Pressure vs Stress - A Comparison

Pressure and stress

Pressure and stress are quantities that have few similarities and also a few differences between them. In this article, pressure and stress are compared.

Also, the similarities and differences are stated.

Similarities 

1. Pressure and stress have the same formulation. Both of them are given by the ratio of the applied force to area.

2. Pressure and stress have the same dimensions and units. The dimensions of pressure or stress are [M] / [L] [t]2 , where [M], [L] and [t] represent mass, length and time respectively. 

3. The fundamental SI unit of pressure or stress is kg/ms2. The derived unit is N/m2, where N stands for Newton.  Newton is the SI unit of force. The next level of the derived unit is Pascal (Pa). The other common units are psi (Pound per Square Inch) and dyn/cm2 (dyne per square centimeter).

The conversions between the units are,

. 1 Pa = 1 N/m2

. 1 Pa = 10 dyn/cm2

. 1 psi = 6.89 KPa (roughly 7 kPa)

Differences

The differences between pressure and stress are tabulated below:

Pressure	Stress
Pressure is the magnitude of an external force applied to a unit area.	Stress is the internal resistance of the object to the force applied to a unit area.
Pressure is a scalar quantity. It has only magnitude.	Stress is a tensor quantity. Tensor has magnitude, direction and also a plane with respect to which stress is defined.
Pressure is equal at all points and in all directions.	Stress is not equal at all points. It varies with angle and direction.
If a surface is subjected to pressure, it always acts perpendicular to the surface. Pressure can be considered as a special case of stress.	The force creating stress on a surface can act in any angle and direction. Normal stress is the component perpendicular to the surface plane and shear stress is the component along the surface plane.
Pressure is always compressive.	Stress can be either compressive or tensile. Compression is the positive normal stress and tension is the negative normal stress.
Pressure is used for fluids (liquids and gases).	Stress is typically used for solids.
Pressure is commonly specified in N/m2.	Stress is commonly expressed in MPa / GPa.
Pressure can be measured by various measuring devices like manometers, bourdon tubes, diaphragms, etc.,	Stress has no measuring device but can be calculated.

Read about the vacuum pressure measurement device - McLeod Gauge