Difference Between Stress and Pressure - A Complete Guide

In this article, we will explore stress, pressure, the difference between stress and pressure, and much more. These two terms, stress and pressure, are often used interchangeably, yet they have distinct definitions. Pressure is defined as the amount of force applied per unit area, while stress refers to the force exerted per unit area that a material experiences internally. Understanding the difference between stress and pressure will help clarify their fundamentals and their unique applications.

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This Story also Contains

1. What is the Definition of Stress?
2. What is Pressure?
3. What is the Difference Between Stress and Pressure?

What is the Definition of Stress?

Stress is a response within a material to restore its original shape and size when deformed. This force, known as the restoring force, acts in opposition to the deforming force. Therefore, stress is defined as the restoring force per unit area within the material. As a tensor quantity, it effectively describes stress and pressure differences in material behaviour.

Stress Formula

Stress is defined as a force per unit of area within a material caused by externally applied forces. It accurately describes and predicts flexible elastic, plastic, and fluid behaviour. Stress formula $(\sigma )$ is given by

$$\sigma =\frac{F}{A}$$

The units for stress depend on force and area measurements. The SI unit of stress is the pascal $(\mathrm{Pa})$, or ${\mathrm{Nm}}^{-2}$, which aligns with pressure in stress calculations.

Types of Stress

In physics, there are various types of stress, however, they are primarily divided into two types: normal stress and tangential or shearing stress. The next sections go over some of the different types of stress.

i) Normal Stress: Normal stress is the one in which the direction of the applied force is normal to the cross-sectional area of the body, stress is said to be normal stress. The stress will be normal as the length of the wire or the volume of the body changes. Normal stress is further divided into two types based on the force dimension. Longitudinal stress Bulk stress, also known as volumetric stress

ii) Longitudinal Stress: Consider the shape of a cylinder.

When two cross-sectional sections of the cylinder are exposed to equal and opposite forces, it undergoes stress. The stress is called a longitudinal stress. Deforming Force / Area of Cross-section = F/A = Longitudinal Stress

The Longitudinal Stress stretches or compresses the thing along its whole length. As a result, based on the direction of deforming force, it can be further categorised into two types:

Tensile stress

Compressive stress

a) Tensile Stress: Tensile stress is defined as stress that occurs when a deforming force or applied force causes an increase in the length of an object. For example, a rod or wire can be stretched by drawing it at both ends with equal and opposite forces (outwards).

b) Compressive Stress: When a deforming force or applied force causes an object's length to shrink, the resulting tension is known as compressive stress. When a rod or wire is compressed/squeezed by pulling it inwards with equal and opposite forces at both ends, for example.

iii) Bulk Stress or Volume Stress

When a deforming force or applied force acts on an object in all dimensions, causing a change in volume, this is referred to as volumetric stress or bulk stress. Volume stress occurs when the volume of a body changes as a result of a deforming force.

iv) Shearing Stress or Tangential Stress

When the applied force is parallel to the cross-sectional area of the object then the stress is called shearing stress or tangential stress. The body's shape changes as a result of this.

What is Pressure?

Pressure is the force exerted per unit area perpendicular to an object's surface, with the pascal (Pa) as its SI unit:

$$P=\frac{F}{A}$$
This formula illustrates that pressure rises with decreased area, explaining stress and pressure differences in applications.

where P denotes the pressure, F denotes the thrust force and A represents the area.

Pressure reduces when the area increases and pressure builds up when the area decreases. The greater the pressure on a surface, the smaller the area. In honour of Blaise Pascal, the S.I. unit of pressure is named the Pascal $(\mathrm{Pa})$. Therefore, $1\mathrm{Pa}=1{\mathrm{Nm}}^2$

1 pascal is described as the pressure exerted on a $1{\mathrm{m}}^2$ surface by a 1 N thrust acting on it.

Different Types Of Pressure

Pressure is classified as follows

• Absolute Pressure: It's a sort of pressure that uses a vacuum or an airless environment as a reference point. Because atmospheric pressure differences alter as the location changes, this sort of pressure varies as well. It eliminates the reference to variable ambient pressure and depends on a defined pressure range for reference by employing a device like an absolute pressure sensor.
• Gauge Pressure: The difference between absolute and atmospheric pressure is known as gauge pressure. Relative pressure is another name for gauge pressure. At sea level, the observed pressure is compared to the standard atmospheric pressure. The vent on pressure sensors used to measure gauge pressure allow the instrument to use ambient pressure as a reference point. Both positive and negative values can be assessed. Overpressure is the term for positive values. Under pressure or partial vacuum occurs when the gauge pressure measurement is negative.
• Differential Pressure: Differential pressure is the difference between two pressures and is a sort of gauge pressure. It aids in determining the pressure difference between two places. Flow and level measurement applications use differential pressure. The suffix "d" is used to represent differential pressure.
• Sealed Pressure: The vacuum is defined as a space with no absolute pressure. Perfect vacuum is difficult to obtain in practice and only exists as a theoretical value. A vacuum is generated when the pressure is lower than the atmospheric pressure. In practice, the vacuum will only be attained partially, which is referred to as a partial vacuum.

A high vacuum indicates a very low absolute pressure. The pressure is measured in a sealed chamber that is kept at atmospheric pressure. The sealed sensor is used to measure the sealed pressure and is made out of an airtight sensing element that prevents further pressure changes owing to changes in ambient conditions. Pressure transducers are protected from damage by sealed pressure. Now let's talk about stress vs pressure in which we are going to see the stress and pressure difference

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What is the Difference Between Stress and Pressure?

Here's the Difference Between Stress and Pressure

Understanding what is the difference between stress and pressure and their calculations allows us to distinguish stress vs pressure for practical and scientific applications.