Question

A widely used weather instrument called a barometer can be built from a long, thin tube of glass that is sealed at one end. The tube is completely filled with mercury and then inverted into a small pool of mercury. The level of the mercury inside the tube drops initially but then stabilizes at some height. A measure of the height of the column of mercury once it stabilizes is a measure of pressure in \(\mathrm{mm} \mathrm{Hg}\) (or torr). Which of the following is the best explanation of how this barometer works? a. Air pressure outside the tube (pressure of the atmosphere) counterbalances the weight of the mercury inside the tube. b. Air pressure inside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal. c. Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal. d. The vacuum that is formed at the top of the tube of mercury (once the mercury level in the tube has dropped some) holds up the mercury. e. I have no idea how a barometer works.

Short answer

The correct answer is option (a): Air pressure outside the tube (pressure of the atmosphere) counterbalances the weight of the mercury inside the tube. Atmospheric pressure on the pool of mercury supports the weight of the mercury column, thus balancing the pressures and determining the height of the column.

Step-by-step solution

Understand the barometer concept

A barometer is a device used to measure atmospheric pressure. The classic barometer consists of a glass tube filled with mercury and inverted into a pool of mercury. The height of the mercury column is maintained due to the balance between the atmospheric pressure pressing down on the mercury pool and the weight of the mercury column.

Analyze the options

To find the correct answer, consider each option given: a. Air pressure outside the tube (pressure of the atmosphere) counterbalances the weight of the mercury inside the tube. b. Air pressure inside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal. c. Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube are equal. d. The vacuum that is formed at the top of the tube of mercury (once the mercury level in the tube has dropped some) holds up the mercury. e. I have no idea how a barometer works.

Choose the correct answer

The correct answer is option (a): Air pressure outside the tube (pressure of the atmosphere) counterbalances the weight of the mercury inside the tube. This is because atmospheric pressure on the pool of mercury supports the weight of the mercury column, thus balancing the pressures and determining the height of the column.

Key concepts

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the atmosphere on a given area. Imagine the air around us as a sea of molecules pressing down on everything, including our clothes, the ground, and in this case, the mercury in a barometer. This pressure is crucial for a barometer to work.
- A barometer measures this atmospheric pressure by balancing it against the weight of a mercury column. - The pressure causes mercury to maintain a particular level in an inverted glass tube. Understanding atmospheric pressure helps explain natural phenomena such as the weather changes and why your ears "pop" when you climb a mountain or take off in an airplane.

Mercury Column

The mercury column in a barometer is simply mercury contained within a glass tube that has one end sealed. When you invert this filled tube into a pool of mercury, the mercury level inside the tube initially drops slightly. - The height of the mercury column that stabilizes is directly affected by atmospheric pressure. - This height provides a measure of atmospheric pressure in units of millimeters of mercury (mmHg) or torr. When the atmospheric pressure changes, the mercury column's height will also change, which is why barometers are useful tools for forecasting weather and studying air pressure variations.

Vacuum in Barometer

The vacuum in a barometer plays a key role in the functioning of this weather instrument. When the mercury is first inverted in the tube, some of it falls back into the lower pool, creating a space at the top without mercury or air. This empty space is what we call a vacuum.
- The vacuum ensures that there is no atmospheric pressure from above pushing down on the mercury column. - With no air pressure inside the tube, the outside atmospheric pressure can support the weight of the mercury column by pressing on the mercury pool outside the tube. This vacuum is important because it ensures that the only force pushing mercury up in the tube is the atmospheric pressure, enabling us to measure it accurately.