Galileo’s Pump Problem
A duke living in Tuscany, Italy, in the 17th century, installed a well in the courtyard of the palace. But, duke saw the pump fail to pull water up. The Duke asked Galileo for this. Galileo discovered that the groundwater level went down too much and could not pull water up.
“We cannot lift water up deeper than about 10 meters.”
Unfortunately, Galileo couldn’t figure out the cause.
Why can’t he pump that wells deeper than about 10 meters?
In 1644, Torricelli took up the problem. After some study of earlier experiments he did one of his own. The apparatus was a glass tube about a meter long, sealed at one end. He filled it with mercury, covered the open end, and inverted it over a dish of mercury. This was not as easy as it sounds today. Glass tubes at the time were fragile and hard to come by. They often broke when filled with a kilogram of mercury. But with the help of a skilled assistant the experiment was done. The mercury in the tube fell and stabilized at a level about 76 centimeters above the level in the dish. Torricelli surmised correctly that the mercury rose in the tube because of the weight of the atmosphere pressing down on the mercury in the dish, and that the space above the mercury column was a vacuum. It was the first time that a vacuum had been created in the laboratory, and understood as such.
In other words, the water was not lift up. It was pushed up by atmospheric pressure like a mercury column. Just as mercury columns can only rise about 76 cm, water could only rise to about 10 meters by atmospheric pressure.
Is the water in the straw coming up? Or is it pushing up?
The stronger the suction force I suck, the more I think I can suck up at any higher place. But that’s not true.
In fact, the fundamental force that moves water is atmospheric pressure.
The water in the straw is pushed up by atmospheric pressure rather than being sucked up.
We have only reduced the pressure inside the straw so that atmospheric pressure can push up the water well.