m tube is filled with mercury up to 76 cm and then closed by a cork. It is inverted in a mercury cup and the cork is removed. The height of mercury column in the tube over the surface in the cup will be

Options

• zero

• 76 cm

• > 76 cm

• < 76 cm

Answer 1

< 76 cm
Because of the trapped air, the pressure at the upper end of the mercury column inside the tube is not zero.
In other words, P₀ > 0.
Using this relation, we get:
P_atm = P₀ + ρgh
Here,
ρ = Density of mercury
h = Height of the mercury column
∵ P₀ > 0
And,
P_atm > ρgh
∴ 76 cm of Hg >ρgh
or, h < 76 cm