In introductory electromagnetism, what is the physical source of an electromagnetic wave in free space?

• A. Gravitational attraction between two masses
• B. Accelerating electric charges (time-varying currents) that produce changing electric and magnetic fields
• C. A uniform, constant magnetic field
• D. Static electric charges that remain at rest

Answer: (B)

Verified step by step guidance

1. Understand that electromagnetic waves are generated by changes in electric and magnetic fields that propagate through space.
2. Recall that static electric charges or constant magnetic fields do not produce changing fields necessary for wave propagation.
3. Recognize that accelerating electric charges (or time-varying currents) create time-varying electric and magnetic fields, which are the source of electromagnetic waves.
4. Use Maxwell's equations to see how a changing electric field produces a magnetic field and vice versa, allowing the wave to sustain itself and travel through free space.
5. Conclude that the physical source of an electromagnetic wave in free space is accelerating electric charges that produce these changing fields.