Question

In a simple barometer, some air is trapped in the space above the mercury column. Let H be the height of the mercury column initially. When the tube is further pushed into the vessel, what happens to the height of the mercury column ‘H’ in the tube?

• A. Increases
• B. Decreases
• C. Remains same
• D. Can’t say

Answer 1

The correct option is B

Decreases

The atmospheric pressure is given by the sum of the pressure of air column in the tube, $P_{air}$ and pressure due to the mercury column in the tube, $P_{H}g$.
$P_{atm}{P}_{air}+P_{Hg}$
As the tube is pressed down into the mercury vessel, the volume of trapped air decreases and thus air pressure inside tube increases. Let it be $P_{ai{r}^{\prime }}$.
$P_{ai{r}^{\prime }}>P_{air}$
The atmospheric pressure still is the same.
$P_{atm}=P_{air}+P_{Hg}=P_{ai{r}^{\prime }}+P_{H{g}^{\prime }}$
As $P_{ai{r}^{\prime }}>P_{ai{r}^{\prime }}{P}_{H{g}^{\prime }}<P_{Hg}$
So the height of mercury column in the tube will decrease.