The Limitations of the Finite Wire Formula

Oct 13, 2025 by wbrum.org 107 views

Why do formulas for current-carrying finite wires not apply to tiny conductors like infinite conductors?

In the realm of electromagnetism, the behavior of electric currents and magnetic fields is governed by various mathematical formulas. One such formula, the magnetic field (Fm) produced by a current-carrying finite wire, is given by:

Fm = (μ * 2I) (Sin(φ1) + Sin(φ2)) / 4πR

where μ is the magnetic permeability, I is the current, φ1 and φ2 are the angles between the wire and the observation point, and R is the distance from the wire to the observation point.

However, when it comes to tiny conductors, this formula does not seem to apply. In fact, the behavior of magnetic fields around tiny conductors is more similar to that of infinite conductors. But why is this the case?

The Limitations of the Finite Wire Formula

The formula for the magnetic field produced by a finite wire is derived by integrating the contributions from infinitesimal elements of the wire. This integration process assumes that the wire is a continuous, finite length, and that the observation point is at a distance R from the wire.

However, when the wire is very small, the assumption of continuity breaks down. The wire can no longer be treated as a continuous object, and the integration process becomes invalid. In this regime, the behavior of the magnetic field is more similar to that of an infinite conductor.

The Infinite Conductor Limit

An infinite conductor is a theoretical object that has no ends or boundaries. It is an idealized model that is used to simplify the analysis of electromagnetic problems. In the case of an infinite conductor, the magnetic field is given by:

Fm = (μ * 2I) / 4πR

This formula is similar to the one for the finite wire, but without the Sin(φ1) + Sin(φ2) term. This term is responsible for the asymmetry in the magnetic field produced by a finite wire.

The Limit of Small Angles

When the angles φ1 and φ2 are small, the Sin(φ1) + Sin(φ2) term can be approximated as:

Sin(φ1) + Sin(φ2) ≈ φ1 + φ2

This approximation is valid when the angles are small, i.e., φ1, φ2 << 1. In this regime, the magnetic field produced by a finite wire is approximately the same as that produced by an infinite conductor.

The Tiny Conductor Regime

When the wire is very small, the angles φ1 and φ2 tend to zero. In this regime, the Sin(φ1) + Sin(φ2) term becomes negligible, and the magnetic field produced by the wire is approximately the same as that produced by an infinite conductor.

In conclusion, the formulas for current-carrying finite wires do not apply to tiny conductors like infinite conductors because the assumptions of continuity and finite length break down in this regime. The behavior of magnetic fields around tiny conductors is more similar to that of infinite conductors, and the Sin(φ1) + Sin(φ2) term becomes negligible.

[1] Jackson, J. D. (1999). Classical Electrodynamics. John Wiley & Sons.
[2] Griffiths, D. J. (2013). Introduction to Electrodynamics. Pearson Education.
[3] Landau, L. D., & Lifshitz, E. M. (1971). The Classical Theory of Fields. Pergamon Press.

The formula for the magnetic field produced by a finite wire is given by:

Fm = (μ * 2I) (Sin(φ1) + Sin(φ2)) / 4πR

where μ is the magnetic permeability, I is the current, φ1 and φ2 are the angles between the wire and the observation point, and R is the distance from the wire to the observation point.

When the angles φ1 and φ2 are small, the Sin(φ1) + Sin(φ2) term can be approximated as:

Sin(φ1) + Sin(φ2) ≈ φ1 + φ2

This approximation is valid when the angles are small, i.e., φ1, φ2 << 1.

When the wire is very small, the angles φ1 and φ2 tend to zero. In this regime, the Sin(φ1) + Sin(φ2) term becomes negligible, and the magnetic field produced by the wire is approximately the same as that produced by an infinite conductor.
Q&A: Why do formulas for current-carrying finite wires not apply to tiny conductors like infinite conductors?

Q: What is the main difference between the magnetic field produced by a finite wire and an infinite conductor? A: The main difference is the presence of the Sin(φ1) + Sin(φ2) term in the formula for the finite wire. This term is responsible for the asymmetry in the magnetic field produced by a finite wire, whereas the magnetic field produced by an infinite conductor is symmetrical.

Q: Why does the Sin(φ1) + Sin(φ2) term become negligible for tiny conductors? A: When the wire is very small, the angles φ1 and φ2 tend to zero. In this regime, the Sin(φ1) + Sin(φ2) term becomes negligible, and the magnetic field produced by the wire is approximately the same as that produced by an infinite conductor.

Q: What is the significance of the approximation Sin(φ1) + Sin(φ2) ≈ φ1 + φ2 for small angles? A: This approximation is valid when the angles are small, i.e., φ1, φ2 << 1. It allows us to simplify the formula for the magnetic field produced by a finite wire and make it more similar to the formula for an infinite conductor.

Q: Can you provide an example of a situation where the formula for the finite wire does not apply? A: Yes, consider a wire with a diameter of 1 μm carrying a current of 1 A. In this case, the wire is very small, and the angles φ1 and φ2 tend to zero. The formula for the finite wire does not apply in this regime, and the magnetic field produced by the wire is approximately the same as that produced by an infinite conductor.

Q: How does the behavior of magnetic fields around tiny conductors compare to that of infinite conductors? A: The behavior of magnetic fields around tiny conductors is more similar to that of infinite conductors. The magnetic field produced by a tiny conductor is symmetrical and does not exhibit the asymmetry seen in the magnetic field produced by a finite wire.

Q: What are the implications of this result for the design of electromagnetic devices? A: This result has significant implications for the design of electromagnetic devices, such as coils, transformers, and inductors. The behavior of magnetic fields around tiny conductors must be taken into account when designing these devices to ensure optimal performance.

Q: Can you provide a mathematical derivation of the formula for the magnetic field produced by a finite wire? A: Yes, the formula for the magnetic field produced by a finite wire can be derived by integrating the contributions from infinitesimal elements of the wire. This integration process assumes that the wire is a continuous, finite length, and that the observation point is at a distance R from the wire.

Q: What are some common applications of the formula for the magnetic field produced by a finite wire? A: The formula for the magnetic field produced by a finite wire has numerous applications in fields such as electromagnetism, electrical engineering, and materials science. Some common applications include the design of electromagnetic devices, the study of magnetic fields in materials, and the analysis of electromagnetic radiation.

Q: Can you provide a comparison of the magnetic field produced by a finite wire and an infinite conductor? A: Yes, the magnetic field produced by a finite wire is asymmetrical and exhibits a dependence on the angles φ1 and φ2, whereas the magnetic field produced by an infinite conductor is symmetrical and does not exhibit this dependence.