How Do Shock Waves Form in Physics?
In Physics, a shock wave is also known as shock waves. It is a strong pressure wave in an elastic medium such as air, water, or any solid material ejected from explosions or lightning, or other phenomena that create variations in pressure.
It is a type of disturbance that propagates at a speed greater than the speed of sound in the medium.
As a rule, like ordinary waves, shock waves carry energy and can propagate through a medium.
Above all, we characterize a shock wave by a sudden, nearly discontinuous, change in pressure, temperature, and density of the medium.
On this page, we will understand the shock wave in detail.
What is Shock Wave?
When an aeroplane travels at a speed less than that of sound, in the first place, the air stays ahead of it.
In the meantime, the air begins to flow out of the way, before the plane approaches it. The pressure waves that aeroplane creates past the air, eventually end up being smooth and gradual.
However, when an aeroplane reaches the speed of sound and catches up to its own pressure waves, the air ahead of it receives no signal of the plane’s approach.
The aeroplane flows through the air, creating a strong pressure wave known as a shock wave.
In this case, when air flows through the shock wave, its pressure, density, and temperature increase sharply and quickly.
Shock Wave Example
A medium carries various characteristics with itself, such as stress, density, and temperature.
When a supersonic aircraft, lightning, or explosions expel a strong pressure wave or a shock wave in an elastic medium, an intense change in pressure occurs inside the medium.
Here, the strong wave pressure from a supersonic aircraft is a cone comprising spherical wavefronts.
Also, the term “supersonic,” itself says that it expels the waves having much more speed than the speed of sound in the air.
The below image describes the spherical wave fronts:
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Shock and Wave
Do you know how a shock wave differs from a sound wave? Well! Shock waves differ from sound waves in the following ways:
In a shock wave, compression takes place in a region of abrupt and violent change in stress, density, and temperature of a medium it propagates/travels.
As a result, shock waves propagate in a manner varying from that of ordinary acoustic waves.
Particularly, shock waves travel faster than sound, and their speed increases consequently with the increase in the amplitude.
However, the intensity of a shock wave also decreases more quickly than that of a sound wave.
Further, some of the energy of the shock wave is dissipated by heating the medium through which it travels.
The air explosion that creates a strong shock wave has a property known as amplitude. This property decreases conversely by the square of the distance until the wave becomes low to obey the laws of acoustic waves.
Shock waves can reshape the electrical, mechanical, and thermal properties of materials.
Thus, we can use shock waves to study the equation of states, such as the relation of pressure, temperature, and volume of any material.
Characteristics of Shock Waves
Comparing supersonic flows, we may achieve an increase in an expansion through an expansion fan. For instance, the known expansion fan is a Prandtl-Meyer expansion fan.
Coupled with, expansion wave may approach, collide and lastly recombine with the shock wave, creating a process of destructive interference.
The sonic boom associates with the passage of a supersonic aircraft, which is a type of sound wave produced as a result of constructive interference.
When a shock wave passes through matter, energy preserves, however, entropy increases. This change in the matter's properties, as a rule, manifests itself as a decrease in energy. However, we can extract work from this energy, also, as a drag force on supersonic objects.
Therefore, shock waves are strongly irreversible processes.
Additionally, we have a few types of shock waves, let’s understand these:
Types of Shock Waves
Concussive Wave
A concussive wave is a blast of energy given off by an exploding object.
Further, the explosion compresses the air immediately surrounding the site of detonation. As a result, the compressed air scatters in all directions away from the source - growing weaker the further, the wave travels.
Incidentally, a person close to the source of detonation can get affected by the blast wave.
Oblique Shockwave
For analyzing shock waves, we attach them to the body. After that, we find that the shock waves deviate at an arbitrary angle from the flow direction.
Meanwhile, we call this shockwave the oblique shock wave.
As a matter of fact, these shock waves require a component vector analysis of the flow.
However, doing so permits the flow treatment in a direction orthogonal to the oblique shock as a normal shock.
Mach Wave
A mach wave is a pressure wave in fluid dynamics. It travels with the speed of sound by a slight change of pressure added to a compressible flow.
FAQs on Shock Waves: Definition, Types, and Examples
1. What is a shock wave in physics?
A shock wave is a type of propagating disturbance that travels faster than the local speed of sound in a medium. It is characterised by an extremely abrupt, almost discontinuous change in the properties of the medium, such as pressure, temperature, and density. Unlike a normal sound wave, a shock wave carries a significant amount of energy and is considered a non-linear phenomenon.
2. What is the main cause for the formation of a shock wave?
A shock wave is formed when a source of sound or disturbance moves through a medium at a speed greater than the speed of sound, a condition known as supersonic speed. As the object moves, the sound waves it generates cannot propagate away from it quickly enough. Instead, they pile up and merge into a single, highly compressed wavefront, creating the shock wave. A common example is the shock wave produced by a supersonic jet.
3. How does a shock wave differ from a regular sound wave?
A shock wave is fundamentally different from a regular sound wave in several key aspects:
- Speed: Shock waves travel at supersonic speeds (faster than sound), while sound waves travel at the speed of sound.
- Pressure Change: A shock wave involves an abrupt, near-instantaneous jump in pressure, whereas a sound wave has a smooth, gradual change in pressure.
- Energy: Shock waves are high-energy phenomena that can cause physical effects, while sound waves carry much less energy.
- Waveform: A shock wave has a sharp, sawtooth-like waveform, unlike the smooth sine wave of a simple sound wave.
4. Why does a supersonic aircraft produce a 'sonic boom'?
A sonic boom is the sound an observer on the ground hears when a shock wave passes over them. A supersonic aircraft continuously generates a cone-shaped shock wave, known as the Mach cone, that trails behind it. The 'boom' is not a one-time event that occurs when the plane 'breaks the sound barrier'. Instead, it is the audible effect of the sudden and intense change in air pressure from the edge of this cone reaching an observer's ears. The sound is perceived as a loud crack or explosion because of this rapid pressure change.
5. How is a shock wave created during an explosion?
An explosion causes a near-instantaneous release of a massive amount of energy, rapidly heating the surrounding gas and causing it to expand outwards at extremely high speeds. This rapidly expanding front of gas acts like a piston, violently compressing the surrounding medium (like air) and pushing it away at supersonic speeds. This creates a powerful, propagating shock wave, often referred to as a blast wave, which is responsible for much of the damage caused by an explosion.
6. What are some examples of shock waves in astrophysics?
Shock waves are common in astrophysical environments. Key examples include:
- Supernovae: The explosion of a star sends a massive shock wave travelling through the interstellar medium at thousands of kilometres per second.
- Bow Shocks: These are formed when the solar wind, a stream of charged particles from the Sun, collides with the magnetic field of a planet like Earth.
- Galactic Collisions: When galaxies merge, the collision of their vast interstellar gas clouds can generate immense shock waves that trigger new star formation.
7. Is a blast wave the same as a shock wave?
While related, the terms are not perfectly interchangeable. A shock wave is the broader, more general term for any disturbance propagating faster than sound. A blast wave is a specific type of shock wave that is generated by a rapid, energetic event like an explosion. Therefore, every blast wave is a shock wave, but not every shock wave (like one from a supersonic aircraft) is a blast wave.
