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.
