Which of the following is a fundamental source of a magnetic field in classical electromagnetism?

• A. A region of uniform electric potential with no charge present
• B. A static gravitational field produced by a mass
• C. Stationary electric charges at rest
• D. Moving electric charges (electric current)

Answer: (D)

Verified step by step guidance

1. Recall that in classical electromagnetism, magnetic fields are generated by moving electric charges or changing electric fields, according to Maxwell's equations.
2. Understand that stationary electric charges produce electric fields but do not create magnetic fields because there is no motion of charge to generate a magnetic effect.
3. Recognize that a region of uniform electric potential with no charge present does not produce any electric or magnetic fields, as there is no source of field variation.
4. Note that a static gravitational field produced by a mass is unrelated to electromagnetic fields and does not generate a magnetic field.
5. Conclude that the fundamental source of a magnetic field is moving electric charges, commonly known as electric current, which creates magnetic fields around the path of the moving charges.