Physics · Electromagnetic Waves · NEET
No. A charge moving at constant velocity is a steady current. It produces a magnetic field, but that field does not change with time, so there is no radiation. Only when the charge speeds up, slows down, or changes direction (that is, accelerates) do the E and B fields become time-varying and travel out as an EM wave. This is the exact NEET trap: constant velocity looks like 'movement', but movement alone is not enough.
A stationary charge makes only a static (unchanging) electric field around it. There is no magnetic field and nothing changes with time, so no wave leaves the charge. NCERT states this plainly: neither stationary charges nor charges in uniform motion can be sources of electromagnetic waves.
Yes. A charge oscillating back and forth (like in SHM) is constantly changing its velocity, so it is always accelerating. NCERT says an oscillating charge is an example of an accelerating charge, and it produces EM waves of the same frequency as its oscillation. This is why an electric dipole (charges oscillating up and down) is the basic source of EM waves.
When a charge accelerates, its electric field must change with time. A changing electric field creates a changing magnetic field (Ampere-Maxwell law), and that changing magnetic field creates a changing electric field again (Faraday's law). The two keep regenerating each other and detach from the charge, travelling outward at speed c. No acceleration means the fields never change, so this self-sustaining chain never starts.
The EM wave has the same frequency as the oscillation of the charge. If a charge oscillates at frequency f, it radiates an EM wave of frequency f. This is why radio waves are made by electrons oscillating in an antenna at the broadcast frequency.
Out of the following options which one can be used to produce a propagating electromagnetic wave?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An accelerating charge. The simplest example is an oscillating charge or an oscillating electric dipole, which radiates an EM wave of the same frequency as its oscillation.
No. A stationary charge gives only a static electric field, and a uniformly moving charge (steady current) gives only a steady magnetic field. Neither field changes with time, so neither radiates an EM wave.
Acceleration makes the fields change with time. A changing electric field creates a changing magnetic field and vice versa, so the two fields sustain each other and travel out as a wave. Without acceleration the fields are steady and no wave forms.
Yes. NEET 2016 asked directly which source produces a propagating EM wave, with 'accelerating charge' as the answer. The common trap option is 'charge moving at constant velocity', so this one line can win you a mark.
