The image shows a pattern of iron particles arranged in a magnetic field. Iron shavings can be used to map out the magnetic field. Each pa...
The image shows a pattern of iron particles arranged in a magnetic field.
Iron shavings can be used to map out the magnetic field. Each particle of iron shavings is a small magnetic dipole, and can align into the local magnetic field. In this way, the tiny particles can form a global view of the magnetic field.
The magnetic field is an abstract entity that describes the influence of magnetic forces in a region. A visual tool is used to represent magnetic fields, it is the magnetic field lines. The magnetic field lines are the trajectories what a (hypothetical) north magnetic monopole would follow when placed in the magnetic field.
Why do iron particles placed in a magnetic field form a physical structure similar to the virtual lines of force used to visualize the magnetic field?
Obviously, it is the magnetic field that creates the structure of iron filings. Apparently, the pattern of the iron particles in the magnetic field draws out the magnetic lines of force. But the magnetic lines of force are an abstraction, a non-physical entity of reality that merely represents the magnetic force acting at a given point in space. Yet the magnetic force is present at every point in the magnetic field. The magnetic field is continuously and consistently changing between any two points in the field without any physically existing lines of force.
How do the scattered magnetizable iron particles and the continuous magnetic field interact with each other, resulting in a structure that resembles a well-ordered pattern of virtual magnetic lines of force in space? How do virtual lines of force become physical reality?
A more general question can be asked: under what conditions can a physical structure form in a force field?
- The (natural) criterion related to the force field: the force field must be continuous, i.e. the force field must be strictly smooth between different points in space and/or different moments in time. (The criterion means that there must be no breakpoints in the mathematical function of the force field. This condition is typically satisfied because force fields that occur in nature are usually characterized as smoothly continuous. The formation of a physical structure in the case of force field having breakpoint is trivial.) Note that physical structures can also form in a constant force field (not varying in space and time) if the mentioned conditions are satisfied, however, non-constant force fields are characteristic of nature.
- Condition on the material that creates the structure: the material that creates the physical structure needs to be present in the force field in a significant amount of particles of negligible size compared to the size of the structure that is forming.
- The (trivial) condition related to the interaction: the material that creates the structure must be able to interact with the surrounding force field.
- Random effect requirement: the particles forming the structure should be subjected to a random force of a magnitude comparable to the strength of their interaction with the force field.
If these conditions are met, physical structures are formed in the continuous force field.
The process of formation of magnetic lines of force as a physical structure:
A bar magnet creates a continuous, smoothly changing magnetic field around itself. Into this force field we place iron filings in a random arrangement. The iron particles are able to interact with the magnetic field and, consequently, with each other in the magnetic field. When the scattered iron filings are shaken slightly, the individual iron particles are subjected to randomly different forces of a magnitude comparable to their interaction with the magnetic field, due to their arrangement relative to each other and their arrangement in the magnetic field, which causes them to move relative to each other.
The changing arrangement causes some iron particles to move closer together and others to move further apart, thus interacting some more and some less strongly with each other and with the surrounding magnetic field. As the shaking continues, the alignment continues, with the shaking affecting the structures that interact more strongly with each other and the surrounding magnetic field less and less, but still affecting the others. The structures that interact with each other become more stable and continue to grow.
During the process, the characteristic shape of iron filings placed in a magnetic field is formed.
Further considerations: What determines the physical form of a structure? For example, the gravitational field creates planetary, stellar, and galaxy systems. How determined the shape that is formed by the interacting force field and the material that makes up the structure?
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