Question

How does air exert pressure? Describe the basic instrument used to measure air pressure. Compare the operation of two different types of instruments discussed.

Short answer

Air pressure is the weight of air molecules pressing down on the Earth. A basic instrument used to measure air pressure is the barometer, including mercury and aneroid barometers. A mercury barometer measures air pressure by balancing the weight of the mercury in the tube against the atmospheric pressure, while an aneroid barometer uses an aneroid cell which expands or contracts with the change in air pressure.

Step-by-step solution

Understanding Air Pressure

Air pressure is created by the weight of air pushing down on the Earth, the ocean, and areas of Earth's atmosphere. The molecules that make up air are in constant motion and, as they move, they push against the surfaces they come into contact with, which is the force we feel as air pressure.

Description of Basic Instrument to Measure Air Pressure

One of the basic tools used to measure air pressure is a barometer. A simple barometer can be formed with a glass tube filled with a liquid like mercury which is then inverted into a container also containing mercury. The height of the mercury column in the tube adjusts until the weight of the mercury equals the weight of the same volume air in the atmosphere.

Comparison of Two Different Instruments

Mercury barometers and aneroid barometers are two of the most common tools for measuring air pressure. A Mercury barometer, as described earlier, employs liquid mercury to measure pressure. On the other hand, an aneroid barometer uses a small, flexible metal box called an aneroid cell. This cell expands or contracts when the air pressure changes; these movements are mechanically linked to a needle which moves over a graduated scale to reflect the changes in pressure.

Key concepts

Barometer

A barometer is an essential instrument for measuring air pressure.
Air pressure is the force exerted by the weight of air in the atmosphere. It is a vital aspect of meteorology and weather prediction. Barometers help us understand these atmospheric conditions.

There are two primary types of barometers: mercury barometers and aneroid barometers.
Both serve the same purpose but operate on different principles and mechanisms.
Learning how they function provides insights into atmospheric pressure changes and how these affect weather patterns.

Mercury Barometer

A mercury barometer is a classic and accurate tool for measuring air pressure. It relies on a liquid column to indicate pressure changes.

Understanding Mercury Barometers

In a mercury barometer, the atmospheric pressure is balanced by a column of mercury in a glass tube.
This tube is sealed at one end and inverted in a mercury-filled dish. The height of the mercury column represents the air pressure.
As atmospheric pressure increases, the mercury in the tube rises, and as it decreases, the mercury falls. This change in height is used to measure the pressure.

How It Works

- The sealed tube ensures no air enters, maintaining a vacuum above the mercury. - Atmospheric pressure pushes on the mercury in the dish, causing movement in the mercury column. - The difference in the mercury height reflects changes in air pressure accurately.

Aneroid Barometer

An aneroid barometer is a more modern and versatile option compared to the mercury barometer. It is often preferred due to its portability and ease of use.

Features of Aneroid Barometers

Unlike mercury barometers, aneroid barometers do not use any liquid.
They utilize an aneroid cell, which is a small, sealed, flexible metal box.
This box expands and contracts in response to air pressure changes.

How Aneroid Barometers Function

- As atmospheric pressure increases, the aneroid cell compresses. - When the pressure decreases, the cell expands. - These movements are transferred to a needle through mechanical linkages. - The needle moves across a dial or scale to indicate the air pressure.

The aneroid barometer's flexibility and absence of liquids make it suitable for various applications, including use in aircraft and portable weather kits.