5.1
Definition and Measurement of Pressure: Atmospheric Pressure, Barometer, and Manometer
Gas pressure is caused by force exerted by gas molecules colliding with the surfaces of objects. Although the force of each collision is very small, a…
Recall that a gas always assumes the shape and volume of the vessel in which it is contained. Microscopically, a gas consists of moving particles that collide with each other and the walls of their container.
Each collision with the wall exerts a small force on the area of contact. The forces resulting from a large number of such collisions add up to its pressure.
Fewer gas particles lead to fewer collisions, a lower force per unit area, and hence, a lower pressure. Pressure, therefore, is directly proportional to the number of particles in a given volume, or the density of the gas.
Air — a mixture of different gaseous atoms and molecules — exerts pressure on everything on the surface of the earth. This is atmospheric pressure, and it is measured using a barometer. A traditional barometer is a mercury-filled glass tube that is inverted into a dish containing mercury.
In response to pressure exerted by the atmosphere, the height of the mercury column rises or falls. Measuring the height provides a measure of the atmospheric pressure.
At sea level, the atmospheric pressure is 1 atm, which raises the column of mercury to a height of 760 millimeters of mercury. At higher altitudes, where the air is less dense, the height of the mercury column falls, indicating a lower atmospheric pressure.
The pressure of a gas trapped in a container is measured using a manometer. A closed-end manometer consists of a mercury-filled U tube. One end is sealed in a vacuum, and the other end is connected to the container that is filled with the gas sample.
The gas particles push down on the mercury in one end of the tube, creating a height difference that corresponds to the pressure exerted by the gas.
In an open-end manometer, one end is left open to the atmosphere. This measures the pressure of the gas sample relative to atmospheric pressure.
When the gas exerts a pressure higher than the atmospheric pressure, the height difference is added to the atmospheric pressure. This yields the pressure of the gas.
Conversely, when the atmosphere exerts a pressure greater than the gas sample, the height difference is subtracted from the atmospheric pressure to get the pressure of the gas.
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Q1: What causes gas pressure at the molecular level?
Gas pressure results from collisions between moving gas particles and container walls. Each collision exerts a small force, but the cumulative effect of numerous collisions across a surface creates measurable pressure. According to basic postulates of kinetic molecular theory particle size energy and collision, pressure is directly proportional to the number of gas particles in a given volume, so fewer particles mean fewer collisions and lower pressure.
Q2: How is atmospheric pressure measured with a barometer?
A barometer is a mercury-filled glass tube inverted into a dish of mercury. Atmospheric pressure pushes on the mercury surface outside the tube, causing the mercury column inside to rise to a height proportional to that pressure. At sea level, standard atmospheric pressure of 1 atm supports a mercury column 760 millimeters high.
Q3: What is the difference between closed-end and open-end manometers?
A closed-end manometer has one sealed end in a vacuum and measures absolute gas pressure directly from the height difference in mercury. An open-end manometer has one end exposed to the atmosphere and measures pressure relative to atmospheric pressure. The gas pressure is calculated by adding or subtracting the height difference from atmospheric pressure depending on whether gas pressure exceeds or falls below it.
Q4: Why is mercury preferred over water in barometers?
Mercury is preferred because it is 13.5 times denser than water. This density difference means atmospheric pressure supports a mercury column only about 0.76 meters tall, whereas a water column would need to be 10.3 meters tall. The shorter mercury column makes barometers practical and convenient for measuring pressure.
Q5: How does pressure relate to force and area?
Pressure is defined as force exerted per unit area. It is directly proportional to force and inversely proportional to area. Pressure increases by applying more force or reducing the area over which force is distributed, and decreases by reducing force or increasing the contact area.
Q6: What are the common units used to express pressure?
The SI unit of pressure is the pascal (Pa), equal to 1 newton per square meter. Larger pressures are often expressed in kilopascals (kPa) or bars, where 1 bar equals 100,000 Pa. Pressure can also be measured in atmospheres (atm), with 1 atm equal to 101,325 Pa or 760 millimeters of mercury.
Q7: How does gas density affect pressure in a container?
Pressure is directly proportional to gas density, the number of particles per unit volume. Higher density means more gas particles occupy the same space, resulting in more frequent collisions with container walls and greater pressure. Conversely, lower density produces fewer collisions and reduced pressure.