A simple barometer is shown. The vertical tube is fixed. a) Determine the atmospheric pressure in SI units. b) Name the part labelled A. c) The tube is accidentally knocked at the top, so that a small

The total height of the liquid in the tube from the reservoir surface is $75 \text{ cm}$. The space above the liquid is $25 \text{ cm}$.
The actual height of the liquid column supported by atmospheric pressure is the difference:

Convert this height to meters:

A simple barometer typically uses mercury. The density of mercury ($\rho_{Hg}$) is $13600 \text{ kg/m}^3$.

Assume the acceleration due to gravity $g = 10 \text{ m/s}^2$.
The atmospheric pressure ($P_{atm}$) is given by:

Substitute the values:

b) Name the part labelled A.

Part A is the space above the liquid column in the sealed tube, which is a vacuum.

c) The tube is accidentally knocked at the top, so that a small crack is caused at the top of the tube. Air leaks in through the crack. State and explain the effect, if any, this has on the level of the liquid in the tube.

Effect: The level of the liquid in the tube will fall.

Explanation: When air leaks into the space above the liquid (the Torricellian vacuum), the trapped air exerts pressure on the surface of the liquid inside the tube. This air pressure adds to the pressure exerted by the liquid column, meaning the external atmospheric pressure now has to support both the air pressure and the liquid column. To maintain equilibrium, the height of the liquid column must decrease.