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ElectricalElectromagnetics Field Theory
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For static magnetic field

A

тИЗ├ЧB=╧Б\nabla \times \mathbf{B} = \rhoтИЗ├ЧB=╧Б

B

тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J

C

тИЗтЛЕB=╬╝0J\nabla \cdot \mathbf{B} = \mu_0 \mathbf{J}тИЗтЛЕB=╬╝0тАЛJ

D

тИЗ├ЧB=0\nabla \times \mathbf{B} = 0тИЗ├ЧB=0

Correct Answer

тЪЩя╕П TE тАв Technical Concept & PrincipleElectricalElectromagnetics Field Theory
Option B

тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J

Quick Summary:

For a static magnetic field, Maxwell's Ampere's Law in differential form is expressed as тИЗ├ЧH=J\nabla \times \mathbf{H} = \mathbf{J}тИЗ├ЧH=J, where H\mathbf{H}H is the magnetic field intensity and J\mathbf{J}J is the current density. Given B=╬╝H\mathbf{B} = \mu \mathbf{H}B=╬╝H, the relation becomes тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J, representing that steady currents are the source of static magnetic fields.

тЪЩя╕ПTETechnical SolutionConcept & Principle
ЁЯТб Explanation

For a static magnetic field, Maxwell's Ampere's Law in differential form is expressed as тИЗ├ЧH=J\nabla \times \mathbf{H} = \mathbf{J}тИЗ├ЧH=J, where H\mathbf{H}H is the magnetic field intensity and J\mathbf{J}J is the current density. Given B=╬╝H\mathbf{B} = \mu \mathbf{H}B=╬╝H, the relation becomes тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J, representing that steady currents are the source of static magnetic fields.

ЁЯФв Key Formulas

тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J тАФ Differential form of Ampere's Law for static fields

тИЗтЛЕB=0\nabla \cdot \mathbf{B} = 0тИЗтЛЕB=0 тАФ Gauss's Law for Magnetism

тЪЩя╕П Working Principle

Ampere's Circuital Law states that the circulation of the magnetic field intensity around a closed loop is equal to the current enclosed by that loop. In differential form, this implies the curl of the magnetic field is equal to the current density. In static conditions, the time-varying displacement current term (тИВDтИВt\frac{\partial \mathbf{D}}{\partial t}тИВtтИВDтАЛ) in the full Maxwell-Ampere law vanishes, leaving the static formulation.

ЁЯУМ Key Points
  • тЦ╕

    The static magnetic field is non-conservative because тИЗ├ЧBтЙа0\nabla \times \mathbf{B} \neq 0тИЗ├ЧBюАа=0.

  • тЦ╕

    Magnetic fields do not have isolated sources (monopoles), hence тИЗтЛЕB=0\nabla \cdot \mathbf{B} = 0тИЗтЛЕB=0.

  • тЦ╕

    The constant ╬╝\mu╬╝ represents the permeability of the medium.

тЬЕ Advantages
  • тЦ╕

    Simplifies calculation of magnetic fields for highly symmetric current distributions.

  • тЦ╕

    Provides a local relationship between current density and magnetic field.

тЭМ Disadvantages / Limitations
  • тЦ╕

    Not applicable to time-varying fields without the displacement current correction.

  • тЦ╕

    Assumes linear, homogeneous, and isotropic medium for simplified permeability.

ЁЯЫая╕П Applications / Uses
  • тЦ╕

    Calculating magnetic fields around long straight wires.

  • тЦ╕

    Designing inductors and transformers operating at low frequencies.

ЁЯУД Additional Information
  • тЦ╕

    Option A (тИЗ├ЧB=╧Б\nabla \times \mathbf{B} = \rhoтИЗ├ЧB=╧Б) is dimensionally incorrect as it equates curl of field to charge density.

  • тЦ╕

    Option D (тИЗ├ЧB=0\nabla \times \mathbf{B} = 0тИЗ├ЧB=0) only holds true in source-free regions (where J=0J=0J=0).

ЁЯУК Diagram / Illustration
Ampere's Law (Static)
тИЗ├ЧB\nabla \times \mathbf{B}тИЗ├ЧB
╬╝J\mu \mathbf{J}╬╝J
тЬЕ

B is correct тАФ The differential form of Ampere's Circuital Law for a static magnetic field is тИЗ├ЧB=╬╝J\nabla \times \mathbf{B} = \mu \mathbf{J}тИЗ├ЧB=╬╝J.

Core Concepts Used
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Ampere's Law Maxwell's Equations Magnetostatics
ЁЯТб EXAM TIP

Always remember that in time-varying fields, the right side of Ampere's Law gains a displacement current term, becoming тИЗ├ЧH=J+тИВDтИВt\nabla \times \mathbf{H} = \mathbf{J} + \frac{\partial \mathbf{D}}{\partial t}тИЗ├ЧH=J+тИВtтИВDтАЛ.

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