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Stress and Pressure are two terms that are often mistaken for one another in Physics. Pressure is defined as the amount of force exerted per unit area, whereas, on the other hand, stress is the amount of force exerted per unit area, experienced by a material. Pressure is a fundamental attribute that is most often associated with fluids. On a micro-scale, it is determined by the momentum transfer between the atoms of a liquid or gas volume (molecules bound within the volume). While stress results from a body's desire to rebound back to its reference state after random deformation, stress is generated whenever a force is applied to distort a body.
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Key Terms: Pressure, Stress, Force, Scalar Quantity, Vector Quantity, Momentum, SI Unit, Thermodynamics, Barometer, Pascal
What is Stress?
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Stress is defined as the internal force acting over a cross-sectional area within the material. Stress is only observed when an external force acts upon and tries to disturb the equilibrium position. Stress is developed within a body to resist an external force.
For instance, when spring is pulled, stress is developed in it, due to which it tries to retain its original position.
The stress formula is given as
\(\color{red}{\text{Stress} = \frac{\text{Resistive Force}}{\text{Area}}}\)
The SI Unit of the stress is Pascal (or N/m²) whereas the FPS unit is lb/ft².
Read More: Strain: Definition, Formula and Derivation
What is Pressure?
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Pressure is termed as the force applied per unit area (commonly used inside a fluid). In other words, it is the force that an object exerts on the area that it influences. It is equal and the same in all directions at a point inside the fluid.
For example, if a ball hits the wall, it exerts pressure on the wall.
The pressure formula is given as
\(\color{red}{\text{Pressure} = \frac{\text{Force}}{\text{Area}}}\)
The SI Unit of the pressure is Pascal (or N/m²) whereas the FPS unit is lb/ft².
Pressure
Read More: Fluid Pressure
Difference Between Stress and Pressure
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Though, stress and pressure are often confused with each other, there are some major differences between both concepts. Here are the main distinctions between stress and pressure:
| Pressure | Stress |
|---|---|
| The intensity of external forces acting at a spot is known as pressure. It is the amount of external force per unit area that is applied. The body is subjected to pressure. | Stress is defined as the amount of internal resistance force created at a specific point due to the application of force or pressure. The amount of internal force per unit area is being applied. Stress is created within the physical body. |
| A unique property of thermodynamics or physics. | A material property. |
| It always acts normal to the surface on which it acts. | Depending on the application of force, it might be in any direction and at any angle. |
| Always try to compress the surface you're applying it to. | Depending on the load type, it might be tensile or compressive. |
| Can be exerted on both liquids and gases. | Can only be exerted on solids. |
| Pressure gauges, barometers, manometers, and other pressure measuring devices or instruments are used to determine the pressure. | There is no technology that can be used to measure stress. These are just mathematical calculations. Strain or elongation is used to calculate stress. |
| It can be only positive. | It can be either a positive or a negative force. |
| It is independent of the area of the contact surface. | Varies with change in surface area. |
| Scalar quantity. | Vector quantity. |
Read More: Difference Between Scalar and Vector Quantities
Things to Remember
- Stress is defined as the amount of force exerted per unit area, experienced by a material whereas Pressure is the amount of force exerted per unit area.
- The main difference between stress and pressure is that the force in pressure is external, however, the force in stress is internal which stops it to change its shape or size.
- There is no instrument for measuring stress as it is not a measurable quantity. Stress is calculated using strain or elongation.
- Pressure is measured using pressure gauges, manometers, and other pressure measurement apparatus or instruments.
- The SI unit of the pressure and stress is Pascal (or N/m²) while the FPS unit is lb/ft².
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Sample Questions
Ques. What exactly is stress? What does this have to do with material engineering? (3 Marks)
Ans. Stress is the internal force exerted on a cross-sectional area within a material. Stress occurs only when an external force acts and attempts to alter the equilibrium posture. For example, when someone tugs on the spring, it develops tension and seeks to return to its original position. Stress is linked to material engineering, which is used for wires and sheets.
Ques. Determine which is scalar and vector; based on stress and pressure. (3 Marks)
Ans. The pressure is a scalar field variable that behaves normally to the surface and is unaffected by direction. The magnitude of the pressure at a given spot remains constant in all directions. Whereas, stress is a vector variable affected by the direction in which a force is applied. The quantity of tension varies depending on which direction it is used.
Ques. What happens to the body when we exert force on it? (3 Marks)
Ans. When we apply force to the body, it attempts to alter its initial shape by changing the interatomic distance. However, the body's natural propensity is to resist external forces to keep its original shape. As a result of this tendency, an opposing force develops inside the body. Stress is the phrase representing the opposing force created per unit.
Ques. Explain the relationship between pressure and vacuum. (3 Marks)
Ans. A very low absolute pressure is indicated by a high vacuum. The pressure is measured in a closed chamber at atmospheric pressure. The sealed sensor measures the sealed pressure and is constructed of an airtight sensing device that prevents further pressure changes due to environmental conditions. Sealed pressure protects pressure transducers from harm.
Ques. Identify the stress and pressure measuring equipment. (3 Marks)
Ans. Because stress is not a quantitative quantity, there is no tool to quantify it. Stress is calculated using strain or elongation. To physically measure the pressure, pressure gauges, manometers, and other pressure measurement devices or instruments are utilised.
Ques. How does stress fluctuate when the surface area changes? (2 Marks)
Ans. The amount of stress fluctuates as the surface area changes. In this case, the force is constant, but an increase in the area causes the stress to decrease, and vice versa. The magnitude of stress is proportional to the surface area on which it acts.
Ques. Determine the difference between hydrostatic and tangential or shearing stress. (2 Marks)
Ans. The comparable stress is called hydrostatic stress when a body is subjected to a uniform and equal force from all sides. The deforming force operating per unit area tangential to the surface is known as tangential or shearing stress.
Ques. Is the pressure in nature usually compressive? (2 Marks)
Ans. Pressure is always compressive in nature and always acts normal to a surface. As a result, all we need to know about it is its magnitude. As a result, the force acting is always normal to the surface, and the amount of the force depends on the area acting.
Ques. Explain the difference between hydrostatic and lithostatic pressure. (3 Marks)
Ans. The tension, or pressure, exerted equally in all directions at places within a confined fluid is known as hydrostatic pressure (liquid or gas). It is the only stress that fluid can experience at rest.
Lithostatic pressure is a pressure in the Earth's crust similar to hydrostatic pressure in fluids. The stress is put on a mass of rock by the surrounding rock. With increasing depth below the Earth's surface, lithostatic pressure rises.
Ques. With the use of an example, describe pressure. (3 Marks)
Ans. A finger can be pressed against a wall without leaving a permanent mark; conversely, the same finger pushing a thumbtack into the wall can readily harm the wall. Even though the force imparted to the surface is the same, the thumbtack exerts higher pressure due to the point's concentration of that force in a smaller area. At every location, pressure is transmitted to solid barriers or through arbitrary lengths of fluid that are parallel to these boundaries or sections. Pressure, unlike stress, is a scalar quantity and the force density is the negative gradient of pressure.
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