Which statement about electromagnetic radiation and time-varying fields is correct?

Which statement about electromagnetic radiation and time-varying fields is correct?

• A. Varying fields with time result in EM radiation.
• B. Electric fields alone cause radiation.
• C. Static fields produce radiation.
• D. Radiation occurs only when the conductors are moving.

Answer: (A)

Electromagnetic radiation arises when fields change with time. Maxwell’s equations show that a changing electric field breeds a changing magnetic field, and a changing magnetic field, in turn, produces a changing electric field. This pair of time-varying fields can propagate through space as an electromagnetic wave, carrying energy away from the source. So the fundamental requirement for radiation is time variation of the fields, not just the presence of an electric field or a conductor moving at a constant speed. Static fields don’t radiate because there’s no time variation to sustain a propagating wave; the energy stays near the source. Likewise, simply moving conductors isn’t enough to guarantee radiation—what matters is that the charge distribution or current varies in time (such as an AC current or an accelerating charge). When those time variations occur, radiation is produced. This is why varying fields with time are the hallmark of EM radiation.

Electromagnetic radiation arises when fields change with time. Maxwell’s equations show that a changing electric field breeds a changing magnetic field, and a changing magnetic field, in turn, produces a changing electric field. This pair of time-varying fields can propagate through space as an electromagnetic wave, carrying energy away from the source. So the fundamental requirement for radiation is time variation of the fields, not just the presence of an electric field or a conductor moving at a constant speed.

Static fields don’t radiate because there’s no time variation to sustain a propagating wave; the energy stays near the source. Likewise, simply moving conductors isn’t enough to guarantee radiation—what matters is that the charge distribution or current varies in time (such as an AC current or an accelerating charge). When those time variations occur, radiation is produced. This is why varying fields with time are the hallmark of EM radiation.