Sources of EM Waves: Accelerating Charges and Oscillating Charges
Physics · Electromagnetic Waves · NEET
Only an accelerating charge (which includes an oscillating charge) can produce electromagnetic waves. A stationary charge makes a fixed electric field, and a charge moving at constant velocity makes a steady magnetic field, but neither of these makes waves. An oscillating charge (like electrons in an antenna) radiates an EM wave of the SAME frequency as its oscillation. Memory hook: "No acceleration, no radiation."
A stationary charge gives only a static E field and a constant-velocity charge only a steady B field, so neither radiates. An oscillating (accelerating) charge produces time-varying E and B fields that sustain each other and travel out as an EM wave of the same frequency.
Your doubts, answered
Does a charge moving at constant velocity produce EM waves?
No. A charge moving at constant velocity is a steady current. It makes a steady (time-independent) magnetic field around it, but this field does not change with time, so no wave leaves the charge. For a wave, you need time-varying E and B fields that keep making each other. Only acceleration (a change in speed or direction) creates those changing fields. So constant-velocity charge = no EM wave.
Why does a stationary charge not radiate EM waves?
A stationary charge makes only a static (unchanging) electric field. There is no changing electric field, so by the Ampere-Maxwell law it produces no magnetic field, and by Faraday's law no new electric field. Nothing changes with time, so nothing propagates away. A wave needs oscillating, self-sustaining E and B fields, which a still charge simply cannot supply.
Is an oscillating charge the same as an accelerating charge?
Yes, an oscillating charge is one special example of an accelerating charge. When a charge oscillates back and forth (like in SHM), its velocity keeps changing direction, so it is always accelerating. That is why NCERT introduces the oscillating charge as the simplest case: it is a charge that is continuously accelerating, so it continuously radiates EM waves.
What frequency does the EM wave have if a charge oscillates with frequency v?
The EM wave has exactly the SAME frequency v as the oscillating charge. If electrons in an antenna oscillate at frequency v, the radiated electromagnetic wave also has frequency v. This is a favourite NEET one-liner: an electric charge oscillating with frequency v produces an EM wave of the same frequency v. An electric dipole is the basic source of EM waves.
Can a chargeless particle produce an electromagnetic wave?
No. EM waves come from the fields of a charge. A chargeless (neutral) particle has no electric field and no magnetic field to begin with, so even if it accelerates, there is nothing to oscillate and radiate. You need a charge AND acceleration together. Remove either one and there is no EM wave.
⚠️ The NEET trap ✗ A charge moving at constant velocity is moving, so it must give out EM waves. ✓ A constant-velocity charge is only a steady current: it makes a time-independent magnetic field, NOT a wave. Only acceleration (changing velocity) radiates EM waves. 🧠 Movement is not enough — the velocity must CHANGE. Constant speed AND constant direction = zero radiation.
Real NEET questions
NEET 2016
Out of the following options which one can be used to produce a propagating electromagnetic wave?
A · A charge moving at constant velocity
B · A stationary charge
C · A chargeless particle
D · An accelerating charge ✓
Solution: Step 1: A stationary charge gives only a static electric field — nothing changes with time, so no wave. Step 2: A charge moving at constant velocity is a steady current — it gives a steady magnetic field, but again no time-variation, so no wave. Step 3: A chargeless particle has no fields at all to radiate. Step 4: An accelerating charge makes time-varying E and B fields that sustain each other and travel out as a wave. Correct answer: D.
NEET 2024
The property which is NOT of an electromagnetic wave travelling in free space is that
A · The energy density in the electric field is equal to the energy density in the magnetic field
B · They travel with a speed equal to 1/sqrt(mu0 x epsilon0)
C · They originate from charges moving with uniform speed ✓
D · They are transverse in nature
Solution: Step 1: Check each option. u_E = u_B is TRUE for a free-space EM wave (equal energy in E and B). Step 2: Speed = 1/sqrt(mu0 x epsilon0) = c is TRUE. Step 3: EM waves are transverse — TRUE. Step 4: 'Originate from charges moving with uniform speed' is FALSE — a uniform-speed charge is a steady current and radiates nothing; EM waves come only from ACCELERATING charges. The question asks which is NOT a property, so the answer is C.
Solved Electromagnetic Waves NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the basic source of electromagnetic waves according to NCERT?
An accelerating charge. NCERT states that accelerated charges radiate electromagnetic waves, and the simplest example is an oscillating charge, which produces a wave of the same frequency. An electric dipole (oscillating charge pair) is called the basic source of EM waves.
Why can neither a stationary charge nor a uniformly moving charge make EM waves?
A stationary charge gives only a static electric field, and a uniformly moving charge (steady current) gives only a steady magnetic field. Both fields are constant in time, so there are no changing E and B fields to sustain a wave. Only acceleration gives the time-varying fields needed.
How is the frequency of the radiated EM wave related to the oscillating charge?
They are equal. A charge oscillating harmonically with frequency v radiates an EM wave of exactly the same frequency v. This is why an antenna oscillating electrons at a chosen frequency broadcasts a radio wave at that frequency.
How does this concept appear in NEET?
Almost every year as a one-line conceptual MCQ: 'which can produce an EM wave' (answer: accelerating charge) or 'which is NOT a property' (answer: originates from uniform-speed charges). It is a guaranteed easy mark if you remember: no acceleration, no radiation.
