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
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.