Question

A pressure equivalent of 1mm is called a?
(A) bar
(B) torr
(C) pascal
(D) atm

Answer 1

Hint: Pressure can be defined as the normal force to the surface of an object per unit area over which the applied force is distributed.
This is denoted by P or p and mathematically pressure can be defined as-
\[P = \dfrac{F}{A}\] ………. (1)

Complete step by step answer:
Step 1: The amount of force exerted on a surface per unit area can be given by the ratio of force to the area over which the applied force is acting.
As from the equation, we can see that the unit of pressure is N/m2.
The SI (International System of units) unit of pressure is ‘Pa’ and it can be defined as given below-
$1 pa= 1N/m^2$
There are various units that are used to express pressure. Out of these various units, one is, the pound-force per square inch, the traditional units of pressure in imperial (another measurement system used).

Step 2: Out of all these various units of pressure is 1mm of Hg column. This is a manometric unit of pressure and previously it is defined as the extra pressure generated by a column of mercury of height $1 mm$.
$1 mm$ of $Hg$ is approximately $\text{1 torr}$.
And 1 bar = 1 standard atmospheric pressure= $105 N/m^2= 105 Pa$.
$1 millibar = 102 Pa$.

And, 1 standard atmospheric pressure = $760mm$ of $Hg$ column.
So, $1\, bar= 760\, torr$,
So, $1\, torr = 1mm$ of $Hg$ column = \[\dfrac{1}{{760}}\] bar (or standard atmospheric pressure).

$\therefore $ A pressure equivalent of 1mm is called a torr. Hence, the correct option is (B).

Note:
(i) $1 torr = 1mm$ of $Hg$ column. Here, $1 mm$ of $Hg$ is a manometric unit of pressure formerly defined as the extra pressure generated by a column of mercury one mm high.
(ii) Pressure is a scalar quantity as it has magnitude but no direction associated with it. For example, if the pressure on the wall of any container containing gas in it, is force per area and it is the same throughout the container. Even if we can shrink the container’s size to a very small point and then also the pressure has a single value at that point.