Density of Air
Nov 01, 2022, 16:45 IST
Density is a measure of mass. The formula of density is d = m / v, where d is density, m is mass, and v is volume. In this session, we will discuss density of air, the value for density of air, and factors affecting the density of air.
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What is Air Density?
Air density is denoted by the rho, ρ (greek letter). The given air density value is at STP. An object of a relatively dense material will have a smaller volume than an object of the same mass made of a less dense substance. The highest known density is for a neutron star. A loaf and a brick of bread are about the same in size, but a brick is heavier - it's denser.
Among the metals, aluminum is less dense than iron. This is why airplanes and some automotive parts are made of aluminum. For the equal volume of material, one metal weighs less than the another. The density of the pure gold nugget on the Earth and the Moon would be the same. But in gases, the density can vary widely because the molecules are free to move. The air density at the earth's surface would be different than the air 50 kilometers above the earth.
Value of Air Density
1.29 grams per litre is the value of air density at 32 degrees Fahrenheit (0 degrees Celsius) at the average sea-level barometric pressure (760 millimetres or 29.92 inches of mercury).
Density of Dry Air
The density of dry air is calculated by using the ideal gas law:
ρ= p / RspecificT
where,
ρ = air density (kg/m3)
p= absolute pressure (Pa)
T = absolute temperature (K)
Rspecific = specific gas constant for dry air (J/(kg.K))
Density of Humid Air
The density of air is reduced by adding water. This is because the molar mass of a water is less than molar mass of dry air. For an ideal gas at a given pressure and temperature, the number of molecules for a given volume of air is constant. The density of the humid air is calculated as follows:
where:
Phumid = density of the humid air
ρd = partial pressure of dry air
Rd = specific gas constant of dry air
T = temperature
ρv = pressure of water vapour
Rv = specific gas constant for water vapour
Md = molar mass of dry air
Mv = molar mass of water vapour
R = universal gas constant
Factors Affecting the Density of Air
Air becomes denser as air pressure increases. The pressure pushes air molecules together, producing more mass in a given volume, and increasing altitude decreases air pressure. The decreased air density means that a climber at high altitudes receives less oxygen when breathing.
Temperature is another factor that affects air density. As the temperature increases, air molecules move faster and spread further apart when colliding. When the air is denser, it generally creates a drag on the objects moving through it. E.g., a golf ball hit on a hot summer day will be farther apart than one hit on a cold day. Higher temperatures and lower air pressure at high altitudes reduce air density.
Humidity, or the amount of atmospheric moisture, also changes air density. More moisture in the air, lower air density.
Frequently Asked Question (FAQs)
Q1. Which instrument is used to measure the air pressure?
Ans. Barometer
Q2. Does density change with pressure?
Ans. When pressure increases, density increases. When the pressure decreases, density decreases.
Q3. Does density remain constant?
Ans. Density (ρ) is the amount of mass (m) per unit volume (V) of a substance. Density is an intensive property, meaning density does not change as the amount of the substance changes.
Q4. Does air density increase with altitude?
Ans. Air density is higher at the lower altitudes. There is more space between the air molecules at a higher altitudes. There is a less oxygen to breathe at the top of a high mountain at sea level.
Q5. Which type of air is more dense?
Ans. Cold air
