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# Pressure and the simple mercury barometer

Unpack the unique properties of gases, their kinetic energy, and their ability to exert pressure on surroundings. Understand the connection between temperature, force, and pressure in gases. Learn about atmospheric pressure and its measurement using a barometer. Get to know gas laws and pressure units such as atmospheres, tor, and pascals. Created by Ryan Scott Patton.

## Want to join the conversation?

• Doesn't the diameter of the tube affect how high the mercury will rise? Increasing the amount of mercury should require more air pressure to make it rise the same distance.
....or have I missed something?
If I am correct, how do we determine what the diameter of the tube should be?
• When the air pressure outside a barometer increases, what happens to the air INSIDE the barometer??
• Hi Kathleen.
This is a great question. When making a simple Torricelli barometer, the tube is initially fully filled with the substance (e.g. Mercury). It is then capped off and inverted into a basin containing the same fluid and the cap is taken off. This method actually causes a vacuum to be formed once the barometer's fluid height changes when the atmospheric pressure is sensed. Here is a link to YouTube that will demonstrate how this is done.
• If we increase the mass of a subject, will that increase overall force?
• I disagree with the above user.

Force is equal to mass x acceleration.

If you increase the mass, you'd increase the force.
• Okay that pressure independs from diameter of the upper tube, but what happens if the volume of mercury of the bottom recipient is not enough to fill the upper tube till 760 mmHg high.

Consider that the upper tube is in vacuum, and We dive him into the bottom recipient, it will suck the mercury but will not have enough volume of mercury to reach the 760 mm.

What happens then?
• What would happen
if you put air inside the barometer.
• Since pressure is force/area, the area of the mercury in the dish exposed to air would affect how high the mercury is pushed up right? So how large does the dish need to be in order for the mercury to rise 760mm at sea level?
• Since the atmospheric pressure at sea level is 760 millimeters of mercury, the dish of mercury would have to large enough to have enough mercury to allow the mercury to rise 760 millimeters up the tube.
(1 vote)
• why did torricelli use mercury for his experiment and why not some other fluid like oil or water?
(1 vote)
• because if we take water then the height of the column increases and we can't have such high columns