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Vacuum

The discovery of atmospheric pressure. Vacuum. In 1643 the Italian scientist E. Torricelli picked up a tube one metre long sealed at one end, filled it with mercury and shut the open end with this finger.

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Vacuum

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  1. The discovery of atmospheric pressure Vacuum In 1643 the Italian scientist E. Torricelli picked up a tube one metre long sealed at one end, filled it with mercury and shut the open end with this finger. Then, he turned the tube upside down, put it in a container of mercury, and took his finger away. He observed that the mercury in the tube descended, and that the height of the mercury inside the tube was independent of the section (width) and of the inclination of the tube. That height was 76 centimetres. 760 mm

  2. Conclusions of the experiment • He demonstrated the existence of the atmosphere, through calculating the pressure it exerted: • Atmospheric pressure is the force exerted by the atmosphere, in all directions, against the surface of all the bodies it contains. • Until then it was thought that the atmosphere, or nature, abhorred a vacuum (this was called horror vacui), and that air tended to occupy any space, however small it was. With his experiment, Torricelli showed how easy it was to generate a vacuum: the empty space left by the mercury when it descends has no air.

  3. Units of pressure The force exerted by the atmosphere is due to the weight of all the air we have above us. Since Torricelli’s experiment was carried out at sea level, we say that the normal atmospheric pressure is 760 mmHg (millimetres of mercury), a quantity which is equivalent to the unit called atmosphere (atm): 1 atm = 760 mmHg In the International System of Units, SI, pressure is measured in pascals, Pa. In meteorological maps another pressure unit is often used: the millibar, mb, and also a multiple of pascal, the hectopascal, hPa, being: 1 atm = 101,325 Pa ; 1 atm = 1,013 mb

  4. Variation in atmospheric pressure The value of atmospheric pressure is not constant: it depends on several factors, the main one of which is altitude. As the quantity of air in the atmosphere decreases as we get higher, the pressure also falls. Thus, at 5,000 m pressure it is approximately 0.5 atm (half the pressure it is at sea level); consequently Torricelli’s experiment would have given a different result at that altitude.

  5. Activities • What is atmospheric pressure? What units of measurement do we use to express it? • If we carry out Torricelli’s experiment on the top of a hill, will the mercury column be higher or lower than 760 mm? Explain your answer. • How high would the mercury column be if we carried out Torricelli’s experiment at an altitude of 5,000 metres above sea level? • Why does the bag of crisps a hiker is carrying inflate as he is climbing a mountain?

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