# Question Video: Determining How a Barometer Changes with Altitude Physics

A barometer is placed on a high cliff. If the barometer is lowered and placed on the ground, the length of the Torricellian vacuum will ＿.

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### Video Transcript

A barometer is placed on a high cliff. If the barometer is lowered and placed on the ground, the length of the Torricellian vacuum will blank. Is it (A) increase, (B) decrease, (C) stay the same, or (D) be indeterminate?

This question is asking us how a barometer’s Torricellian vacuum will behave when the barometer is moved from a high altitude to a lower altitude. First, let’s remind ourselves of how a barometer works.

A barometer consists of a dish of mercury and a tube full of mercury. The tube of mercury 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 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. When the mercury drains out of the tube, it leaves this space up here completely empty. A volume that is completely empty like this is called a vacuum. Because this barometer was invented by someone called Torricelli, this is sometimes called the Torricellian vacuum.

To answer this question, we need to work out how the length of the Torricellian vacuum will change when the barometer is moved from a cliff top to a lower altitude. In other words, what will happen to the mercury in the tube?

Let’s think about how the atmospheric pressure at the top of a cliff might compare to atmospheric pressure at a lower altitude. Recall that atmospheric pressure is the pressure exerted on an object by all of the air in the air column above it. At high altitude, like the top of a cliff, atmospheric pressure is lower than at sea level. This is because when an object is high up, the air column above it is much shorter. Also, the air at high altitudes is less dense than that at lower altitudes. Both of these things mean there is less air above the object to exert any pressure. Hence, atmospheric pressure is lower at higher altitudes.

So, when the barometer moves from the top of the cliff to a lower altitude, it moves from a lower atmospheric pressure to a higher atmospheric pressure. In other words, the atmospheric pressure it experiences increases.

So how does this increase in atmospheric pressure affect the length of the vacuum? Remember that atmospheric pressure causes the mercury in the dish to be pushed up the tube. The greater the atmospheric pressure, the higher the level of mercury in the tube, and the shorter the length of the vacuum. So when the barometer is moved from the cliff top to lower ground, the atmospheric pressure it experiences increases. This causes the mercury in the barometer to be pushed up the tube, which decreases the length of the Torricellian vacuum.

If we look at the answer options, we see that this corresponds to option (B) decreases. If the barometer is lowered and placed on the ground, the length of the Torricellian vacuum will decrease.