Which statement correctly describes a condition for radiation to occur when a single wire carries current?

Which statement correctly describes a condition for radiation to occur when a single wire carries current?

• A. Radiation occurs if the current is oscillating
• B. Radiation occurs only if the wire is straight
• C. Radiation occurs only when the wire is at rest
• D. Radiation never occurs from a single wire

Answer: (A)

Radiation from a conductor happens when the current changes in time. When electrons accelerate back and forth in an alternating current, the magnetic field around the wire also changes with time. Maxwell’s equations tell us that a time-varying magnetic field (and the accompanying electric field) propagates away as electromagnetic waves. So an oscillating current produces radiation, while a steady direct current does not produce a far-field radiated wave in the idealized, long-wire case. This means the statement is correct because the essential condition for radiation here is the current’s time variation, not the wire’s shape or whether the wire is at rest. The other options aren’t correct because radiation isn’t dependent on the wire being straight, at rest, or on the claim that a single wire can’t radiate. If the current isn’t changing, there’s no radiated EM wave; if it is changing, radiation can occur even from a single wire, though its strength depends on length relative to the wavelength.

Radiation from a conductor happens when the current changes in time. When electrons accelerate back and forth in an alternating current, the magnetic field around the wire also changes with time. Maxwell’s equations tell us that a time-varying magnetic field (and the accompanying electric field) propagates away as electromagnetic waves. So an oscillating current produces radiation, while a steady direct current does not produce a far-field radiated wave in the idealized, long-wire case.

This means the statement is correct because the essential condition for radiation here is the current’s time variation, not the wire’s shape or whether the wire is at rest. The other options aren’t correct because radiation isn’t dependent on the wire being straight, at rest, or on the claim that a single wire can’t radiate. If the current isn’t changing, there’s no radiated EM wave; if it is changing, radiation can occur even from a single wire, though its strength depends on length relative to the wavelength.