Vol 2 - Landau, Lifshitz - The classical theory of fields (4th, 1994) (1) · Problem 9.1

Radiation of Electromagnetic Waves

Physics — Landau & Lifshitz · electric-dipole-radiation

Problem

An oscillating electric dipole has p0=10−10p_0 = 10^{-10} C·m and ω=1010\omega = 10^{10} rad/s. Find the total radiated power.
Intuitive explanation

The key is recognizing that an oscillating dipole radiates electromagnetic energy, and the power depends on how quickly the dipole moment changes. The crucial formula connects radiated power to the second time derivative of the dipole moment—essentially, the acceleration of the charge distribution. Since p(t)=p0cos⁡(ωt)p(t) = p_0 \cos(\omega t), its second derivative gives you p¨∼ω2p0\ddot{p} \sim \omega^2 p_0, which scales as the fourth power of frequency. Plug in the numbers using the dipole radiation formula and you'll find the answer.

Intuitive explanation

The key is recognizing that an oscillating dipole radiates electromagnetic energy, and the power depends on how quickly the dipole moment changes. The crucial formula connects radiated power to the second time derivative of the dipole moment—essentially, the acceleration of the charge distribution. Since p(t)=p0cos⁡(ωt)p(t) = p_0 \cos(\omega t), its second derivative gives you p¨∼ω2p0\ddot{p} \sim \omega^2 p_0, which scales as the fourth power of frequency. Plug in the numbers using the dipole radiation formula and you'll find the answer.

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