# Question Video: Determining Pressure from a Barometer Physics

When the atmospheric pressure decreases, the distance ＿ increases in the barometer shown in the diagram.

03:50

### Video Transcript

When the atmospheric pressure decreases, the distance blank increases in the barometer shown in the diagram. Is it (A) 𝐷𝐸, (B) 𝐵𝐷, (C) 𝐴𝐵, or (D) 𝐴𝐸?

In this question, we’re given a diagram of a barometer that consists of a mercury column. We’re asked to work out how a decrease in atmospheric pressure will affect the barometer.

First, let’s remind ourselves of how a barometer works. A barometer consists of a dish of mercury, as well as a tube full of mercury that is turned upside down and placed in the dish. When the tube is placed in the dish like this, some of the mercury drains out of the tube and into the dish. However, not all of the mercury in the tube drains out. This is because of atmospheric pressure.

Atmospheric pressure exerts a force on the mercury in the dish, which pushes it up the tube. The greater the atmospheric pressure, the smaller the amount of mercury that drains from the tube. By measuring the height of the mercury that is left in the tube, the value of atmospheric pressure can be deduced.

Now that we know how a barometer works, let’s think about the question that we have been asked. We need to work out which of these labeled distances increases when atmospheric pressure decreases. The distance 𝐷𝐸 corresponds to the distance between the bottom of the tube and the bottom of the dish of mercury. This distance is not affected by changes in atmospheric pressure, so option (A) is incorrect.

Similarly, the distance 𝐴𝐸 corresponds to the distance between the top of the tube and the bottom of the dish of mercury. This distance is also not affected by changes in atmospheric pressure, so option (D) is incorrect.

Now let’s think about how atmospheric pressure acts on this system. We have already said that atmospheric pressure pushes down on the mercury in the dish and stops all the mercury in the tube from draining away. But if atmospheric pressure decreases, this effect becomes smaller and more mercury will drain out of the tube. This means that when atmospheric pressure decreases, the level of mercury in the tube also decreases. The distance 𝐵𝐷 corresponds to the distance between the level of the mercury and the bottom of the tube. We have shown that the level of mercury in the tube decreases when the atmospheric pressure decreases. This means that the distance 𝐵𝐷 will decrease when the atmospheric pressure decreases, so option (B) is incorrect.

However, when the level of the mercury in the tube decreases, the distance between the top of the tube and the top of the mercury in the tube increases. This corresponds to the distance 𝐴𝐵 on the diagram. So, the correct answer to this question is option (C). When the atmospheric pressure decreases, the distance 𝐴𝐵 increases in the barometer shown in the diagram.