Difference Between Stress and Pressure

Dhristi JEE 2022-24

Difference Between Stress and Pressure in Physics

Pressure is something that you observe in Pressure cookers, exam pressure, work pressure, and so on; however, if you talk about pressure in Physics, it makes a difference but how?


Well, in Physics, we define pressure as the physical force exerted on the object.


Also, we can define it as the perpendicular force applied per unit area of the object. And, when we say, a perpendicular force, we mean to say, it is stress.


In this article, we will start with the pressure and stress difference following the explanation.


Note: One thing to note is when pressure and stress are caused by applying a force, then how we can differentiate between stress and pressure? Quite confusing, right?


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We will proceed with the major difference in pressure and stress, following the real-life examples, and further explain these two in detail.

 

Difference Between Stress and Pressure

Stress

Pressure

Stress applies at the internal level.

For example, when tensile stress is applied to a block of mass ‘m’, the interatomic force of attraction tries to bring these molecules to their lattice points, so the restoring force acting inside is the stress.

The pressure is applied externally to the surface.

For example, pressure on the ball or pressure on the surface of the fluid.

Stress acts on the solids only.

Pressure occurs on liquids, fluids, and gases

Stress is of two types viz: normal and shear

Normal stress is when you apply a force perpendicularly to the block of mass ‘m’.

Shear stress is a force applied parallel to the block.

Pressure occurs normally on the surface. For example, the normal pressure difference in the airfoil creates a lift.

Stress is a vector quantity. It means stress has both magnitude and direction.

However, one more term adds to this physical quantity and that is the point of application.

Stress is called the second-degree tensor.

The pressure is a scalar quantity. 

It means we are not sure if the pressure acts along x, y, or z-axis (direction), we only know the magnitude of the pressure applied in bar or atm units.

Though the unit for pressure and stress is the same, i.e., Nm-2; however, the difference between stress and pressure can be explained by the formulas mentioned below:

In technical or mathematical terms, the formula for stress is:

Internal resistance = force/area

In technical terms, the formula for pressure is:

External force / area

Stress is the intensity of internal resisting forces developing at a point in the plane of the object. 

And, the point at which stress acts is the point of application.

In simple terms, pressure is the intensity of external forces acting at a point.

Stress is an immeasurable quantity. 

We can measure the pressure by the measuring device.


Differentiate Pressure from Stress

Pressure

Let’s suppose that you have a vessel filled with water and keep it on the fire and cover the vessel with a lid. Now as the temperature rises, the lid starts vibrating. You might have observed this happening while preparing a steamed food item.


The science behind it is, at the microscopic level, water has millions of molecules bonded together with the interatomic force of attraction, as the temperature rises, the bond between the molecules break and they start colliding with each other and at the surface of the vessel, which, in turn, creates pressure at the walls of the vessel.


Even if you take the case of a U-shaped jar, fill it with gas, and attach a piston to its top. Now, as you push the piston inwards, the gas molecules expand because of the collision between them.


In Bernoulli’s principle, we learned that the velocity difference between the upper and the lower stream of the fluid creates a pressure difference, and this principle is useful in generating an airlift.


According to these three examples, we understand that pressure occurs in three things viz: liquid, gas, and fluids.


The different types of pressure are absolute pressure, atmospheric pressure, differential pressure, overpressure, or gauge pressure.


Stress

If we talk about stress, it is a force applied perpendicular to the area, so it is clearly related to the matter.


Let’s suppose that there is a block of mass ‘m’ placed horizontally. Now, if you apply a force, stress acts at a point of the plane. Since stress acts at a point on the plane of the block, that’s why we consider stress as the force acting per unit area of the plane.


Since stress has three parameters to define it as the vector quantity. These are the magnitude, direction, and point of application. We also called stress the ‘Second-order tensor’.


Task to Do

So, the difference between pressure and stress is easy-to-comprehend when we consider the real-life examples, as we discussed above. So, try to think of a few more real-life examples to understand the pressure stress difference.

FAQs on Difference Between Stress and Pressure

1. What type of quantity is pressure? List the different types of units of pressure.

Pressure is a scalar quantity because we are not sure of the direction the pressure is applied to, on the surface; however, we can define its magnitude or value in different units. Five types of units of pressure are used commonly. These are:


mmHg - We use this unit to measure blood pressure.


Psi - Pound per square inch.


Pascal (Pa), KiloPascal (kPa), MegaPascal (mPa)


Atmospheric (atm)


Bar

2. Mention the different types of Pressure.

The different types of pressure are as follows:


Absolute Pressure


This type of pressure takes reference from vacuum or air-free space. It changes when there is a change in location, as the atmospheric pressure changes along with it. Using an absolute pressure sensor eliminates the possibility of varying atmospheric pressure. This helps rely on a specific reference range for pressure.


Atmospheric pressure/Sealed pressure


In a vacuum, the absolute pressure is zero. Vacuum is created when the pressure is lower than the atmospheric pressure. This is not practically achievable and is only valuable theoretically. Practically, the vacuum will only be achieved partially and is known as the partial vacuum. 


In this case, the pressure is measured with reference to a sealed chamber closed with atmospheric pressure. The sealed sensor (with a sensing element that is sealed to make it air-tight) is used to measure the sealed pressure. Sealed pressure is used to avoid damage to pressure transducers.


Gauge Pressure


Gauge pressure, also known as relative pressure, is the difference between absolute pressure and atmospheric pressure at sea level.


Sensors used to measure the gauge pressure have a vent that lets it use the atmospheric pressure for reference. The measured value can be either positive or negative. While the positive values are called overpressure, the negative values are called under pressure.


Differential Pressure


Differential pressure is a type of gauge pressure that measures the pressure difference between two different points.

3. What are the different types of Stress?

The different types of stress are:

Normal Stress: Normal stress is that in which a body acts perpendicular to the area.


Tensile Stress: The stress-induced in an object when it is subjected to two equal and opposite pulls. Tensile stress causes an increase in the length of the object and a decrease in the cross-section area. It is a type of normal stress so it acts at 90 degrees.


Shearing or Tangential Stress: Shear stress is induced in an object when it is subjected to two equal and opposite forces, acting tangentially to the area It is the ratio of shear resistance to the shear area. It causes a change in the shape of the body. 


Longitudinal Stress: Longitudinal stress is stress that is produced when a body is subjected to internal pressure. The longitudinal stress acts in the direction of the pipe, which means it is parallel to the longitudinal axis.


Bulk Stress or Volumetric Stress: Bulk stress causes a change in the volume of the body. An example of this could be a solid sphere placed in a fluid under high pressure. 


Compressive Stress: When an object receives two equal and opposite pushes it  is known as compressive stress. This kind of stress decreases the length of the body and increases its cross-section area. Compressive stress acts at a 90-degree angle because it is also a type of normal stress. 


The types of stress mentioned above affect the object (strain) in different ways when an internal resisting force acts on it.

4. What is meant by pressure exerted by a liquid?

The pressure exerted by the liquid is called the hydrostatic pressure The hydrostatic pressure is the pressure exerted by a fluid (gas/liquid) at any point in space within that fluid, assuming that the fluid is at rest.

5. What are the examples of different types of stress in everyday life?

Here are a few everyday examples of the different types of stress, i.e. normal, tensile, shear, compressive and longitudinal stress. 


Tensile Stress: When a rod, wire or elastic material like rubber is stretched by pulling it with equal and opposite forces at both ends, it is an example of tensile stress.


Compressive Stress: When a rod or wire is compressed by pushing it inward with equal and opposite forces at both ends.


Shear Stress: Some of the many examples of shear stress are cutting vegetables, writing on blackboard, painting, applying creams/soaps etc, chewing food, walking or running. Other examples include when water flows river beds experience shear stress. 


Bulk Stress: When a diver goes deep into the ocean, the water exerts an almost uniform pressure from all sides on the body or the suit. This will squeeze the diver to a slightly smaller volume. This kind of uniform pressure put on the diver from all sides is called bulk stress or volume stress.

6. List the types of pressure.

Various types of pressure are as follows:

  • Absolute pressure

  • Atmospheric pressure

  • Differential pressure

  • Overpressure / Gauge pressure

7. List the types of stress.

The list of the types of stress are:

  • Tensile stress

  • Normal stress

  • Shearing or Tangential stress

  • Longitudinal stress

  • Bulk stress or Volumetric stress

  • Compressive stress

The types of stress mentioned above affect the object (strain) in different ways when an internal resisting force acts on it.

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