Physics · Electromagnetic Waves · NEET
No, and this is the most common mix-up. Energy density u is energy stored per unit VOLUME (J/m³) — it is a snapshot of energy sitting in space. Intensity I is energy crossing per unit AREA per unit TIME (W/m² = J/m²/s) — it is energy flowing. They are linked by the wave speed: I = c·u_avg. Think of it as water in a pipe: density is how much water sits in each litre of pipe, flux (intensity) is how many litres cross a ring each second.
Imagine a beam hitting an area A. In one second, all the energy inside a cylinder of length c (the distance light travels in 1 s) and cross-section A passes through the surface. Volume of that cylinder = c × A, energy inside = u_avg × c × A. Energy per area per second = (u_avg × c × A) / A = c·u_avg. So the c just converts 'energy stored in a length' into 'energy delivered per second'.
Both work if you match them correctly. With PEAK value: I = (1/2) c ε0 E0². With RMS value: I = c ε0 Erms², because Erms² = E0²/2, so the (1/2) is already absorbed. The (1/2) in the peak version comes from averaging sin² over a cycle, which gives 1/2. NEET traps you by giving E0 and expecting the (1/2) — never forget it when you use the peak amplitude.
Yes — this is exactly what NEET 2020 tested. u_E = (1/2)ε0 E² and u_B = B²/(2µ0). Because E = cB and c = 1/√(µ0ε0), substituting gives u_E = u_B. So the electric and magnetic parts each contribute HALF the total energy, and their contributions to intensity are in the ratio 1 : 1.
Power P is total energy per second (watt) from the whole source. Intensity I is power spread over area, P/A (W/m²). For a point source radiating equally in all directions, I = P / (4πr²), so intensity falls off as 1/r² even though the total power stays the same. Use area geometry: flat beam → divide by beam area; point source → divide by 4πr².
The ratio of contributions made by the electric field and magnetic field components to the intensity of an electromagnetic wave is (c = speed of electromagnetic waves)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
I = c · u_avg = (1/2) c ε0 E0² = (1/2) c B0² / µ0, where E0 and B0 are peak amplitudes. In RMS form, I = c ε0 Erms². Unit is W/m².
Average energy flux is the average energy passing through unit area per unit time — it is exactly the intensity of the wave, measured in W/m². 'Flux' here means flow of energy across a surface.
About 1.4 kW/m² (the solar constant, roughly 1360–1400 W/m²). NEET problems use this value to ask you to work backward for E0 or B0 using I = (1/2)cε0E0².
For a point source radiating equally in all directions, I = P/(4πr²). Intensity follows the inverse-square law: double the distance, intensity drops to one-fourth.
Yes. Because E = cB, the electric energy density equals the magnetic energy density, so each contributes half. Their ratio is 1 : 1 (NEET 2020).
