Which one of the following is NOT a pressure unit?
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To avoid confusion in unit classification, always remember the dimensional formulas:
- Force (Newton) has dimensions \( [\text{M L T}^{-2}] \).
- Pressure (Pascal, Torr, atm) has dimensions \( [\text{M L}^{-1} \text{T}^{-2}] \).
Step 1: Understanding the Concept:
Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed.
The physical unit of pressure is derived from the formula:
\[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} \]
Therefore, any standard unit of pressure must represent a unit of force divided by a unit of area, or be an empirically established equivalent. Step 2: Detailed Explanation:
Let us analyze each of the options provided:
- Atmosphere (atm): This is a standard unit of pressure defined as being equal to \( 101,325 \text{ Pa} \) or \( 760 \text{ mmHg} \).
It represents the average atmospheric pressure at sea level. Thus, it is a valid unit of pressure.
- Pascal (Pa): This is the SI derived unit of pressure.
One Pascal is defined as one Newton per square meter:
\[ 1 \text{ Pa} = 1 \text{ N/m}^2 \]
Thus, it is a valid unit of pressure.
- Newton (N): This is the SI derived unit of force, named after Sir Isaac Newton.
One Newton is the force needed to accelerate a mass of one kilogram at a rate of one meter per second squared:
\[ 1 \text{ N} = 1 \text{ kg}\cdot\text{m/s}^2 \]
It is not a measure of force per unit area and is therefore not a unit of pressure.
- Torr: This is a non-SI unit of pressure defined as exactly \( 1/760 \) of a standard atmosphere.
It is historically and practically equivalent to the pressure exerted by a millimeter of mercury (\( \text{mmHg} \)).
Therefore, Newton is the only unit in the list that is not a pressure unit. Step 3: Final Answer:
The physical quantity that is not a pressure unit is Newton.
