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What is Pressure?

Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. The SI unit of pressure, the pascal (Pa). This article is on atmospheric Pressure.


F- Magnitude of normal force

A- area of the surface on contact

p- pressure

The minus sign comes from the fact that the force is considered towards the surface element, while the normal vector points outward.

Absolute Pressure

Barometric pressure is an absolute pressure type. When measuring absolute pressure, the measured pressure is being compared to a perfect (absolute) vacuum, where there are no air molecules left and therefore no pressure.

In comparison, the common gauge pressure is referred to as current barometric/atmospheric pressure.

Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.

Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the value may be appended with the word “vacuum” or the gauge may be labeled a “vacuum gauge.”

What is Atmospheric Pressure?

Atmospheric pressure is sometimes also referred to as barometric pressure. Atmospheric pressure is the pressure caused by the weight of the air above us.

The earth’s atmosphere above us contains air, and although it is relatively light, having that much of it, it starts to have some weight as gravity pulls the air molecules. The air around us, comprises of about 78% of nitrogen, 21% of oxygen, under 1 % argon and small traces of other gases.

Image result for barometric pressure images

The nominal atmospheric pressure on earth is agreed to be 101.325 kPa which means that there is about 1.03 kilogram-force per every square centimeter ( 14.7 pound force per every square inch) typically on earth’s surface caused by the weight of the air.
In practice, the Atmospheric pressure very rarely is exactly that nominal value, as it is changing all the time and varies at different locations.

The atmospheric pressure  varies based on altitude. The higher you are, the smaller the atmospheric pressure, It is  because when you move to a higher altitude, there is less air on top of you i.e. the air gets thinner. The air at higher altitudes also contains fewer molecules, making it lighter than it would be at a lower altitude. The gravity also decreases at these heights. Due to these reasons, the atmospheric pressure is smaller at higher altitudes.

The Atmospheric Pressure also changes with weather. If humidity is high the Pressure also increases, because of the increase in Water molecules in the air.

Ideal Gas Laws

An ideal gas is a theoretical gas! It does not exist in reality but is assumed to exist for the purpose of simplifying calculations. It also generates a reference point in relation to which behaviour of other gases can be studied.

An ideal gas is defined as a gas composed of randomly moving particles as all gases do, the only difference being that for an ideal gas when its particles collide with each other, these collisions are assumed to be perfectly elastic which means no energy of either of these particles is wasted.

In reality, however, when actual gas particles collide with each other, some of their energy is wasted in changing directions and overcoming friction. However, at STP ( defined below) conditions most natural gases act just like an ideal gas subjected to reasonable restrictions.

Generally, any gas behaves similarly to an ideal gas under the conditions of high temperature and low pressure. To derive correctly the ideal gas law, we will learn some of the other very important laws for gases.

There are a few laws that govern the Ideal Gas laws, they are:

Boyle’s law:

Boyle’s Law states that ‘The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if both the temperature and amount of gas remain unchanged’. In mathematical terms this law is given as:

 1/V     or that  PV= K

where P=  Pressure of the gas; V=Volume of the gas; K= constant. It means that both pressure and volume of a given mass of gas are inversely proportional to each other at a constant temperature. Furthermore, it also expresses that  product of pressure and volume for any gas is a constant and thus can be used to study the  comparison of the gas under different conditions as:

P’V’ = P”V”

where both the products are for the same gas but under different pressures and volumes.’

Charles’ Law:

Charles’ law states that ‘ When the pressure of a sample of air is held constant, then the volume of the gas is directly proportional to its temperature’, that is


where V= Volume of a gas sample; T= Absolute temperature. Quite simply put, it says that Gases expand on heating and contract on cooling.

Avogadro’s Law:

Avogadro’s law states that ‘Equal volumes of all gases at conditions of same temperature and pressure have the same number of molecules’, written as:

 n or V/n =K

where V=volume of gas;  n= Number of moles  ( 1 mole=6.022 x 1023 molecules). It implies that under similar conditions of pressure, volume and temperatures all gases will have an equal number of molecules, independent of the weight and density of the gas.


The Ideal Gas Equation

If we combine the results of all the above gas laws we get an equation which holds true for ideal gas. The most common form of this equation is, since PV= K and V/T =k then

PV/T = constant

Thus, Ideal Gas Equation is given as

PV = nRT

where, P= pressure of the gas;  V=volume of the gas;  n= Number of Moles;  T=Absolute temperature; R=Ideal Gas constant also known as Boltzmann Constant = 0.082057 L atm K-1 mol-1.

Using this equation, the study of any gas is possible under assumptions of STP conditions and subjecting the gas to reasonable restrictions to make it behave similarly to an Ideal gas.

What is a barometer and its applications?

A barometer is any device used to measure barometric pressure/Atmospheric pressure. Barometers and pressure altimeters (the most basic and common type of altimeter) are essentially the same instrument, but used for different purposes.

An altimeter is intended to be transported from place to place matching the atmospheric pressure to the corresponding altitude, while a barometer is kept stationary and measures subtle pressure changes caused by weather. The main exception to this is ships at sea, which can use a barometer because their elevation does not change.

Barometers are primarily used for the prediction of weather, and maintaining health and well-being.

  • Weather Observation – Along with temperature, humidity, wind speed, wind direction, and rainfall, Atmospheric pressure is a key weather metric, particularly with regard to a Atmospheric pressure forecast.
  • Maintaining Health and Well-being – Many believe the weather has an impact on our health, and Atmospheric pressure is a contributor to that. As such, tracking Atmospheric pressure and how it makes you feel can help you take preventative actions like getting indoors, using a prescribed medication before a weather event arrives, and so forth.
  • Fishing and Hunting – More and more scientists, wildlife experts, and highly-experienced fishermen and hunters are starting to recognize a correlation between Atmospheric pressure and animal activity. Using a fishing barometer/hunting barometer to harness the effects weather can have on your chosen game or favorite fish can help ensure success.
  • Used in Coal Mine– falling air pressure, gases trapped within the coal in deep mines can escape more freely. Thus low pressure increases the risk of firedamp accumulating. Collieries therefore keep track of the pressure.

Why Is It Beneficial to Have a Barometer?

Atmospheric pressure monitoring benefits, depending on how you use your barometer, can include planning outdoor activities, as well as management of headaches, joint pain, blood sugar, headaches, and blood pressure.

  • Planning Outdoor Activities –A drop in Atmospheric pressure usually means a storm is coming. Conversely, a steady rise in pressure typically is an indicator of dry and stable weather. As such, keeping an eye on the Atmospheric pressure forecast can be helpful when putting together an afternoon cookout, or knowing to grab a raincoat before heading to work in the morning.
  • Managing Joint Pain – If you hear someone talk about his “trick knee,” and how it lets him know bad weather is coming, there could be some truth to that. If that trick knee belongs to you, watching the barometer can give you a heads up to take medication before the onset of pain and discomfort.
  • Managing Headaches – Many studies have suggested that changes in weather can trigger migraines and other types of headaches. As with joint pain, watching Atmospheric pressure can provide advance warning to have your headache medication nearby. In addition, keeping a headache diary, and tracking how different types of weather affect your pain, can be highly useful in understanding and mitigating your symptoms.
  • Managing Blood Pressure – A drop in Atmospheric pressure can also mean a drop in blood pressure, some scientists say. Depending on your health, this could trigger dizziness, blurry vision, and other symptoms. It also may change how you and your doctor may manage your blood pressure depending on the season.
  • Managing Blood Sugar – According to the American Diabetes Association, atmospheric pressure can affect the operation of a diabetic’s insulin pump. This can be something to think about when traveling, but also when at home and drastic changes in the weather occur.
  • Fishing and Hunting – Understanding the relationship between Atmospheric pressure and animal activity can help you pick the best times to paddle out on the lake, or venture into the woods. To help take the guesswork out, some fishing barometer/hunting barometer products are available to let you know when conditions are likely optimal, using Atmospheric pressure, moon phase, and other variables.

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