Physics · Electromagnetic Waves · NEET
You DIVIDE. The rule is E0 = c B0, so B0 = E0 / c. Because c = 3 x 10^8 is a huge number, dividing makes B0 very small. If you accidentally multiply (B0 = E0 x c) you get a giant wrong number like 10^10 — a clear sign you used the wrong operation. Quick check: B0 must always come out tiny (around 10^-7 or 10^-8 T).
Yes, just rearrange it. From E0 = c B0, if B0 is given you MULTIPLY: E0 = c x B0. Example: B0 = 2 x 10^-7 T gives E0 = 3 x 10^8 x 2 x 10^-7 = 60 V/m. So going E0 to B0 you divide by c, and going B0 to E0 you multiply by c.
No. B0 = E0 / c needs only E0 and c. Questions often give frequency (like 2 x 10^10 Hz) as extra bait to confuse you. Ignore it for finding B0. Frequency only matters if the question also asks for wavelength (lambda = c/f) or the full wave equation.
It holds for ALL of them because E = cB at every instant. So E0/B0 = c (peaks), and E_rms/B_rms = c (rms values) too. Just keep both sides the same type. If a question gives E_rms and asks for peak B0, first do B_rms = E_rms/c, then B0 = sqrt(2) x B_rms.
Because they differ by the factor c = 3 x 10^8. B0 = E0/c, so B0 is about 300 million times smaller in number than E0. This does NOT mean the magnetic field is weaker in energy — in an EM wave the electric and magnetic parts carry EQUAL energy. The number is just small because of the units.
In a plane electromagnetic wave travelling in free space, the electric field component oscillates sinusoidally at a frequency of 2.0 x 10^10 Hz and amplitude 48 V/m. Then the amplitude of the oscillating magnetic field is (speed of light in free space = 3 x 10^8 m/s)
The electric field in a plane electromagnetic wave is given by E_z = 60 cos(5x + 1.5 x 10^9 t) V/m. Then the expression for the corresponding magnetic field is (subscripts denote field direction)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
B0 = E0 / c, where c = 3 x 10^8 m/s is the speed of light. This comes from the relation E0 = c B0, which holds for every plane EM wave in free space.
Very small — usually around 10^-7 to 10^-8 tesla. For example, E0 = 48 V/m gives B0 = 1.6 x 10^-7 T. The small size comes from dividing by c = 3 x 10^8.
Multiply by c: E0 = c x B0. For example, B0 = 2 x 10^-7 T gives E0 = 3 x 10^8 x 2 x 10^-7 = 60 V/m.
Yes. In free space E and B oscillate in phase — they reach their maximum and zero at the same place and time. So B copies the exact sin or cos function of E, only the amplitude changes to E0/c.
In a medium the relation becomes E0 = v B0, where v = c/n is the slower speed in that medium. In free space (vacuum) v = c, giving E0 = c B0. For NEET, most amplitude numericals are in free space, so use c.
