Sectieoverzicht
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What is atmospheric pressure?
The earth's surface is surrounded by a layer of air (which contains a mixture of gases) called the atmosphere. The weight of this layer of air is known as atmospheric pressure.
At sea level, the pressure exerted by the weight of the atmosphere has a mean value of 101,325 pascals i.e. 101,325 Pa or 101 kPa.
A barometer is the instrument used to measure atmospheric pressure. Under normal circumstances and at sea level, we don't feel atmospheric pressure since the fluids in our bodies exert a pressure which is greater than that of atmospheric pressure.
The higher the altitude the lower the atmospheric pressure i.e. atmospheric pressure decreases the higher the altitude. Let's look at an example: Mount Kilimanjaro in Tanzania.
The summit of Mount Kilimanjaro is 5895 m above sea level. The atmospheric pressure at the summit is measured at 50 kPa. When compared to the 101.3 kPa at sea level, the atmospheric pressure at the summit is only 49% of that at sea level. Only 49% of oxygen is available at the summit compared to 100% at sea level. This is due to the fact that the density of air is greater at sea level. The higher the altitude the lower the density of air and the lower the atmospheric pressure. An interesting application of this fact is the training of athletes at high altitudes. For example, East African middle distance and endurance runners have dominated races across the world. One of the reasons attributed to this dominance is that they train at high altitudes. Their bodies are having to function and create energy with less oxygen available. This is achieved by their bodies creating more red blood cells which results in their blood being able to carry more oxygen.
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Review the table below which illustrates the point that atmospheric pressure does indeed increase the lower the altitude.
An interesting observation is the Dead Sea. The Dead Sea is actually a landlocked lake that is situated between Jordan and Israel. The Dead Sea is approximately 430 m BELOW sea level. This lake has an extremely high salt content making it very dense. Therefore humans are able to float in the Dead Sea since the human body is less dense than the lake.
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Let's now look at some examples which proves that atmospheric pressure does indeed exist!
Magdeburg hemisphere
The Magdeburg hemisphere consists of two half spheres which are joined together forming an airtight sphere. The air is merely trapped inside the hemisphere and not compressed. Therefore the pressure outside the sphere is the same as the pressure inside the sphere, as indicated by the red arrows in the image. The spheres can therefore be pulled apart easily and without any real resistance.
Now, what happens if the air inside the sphere is removed, thereby creating a partial vacuum inside? The pressure outside the sphere will therefore be greater than the pressure inside the sphere resulting in the spheres being pressed tightly together (as indicated by the arrows in the image to the right). It will be difficult to pull the spheres apart.
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Measuring atmospheric pressure
As mentioned earlier, atmospheric pressure is measured using a barometer. In this section, we will explore three types of barometers viz, the mercury barometer, the aneroid barometer and the digital barometer.
Mercury barometer
This is the oldest barometer in existence, having been invented by Evangelista Torricelli in 1643.
It consists of a glass tube which is closed at the top and open at the bottom. It sits in a pool of mercury. As the atmospheric pressures rises, the mercury is forced by this increasing atmospheric pressure to rise and vice versa.
Aneroid barometer
The aneroid barometer was invented in 1844 by Lucien Vidi. This barometer measures atmospheric pressure without the use of any liquid. Instead, a sealed metal chamber expands and contracts based on the level of atmospheric pressure around it. A lever which is connected to the chamber moves a pointer which displays the measurement of the atmospheric pressure.
Digital barometer
The digital barometer is the most commonly used barometer these days, particularly since it displays the data quickly and more accurately. This information can then be downloaded and used in data analysis.
For further background on the history of the mercury barometer, watch the short YouTube video below.
TedEd. (2014). The history of the barometer (Standard YouTube licence)
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Siphon
A siphon is a continuous tube which enables liquid to drain from a higher point to a lower point due to the difference in pressure. No pumps are required.
The liquid rises in the tube due to the atmospheric pressure which is pushing down on the liquid. The long arm of the tube carries more liquid and is therefore heavier. The force of gravity draws the liquid through the long side of the tube.
This liquid will continue to flow until the level of liquid is lower than the intake point.
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Vacuum cleaners
The vacuum cleaner works in a similar way to when you suck juice from a straw. The rotating fan within the vacuum cleaner creates a vacuum and will then begin to suck in air (and dust / dirt) through the nozzle due to the higher pressure being 'outside' the vacuum cleaner.
Syringe
Have you ever wondered how the syringe is able to draw blood or any other liquid?
When the plunger is pulled back this creates a lower pressure inside the syringe than the outside pressure, causing the fluid to move into the syringe. When the plunger is pushed into the tube, the pressure inside is less than the pressure outside the tube, therefore the liquid squirts out. -
Summary
The following topics were covered in this course:
- What is pressure?
- Pressure due to solids
- Pressure in a liquid
- Atmospheric pressure
You will now know that pressure is the amount of force which is exerted over a specified area.
Using the above formula you are now able to determine that:
P = hgp
Pressure is measured in Pascal's where 1 Pa = 1 N/m2
You now understand that pressure in a solid will be higher the smaller the area e.g. walking in high heels on grass will cause the heels to sink into the grass (due to the force over a small area) as opposed to walking in flat shoes on grass (the downward force is spread over a larger area).
You also determined that pressure in a liquid is determined by the density and depth of the liquid. Pascal's principle was introduced: any pressure change on the surface of an enclosed fluid will be transmitted equally throughout the fluid.
Atmospheric pressure at sea level is 101 kPa. The higher the altitude the lower the atmospheric pressure i.e. atmospheric pressure is dependent on the density of the air and the height of the air column. At sea level, the air is more dense, there are more oxygen particles in the air. The higher the altitude, the less dense the air is.
We encourage you to reflect on everyday examples where the use of pressure is demonstrated.
Finally, to test your understanding of this topic, answer the questions in the short quiz below. A pass mark of 50% is deemed to be successful - good luck!
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