Question

Figure \(5.2\) shows a picture of a barometer. Which of the following statements is the best explanation of how this barometer works? a. Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal. b. Air pressure inside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal. c. Air pressure outside the tube counterbalances the weight of the mercury in the tube. d. Capillary action of the mercury causes the mercury to go up the tube. e. The vacuum that is formed at the top of the tube holds up the mercury. Justify your choice, and for the choices you did not pick, explain what is wrong with them. Pictures help!

Short answer

The best explanation of how the barometer works is given by statement a: "Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal." This is because the air pressure outside the tube directly affects the mercury level inside the tube, while the other statements are either incorrect or only partially correct in describing the barometer's functioning.

Step-by-step solution

Understanding a barometer

A barometer is an instrument used to measure air pressure. It consists of a long, closed tube filled with a liquid such as mercury. The closed end of the tube is a vacuum, while the open end is immersed in a reservoir of the liquid. As air pressure changes, the liquid level in the tube rises or falls. This change in liquid level helps measure the atmospheric pressure outside the tube.

Evaluate statement a

Statement a states that air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal. This statement is CORRECT. The air pressure outside the tube affects the mercury level inside the tube and when the air pressure inside the tube matches the outside air pressure, the mercury level remains constant and the barometer shows the current atmospheric pressure.

Evaluate statement b

Statement b says that air pressure inside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal. This statement is INCORRECT because the inside of the tube at the closed end is a vacuum, so there cannot be air pressure causing the mercury to move.

Evaluate statement c

Statement c suggests that air pressure outside the tube counterbalances the weight of the mercury in the tube. This statement is PARTIALLY CORRECT. The air pressure outside the tube does counterbalance the weight of the mercury in the tube. However, statement a better explains how the barometer works, as it connects the air pressure inside the tube to the air pressure outside the tube.

Evaluate statement d

Statement d claims that capillary action of the mercury causes the mercury to go up the tube. This statement is INCORRECT because capillary action is not responsible for the rise and fall of mercury in the barometer. It is the air pressure outside the tube that causes the change in mercury levels, as explained in statement a.

Evaluate statement e

Statement e states that the vacuum that is formed at the top of the tube holds up the mercury. This statement is INCORRECT because the vacuum only indirectly affects the mercury level in the tube. The vacuum ensures that it is the air pressure outside the tube that impacts the mercury level, not air pressure inside the tube. In conclusion, the best explanation of how the barometer works is given by statement a: "Air pressure outside the tube causes the mercury to move in the tube until the air pressure inside and outside the tube is equal."