Pressure Gradients: No Sucking Involved
    By Enrico Uva | September 20th 2011 01:08 PM | 5 comments | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    The word "suck" sucks in science. Although much has been written about this misconception, I'll try to string together several everyday examples to further tackle a sucking illusion that persists despite the nearly four hundred year old discovery of atmospheric pressure.

    If you place one end of a straw into a juice and its other end loosely into your mouth, and you do nothing, the juice does not rise into your mouth. The only reason there's a bit of liquid in the straw is because the tube is narrow enough and capillary action acts against gravity. If you had inverted a glass in water, however, the inside of the glass would only contain air.

    Air molecules are squeezed against each other by the overhanging weight of other air molecules in the atmosphere. The ratio of the impacting force of their molecules to the area that they're acting upon is their pressure. But the pressure of the air above the juice and the air in the straw or glass are equal. This balance results in no net force. The nonzero net force from surface tension plus liquid-straw adhesion(capillary action) minus gravity is not strong enough to allow the juice to reach you.

    When you "suck", you are really inhaling with your lips tightly sealed around the straw. As your chest cavity expands from the downward movement of the diaphragm, the air molecules inside your lungs have more room to move about. This decreases their frequency collision with the walls of the air sacs. The imbalance in pressure causes the air to move from your mouth into your lungs. But the atmospheric pressure above your juice hasn't changed. The now less-hindered external air pressure is what pushes the liquid into the straw and into your mouth. As you swallow, you usually break the seal between your lips and straw and allow air into your mouth, which erases the pressure gradient, and you have to repeat the cycle.

    A sucking kiss over the ear can be an unpleasant experience. By the same drinking straw mechanism, the air pressure between the external ear and the eardrum becomes weaker than the air pressure between the eardrum and the pharynx. This leads to a net force(see red arrow in diagram) which can damage the fragile membrane if the air in the Eustachian tube does not escape fast enough through the mouth or nasal passages.

    The familiar change in airplane cabin pressure during takeoff and landing causes the eardrum to experience a similar but less intense stress.

    Hold both ends of a hose towards the air and completely fill the hose with water. Cover both ends with your thumbs. There will still be a small amount of air in the hose, but as the water moves towards the opposite end, it will create more room for that small amount of air. The subsequent decrease in pressure inside the hose will allow the atmospheric pressure to push along the edges of the seal between the thumb and the edge of the hose.(It was always pushing, but in vain, so to speak, when it was up against an equal force per area.) If what I described has happened, you will feel it. With your thumb still sealing both ends, you can then lower one end into a tub of water and place the other end below the level of the inlet. Water will drain out of the tub even though you did not suck on the hose.

    In a test tube, place a small amount of iron powder and a bit of vinegar in it to accelerate its corrosion. Invert the test tube in a beaker of water. Soon, while the consumption of oxygen inside the tube decreases pressure, water begins to be pushed into the test tube by the unaffected atmospheric pressure above the water in the beaker. After several hours, 21% of the test tube will be filled with water, corresponding to the percentage of oxygen in the air. At this point, there is no further rise in the test tube's water level because of the pressure of the remaining nitrogen gas.

    A sea breeze is also caused by pressure differences. During the day, land in the sun warms up faster than water. This is partly because water's strong network of hydrogen bonding makes it difficult for its molecules to be agitated.  The warmer air is less dense than cool air, and its buoyant force slightly reduces atmospheric pressure above the land. But the unreduced impact of air molecules above the sea is now stronger than those above the land, allowing movement of air from the sea. At night, water cools more slowly than land. It is now air above the sea that experiences a buoyant force, so the breeze comes from the land.

    It's because the pressure gradient is so small that only a breeze results. Although the pressure differences in a hurricane are similarly initiated but altered by other mechanisms, the fact they are more pronounced is the main reason hurricane winds are so much stronger than a breeze. A typical sea breeze is caused by an approximately 0.2% reduction in air pressure, resulting in winds of about 5 to 10 miles per hour. The strongest hurricanes involve 10-11% drops in pressure, and the net force from the imbalance can bring wind speeds up to about 180 mph.


    And Nuclear bomb over pressure is measured in atmospheres!

    But, this also applies (of course) to internal combustion engines, but in their case it's driven by finite impulse waves. I've also wondered if they're (finite impulse waves) the source of pressure fronts in global weather.
    Never is a long time.
    Nice one, Enrico!

    Next article – the vacuum, maybe?

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Glad you enjoyed it Robert. I had an idea for the next article, but thanks to the flurry of today's events, it was pushed into a partial vacuum.
    "compared to sand and rocks, water has a stronger network of intermolecular forces, which makes it more difficult to agitate them"

    This is an unhappy way to explain the effect. Crystal forces in rocks are much stronger than hydrogen bonding. If your students are not up to the concept of entropy, call it heat capacity.

    Yes. You're right. My sentence is misleading. The term "crystal forces", though, won't explain much, and I don't think entropy is that relevant here. Rocks can actually heat up at different rates because of different albedos and different thermal conductivities. Compared to water, rocks reflect less light. And of course water's high specific heat, due hydrogen bonding definitely contributes to the fact that it warms up slowly.