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 a barometer works is option (c): Air pressure outside the tube counterbalances the weight of the mercury in the tube. The other options are incorrect because (a) the air pressure inside the tube is effectively nil due to the vacuum at the top, (b) there is no air pressure inside the tube to balance out the external air pressure, (d) capillary action is not relevant to a barometer's functioning, and (e) the vacuum at the top of the tube does not hold up the mercury; rather, it's the external air pressure that causes the mercury to rise and balance itself.

Step-by-step solution

Understand the Barometer

A barometer is an instrument used to measure atmospheric pressure. A simple barometer consists of a glass tube (closed at one end and filled with mercury), which is inverted and placed in a container of mercury. The height of the mercury column inside the tube is directly affected by the surrounding atmospheric pressure. The higher the atmospheric pressure, the higher the mercury column inside the tube.

Examining the Options

Let's analyze each of the given options: 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.

Choose the Correct Answer

The best explanation for the functioning of the barometer is option (c). Air pressure outside the tube counterbalances the weight of the mercury in the tube, creating a stable interface at the height of the mercury column.

Explaining the Incorrect Options

Here are the reasons why the other options are incorrect: a. The air pressure inside the tube is effectively nil, as the top of the tube has a vacuum. So this statement is incorrect. b. There is no air pressure inside the tube, as a vacuum is formed at the top due to the absence of air in the tube. The air pressure outside is what causes the mercury to rise and balance itself. d. Capillary action is irrelevant in this context, as it refers to the tendency of a liquid to rise by surface tension when in contact with a solid. A barometer's working principle relies on the balance between atmospheric pressure and the weight of the mercury column rather than capillary action. e. While there is a vacuum at the top of the tube, it does not actually hold up the mercury. The external air pressure exerting force on the mercury in the container is what causes the mercury to rise in the tube, and this balances the weight of the mercury column.

Key concepts

Atmospheric Pressure

Understanding atmospheric pressure is fundamental when discussing barometers. Atmospheric pressure is the force exerted by the weight of the air in the atmosphere on a surface. This pressure affects everything on Earth, including how a barometer functions.

• The weight of the air above us presses down on the mercury in the open container of a barometer.
• This pressure pushes mercury up into the tube, creating a balance between the mercury column height and atmospheric pressure's weight.
• The higher the atmospheric pressure, the higher the mercury will rise in the tube.

So, when meteorologists refer to 'high' or 'low' pressure systems, they're describing changes in atmospheric pressure that can be observed with a barometer.

Mercury Column

The mercury column is a key component of a barometer, as it visually represents atmospheric pressure. A barometer often consists of a long, upright glass tube filled with mercury.
• Mercury is chosen for its significant density and non-evaporative nature under most conditions, allowing for a practical column height. • As atmospheric pressure changes, the mercury column adjusts accordingly: it rises with increased pressure and falls with decreased pressure. This direct relationship provides an easy and reliable indication of pressure changes.

Vacuum Formation

In a barometer, the vacuum formation occurs at the top of the mercury column. When the barometer tube is inverted into a mercury reservoir, some space at the tube's top becomes devoid of air, forming a vacuum.
• This vacuum is crucial because it ensures that the air pressure inside the tube is essentially zero, allowing external atmospheric pressure to be the only external force affecting the mercury.
• Without this vacuum, there would be additional air pressure counteracting the atmospheric force, disrupting the accurate measurement of atmospheric pressure. Thus, creating a vacuum in the barometer tube is vital for its correct functionality.

Capillary Action

Capillary action refers to the ability of a liquid to flow in narrow spaces without assistance, often against gravity. It is a concept related to the interaction between liquid and solid surfaces.

• In the case of mercury in a glass tube, capillary action is usually negligible because mercury is non-wetting, meaning it does not readily adhere to the glass surface.
• Hence, capillary action does not play a significant role in the functioning of a barometer.

Barometers depend on external air pressure, rather than capillary forces, to push mercury up the tube. Thus, mentioning capillary action in a barometer context is misleading.

Air Pressure Balance

The concept of air pressure balance in a barometer is fundamental to its operation. This balance occurs between the atmospheric pressure outside the tube and the weight of the mercury column inside.
• Atmospheric pressure pushes on the mercury in the reservoir, driving it up the tube until it reaches a point where its weight balances this external pressure.
• This equilibrium forms a stable height of the mercury column, which can be measured to determine atmospheric pressure. Understanding this balance helps to explain how barometers accurately gauge changes in weather conditions by noting shifts in atmospheric pressure.