You live your life under constant pressure. The atmosphere weighs down on you with a force of 6.67 Kilogram per square inch, or about a ton per square foot. We're largely unaware of this tremendous pressure because it's been with us since we were born. A simple experiment with a playing card and a glass of water, however, demonstrates the power of atmospheric pressure in action.
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Take a glass of water and cover the mouth of the glass with a playing card or a piece of cardboard. If you hold the card in place as you invert the glass, then slowly remove your hand, the water will remain in the glass rather than falling as it would if the card were removed. The card is held up against the weight of the water by atmospheric pressure.
The molecules in the air around you are in constant motion; the speed at which they move is a function of their size and the temperature. The average speed of molecules in a gas is the square root of (3kT / m) , where k is a constant, T is the temperature in Kelvins and m is the mass. Since air is mostly nitrogen and oxygen, this fairly simple calculation works out to an average speed in air at room temperature of more than a thousand miles an hour. As the air molecules collide with other objects and bounce off, they exert force, a little like a stream of tennis balls striking a target. This force is the pressure, which is defined as force per unit area.
The gravitational attraction between the water and the Earth also exerts a downward force on the water in the glass. The water remains in the glass if the force of atmospheric pressure is sufficient to counterbalance the weight of the water.
Atmospheric pressure, however, isn't the only force at work. Water is a polar molecule, meaning that one end has a partial negative charge and the other end has a partial positive charge. It's able to form weak bonds between molecules called hydrogen bonds. These properties give water a high surface tension, which helps form a seal around the glass rim. The glass may also have contained both water and air in it before you turned it over. If any water leaks out as you turn it over, the air inside the glass must expand to fill the space left vacant by the water. When the same amount of gas expands to fill a greater volume, its pressure decreases--and thus the air pressure outside the glass is greater than the air pressure on the water inside.
The trick only works as long as air can't seep inside to replace any of the water in the glass. Once you separate the card from the rim of the cup, the water flows out, air rushes in to replace it and the glass quickly empties.
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