In which direction do gases exert pressure?

Pressure in Physics: Simply Explained

The pressure indicates the force with which a body acts on a surface. The rule here is that the greater the force acting on the surface or the smaller the surface, the greater it is. Two factors determine the amount of pressure that acts on a surface. The pressure is all the greater

  • the greater the force acting on the surface
  • or the smaller the area.

Pressure can also be positive or negative. The pressure acting on a surface is positive if the force is directed towards the surface. On the other hand, the pressure is negative when the force, turned away from the surface or the body, acts as a pull.

Pressure can be divided into hydrostatic pressure, dynamic pressure and total pressure.

Hydrostatic pressure: Hydrostatic pressure builds up in stationary fluids, i.e. liquids and gases, due to gravity on every body in the fluid. The pressure acts on all sides and grows with the depth. For example, air pressure or pressure under water is hydrostatic pressure.

Dynamic pressure: Dynamic pressure, on the other hand, detects pressure that builds up in a liquid or gas when the fluid is in motion. For example, water flows through pipes at dynamic pressure.

Total pressure: Total pressure (also: total pressure) is the pressure that arises when the flow velocity of a fluid is reduced without loss (isentropic) until it almost comes to a standstill.

Pressure and its various units

In physics, Pressure measurements usually the Unit Pascal (Pa) used - named after the French mathematician and physicist Blaise Pascal (1623-1662), where 1 Pascal corresponds to the force of one Newton per square meter. That is in equations Formula symbol p for printing, derived from the Latin word "pressio". The multiple of the pressure unit Pascal are the pressure units Kilopascal (kPa) and Megapascal (mPa). 1 kPa = 1000 Pa and 1 mPa = 1 000 000 Pa.

Pressure occurs in liquids and gases, but solids can also exert pressure on other bodies. In everyday life it plays a role in car tires, soccer balls, water pipes or air pressure measurements. Older units, some of which are still in use, are Atmosphere (at), bar (bar) or Torr. The latter corresponds to the pressure of 1 mm Mercury column (mmHg) and was used, among other things, in physics and meteorology for air pressure.

In some European countries Blood pressure and pressure of other body fluids are also given in mmHg. The torr is after the Italian Evangelista Torricelli named who invented the mercury barometer as Galileo Galileo's assistant. The following applies for the conversion: 1 bar = 100,000 Pa; 1 at = 98,100 Pa; 1 Torr = 133.32 Pa.

Calculate and convert pressure

The force acting vertically as pressure on a certain surface can be calculated using the following equation:

p = F / A

  • p = pressure
  • F = acting force
  • A = area

The following applies to the conversion of the various pressure units:

Air pressure: The hydrostatic pressure of the air

The lawyer, physicist and inventor demonstrated for the first time that air exerts a compressive force on objectsOtto von Guericke followed up in 1663 with the famous experiment with the "Magdeburg hemispheres". These were two half hollow spheres pressed close together, from which von Guericke pumped the air. As soon as a sufficiently strong negative pressure was created, two teams of horses pulling in opposite directions could no longer separate the hemispheres.

In fact, the air pressure results from the weight of the column of air that rests on the surface of the earth or on a body. It reaches from the ground to a height of about 10 kilometers and weighs about 10 tons (one cubic meter of air is about 1 kilogram). The air pressure differs from place to place. On average, it is 100.013 Pa = 1.013 bar at sea level.

In meteorology it is in Hectopascal (hPa) It is shown on weather maps in the form of isobars, i.e. the lines that enclose areas with the same air pressure. Depending on the weather, it is subject to greater fluctuations. In a strong high pressure area - such as the Siberian high - it can essentially rise to 1040 to 1065 hPa, in extreme cases even up to 1080 hPa. In the case of low pressure areas, on the other hand, it falls. In Central Europe it is usually 990 to 1000 hPa, in hurricane lows it is 950 to 970 hPa. In a typhoon in 1979, an extreme value of 870 hPa was measured.

Pressure signals change in weather

The pressure decreases rapidly with altitude - at sea level by about 1 hPa per 8 meters. This is because the density of air decreases with altitude, making it lighter per unit of volume. The temperature also influences the density of the air. At the summit of the GroƟglockner around (3798 meters) the air pressure is 623 hPa, at the cruising altitude of an airplane (13 kilometers) it is 191 hPa.

Air pressure plays a major role in both aviation and shipping. In aircraft, the flight altitude is determined by the respective pressure according to the so-called barometric altitude formula, which includes factors such as altitude, temperature and density of the air.

At sea, rapid changes in air pressure usually signal an imminent change in weather. Rapidly falling pressure usually means there is a risk of strong winds or storms.

Pressure in liquids

Pressure in liquids is used for a number of technical devices, examples are hydraulic presses or vehicle brakes. In general, the following applies: In a closed vessel, the pressure in a liquid is almost the same everywhere and spreads out on all sides. In the water, a column ten meters high exerts a pressure of 981 hPa on a body, which corresponds approximately to the air pressure at sea level. A diver who dives ten meters into fresh water at sea level has twice the atmospheric pressure on the lungs. The pressure generated by the weight of a column of liquid above is also called gravitational pressure.

In addition, the boiling point of water changes with the air pressure. Since this decreases with increasing altitude, water boils at ever lower temperatures and no longer at 100 degrees Celsius as on the earth's surface. The rule of thumb is that the boiling point drops by one degree for every 300 meters of altitude.

Pressure in gases

Gas pressure is important for tires, balls, life jackets, air pumps or gas cylinders, among other things, as well as for supplying patients with oxygen in medicine. As with liquids, the pressure in a gas that is in a closed vessel is approximately the same everywhere and spreads out on all sides. But at Changes in pressure also change the volume of gasesbecause, unlike liquids, they are compressible.

The state of a gas can therefore be expressed in terms of the three quantities Pressure, volume and temperature describe. Naturalists recognized this in the 19th century Robert Boyle, Edme Mariotte and Joseph Gay-Lussac.

General gas equation

To calculate the state of a gas, they devised the laws named after them. These describe thermal equations of state, i.e. the relationship between the thermal parameters pressure, volume and temperature of ideal gases.

The law of Boyle and Mariotte states that in an air pump, for example, there is a certain pressure with a certain volume of enclosed air. When the piston is pushed into the cylinder, the volume decreases, but the pressure increases. The following applies: The smaller the volume of the enclosed air, the greater the pressure in the air. But in the case of non-rigid bodies such as beach balls or air mattresses, the volume increases with increasing pressure. The pressure of ideal gases is therefore inversely proportional to the volume. The prerequisite for this is a constant temperature and constant amount of substance. The increase in pressure then leads to a reduction in the gas volume; if the pressure decreases, the gas expands.

Gay-Lussac's Law again means: the higher the temperature, the greater the volume that an ideal gas occupies at constant pressure and constant amount of substance. If the temperature drops, an ideal gas contracts and takes up a smaller volume. They show the temperature of a room or a measurement object through the expansion or shrinkage of a gas at constant pressure or pressure increase at constant volume.

Pressure in fluids and the particle model

The pressure in liquids and gases is good with that Particle model To explain: In liquids, the force of the particles (molecules) on each other and on the vessel walls creates the pressure.

In gases, the freely moving molecules meet and hit the vessel walls. The forces that they exert on their surfaces make themselves felt as pressure.

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