How Changing Electric and Magnetic Fields Produce EM Waves
Physics · Electromagnetic Waves · NEET
A changing electric field creates a magnetic field (Ampere-Maxwell law) and a changing magnetic field creates an electric field (Faraday's law). Each field keeps producing the other, so once started by an accelerating charge, the two fields feed each other and travel forward together as a self-sustaining electromagnetic wave. Memory hook: "E feeds B, B feeds E" — like two hands drawing each other, the wave never needs a medium to keep going.
In a plane EM wave the electric field E (red) and magnetic field B (blue, dashed) oscillate in phase and perpendicular to each other. A changing E creates B and a changing B creates E, so the two fields sustain each other and move together along x.
Your doubts, answered
Does a changing electric field really produce a magnetic field?
Yes. This was Maxwell's key idea. A time-varying electric field acts like a current (called displacement current) and produces a magnetic field around it, given by the Ampere-Maxwell law. NCERT shows this inside a charging capacitor: there is no moving charge in the gap, yet a magnetic field appears because the electric field there is changing with time.
Why does an EM wave keep travelling even after the source stops?
Because the two fields sustain each other. A changing E field at one point creates a B field just ahead of it; that B field is also changing in time, so it creates an E field further ahead; that new E field creates a new B field, and so on. This continuous hand-off carries the disturbance forward on its own, so the wave is self-sustaining and does not need a wire or medium.
Which law gives changing-B to E, and which gives changing-E to B?
Faraday's law of induction says a changing magnetic flux produces an electric field: oint E.dl = -dΦB/dt. The Ampere-Maxwell law says a changing electric flux (plus any conduction current) produces a magnetic field: oint B.dl = µ0 ic + µ0 ε0 (dΦE/dt). Together these two laws are the engine of the wave.
Why must the charge accelerate to start an EM wave?
A stationary charge gives only a static electric field (nothing changes with time). A charge moving at constant velocity gives a steady current and a steady magnetic field (still no change in time). Only an accelerating charge produces fields that change with time, and only time-varying fields can feed each other. So acceleration is what launches the wave.
Do EM waves need a medium like sound waves do?
No. Sound needs air or another material to vibrate. EM waves are made of the fields E and B themselves feeding each other, so they travel through vacuum. That is why sunlight reaches Earth across empty space at speed c = 1/sqrt(µ0 ε0) = 3 x 10^8 m/s.
⚠️ The NEET trap ✗ A charge moving with constant velocity radiates an EM wave because moving charges make magnetic fields. ✓ Only an ACCELERATING charge radiates. Constant-velocity charge gives a steady (time-unchanging) magnetic field, so no wave is launched. 🧠 Ask: is anything changing with TIME? No change in time means no wave. Only acceleration makes E and B change in time so they can feed each other.
Real NEET questions
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 a static electric field only — nothing changes in time, so no wave. Step 2: A charge moving at constant velocity gives a steady current and a steady (time-unchanging) magnetic field — still no wave. Step 3: A chargeless particle produces no E or B field at all. Step 4: An accelerating charge produces electric and magnetic fields that change with time. These time-varying fields feed each other (changing E makes B, changing B makes E) and propagate outward. Answer: (D) An accelerating charge.
2025
A parallel plate capacitor made of circular plates is being charged such that the surface charge density on its plates is increasing at a constant rate with time. The magnetic field arising due to the displacement current is
A · Non-zero everywhere with maximum at the imaginary cylindrical surface connecting the peripheries of the plates ✓
B · Zero between the plates and non-zero outside
C · Zero at all places
D · Constant between the plates and zero outside the plates
Solution: Step 1: Charge is increasing, so the electric field between the plates is changing with time. A changing E field acts as a displacement current id = ε0(dΦE/dt). Step 2: This displacement current produces a magnetic field circling the axis, exactly like a real current would (Ampere-Maxwell law). Step 3: Inside the plate region (r < R) only the flux through radius r matters, giving B ∝ r (B grows with r). Step 4: Outside (r > R) the full changing flux is enclosed, giving B ∝ 1/r (B falls with r). Step 5: So B is non-zero both inside and outside and is maximum at the rim r = R (the cylindrical surface joining the plate edges). Answer: (A). This directly shows a CHANGING electric field producing a magnetic field.
Solved Electromagnetic Waves NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An accelerating charge makes time-varying E and B fields; a changing E makes B and a changing B makes E, so they feed each other and travel forward as a self-sustaining wave.
What is the role of displacement current here?
Displacement current is Maxwell's way of saying a changing electric field behaves like a current and produces a magnetic field. It is the missing link that lets E create B, completing the feedback loop needed for the wave.
Are E and B in phase in the wave?
Yes. In a plane EM wave in vacuum, E and B reach their maxima and zeros at the same time and place. They are perpendicular to each other and to the direction of travel, with E0/B0 = c.
Why is the speed exactly c = 1/sqrt(µ0 ε0)?
The two field laws contain the constants µ0 and ε0. When you combine Faraday's law and the Ampere-Maxwell law into a wave equation, the wave speed comes out as 1/sqrt(µ0 ε0) = 3 x 10^8 m/s, which equals the measured speed of light — proving light is an EM wave.
