Understanding Pressure from a Mercury Column

Pressure is defined as force per unit area. In physics and chemistry, pressure is often measured in various units. One common way to demonstrate and measure pressure, especially atmospheric pressure, is using a column of liquid like mercury in a barometer.

Defining Standard Atmospheric Pressure

Standard atmospheric pressure is a specific value used as a reference. It is historically defined based on the pressure exerted by a column of mercury under standard conditions.

The standard atmospheric pressure corresponds to the pressure capable of supporting a column of mercury exactly 760 millimeters (mm) high at 0 °C under standard gravity.

This specific pressure value is formally defined as 1 atmosphere (atm).

• A mercury column's height is directly proportional to the pressure it exerts at its base (or the pressure balancing it).
• The density of mercury changes with temperature, so the temperature (0 °C in this case) is crucial for defining the standard value.
• Standard gravity is also specified to ensure consistency in the force calculation \(F = mg\).

Comparing Pressure Units

Let's look at why the pressure exerted by a 760 mm column of mercury at 0 °C is called 1 atmosphere and why the other options are not correct in this specific context.

Here are the options provided and their relationship to atmospheric pressure:

• 1 Pascal (Pa): The Pascal is the SI unit of pressure, defined as one Newton per square meter \(1 \, \text{N/m}^2\). 1 atmosphere is approximately \(101325 \, \text{Pa}\). A Pascal is a much smaller unit than 1 atmosphere.
• 1 atmosphere (atm): As discussed, 1 atmosphere is defined as the pressure exerted by a 760 mm column of mercury at 0 °C under standard gravity. This matches the description in the question.
• 1 bar: The bar is another unit of pressure, widely used in meteorology and other fields. 1 bar is defined as exactly \(100000 \, \text{Pa}\). While close to 1 atmosphere, 1 bar is slightly less than 1 atm (\(1 \, \text{atm} \approx 1.01325 \, \text{bar}\)).
• 1 poise (P): Poise is a unit of dynamic viscosity, not pressure. Viscosity is a measure of a fluid's resistance to flow. Therefore, poise is completely unrelated to the pressure exerted by a mercury column.

Based on the definitions, the pressure exerted by a 760 mm column of mercury at 0 °C under standard gravity is precisely 1 atmosphere.

Revision Table: Pressure Units

Additional Information on Atmospheric Pressure Measurement

The instrument used to measure atmospheric pressure is called a barometer. The most basic type is the mercury barometer, invented by Evangelista Torricelli.

• A simple mercury barometer consists of a glass tube, sealed at one end, filled with mercury, and inverted into a dish of mercury.
• The space above the mercury column in the tube is a near vacuum (Torricellian vacuum).
• The atmospheric pressure acting on the surface of the mercury in the dish supports the column of mercury in the tube.
• The height of the mercury column above the level in the dish is a direct measure of the atmospheric pressure.
• The value of 760 mm Hg became the standard representation of average atmospheric pressure at sea level.

Understanding standard atmospheric pressure and its definition based on the mercury column is fundamental in fields like physics, chemistry, and meteorology.