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Chapter 11 Radiation. 11.1 Dipole Radiation 11.2 Point Charges. 11.1 Dipole Radiation. 11.1.1 What is Radiation? 11.1.2 Electric Dipole Radiation. 11.1.1 What is Radiation?. Static field does not radiate. The source of EM waves is from the radiation of accelerating
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Chapter 11 Radiation 11.1 Dipole Radiation 11.2 Point Charges
11.1 Dipole Radiation 11.1.1 What is Radiation? 11.1.2 Electric Dipole Radiation
11.1.1 What is Radiation? Static field does not radiate. The source of EM waves is from the radiation of accelerating charges and changing currents. The signature of radiation is the irreversible flow of energy away from the source. Assume that the source is localized near the origin. The total power passing out through the surface at radius r is The power radiated is For static fields,
11.1.1 (2) In the and terms, But, Jefimenko’s equations: These terms are responsible for EM radiation
11.1.2 Electric Dipole Radiation approximation 1:
11.1.2 (2) approximation 2: approximation 3:
11.1.2 (3) Keep only zeroth order of [ approximation 1: approximation 2: ]
11.1.2 (4) [ approximation 3]
11.1.2 (6) The energy radiated by an oscillating electric dipole is determined by the Poynting vector: The total power radiated is
11.1.2 (7) Example 11.1 (a) Why the sky is blue during the day ? Answer : • Sun’s radiation is white-light. • The absorption of the white-light • by the atmosphere atoms is more • on higher frequency. • The reradiation • so more on the blue than the red. • The sky is blue .
11.1.2 (8) (b)Why, during the sunset, the sky is more red ? Longer, more scattering, especially for Blue lights. more red-lights reach.
11.2 Point Charges The fields of a point charge q in arbitrary motion Velocity field Acceleration field Radiation field The Poynting vector
11.2 (2) The radiation energy is the stuff that detaches itself from the charge and propagates off to infinity. Only terms of remain (the terms ) That is, only the terms of survive.
11.2 (4) The total power radiated Larmor formula
11.2 (5) if Rate of gun shoot Rate of strike on the target
11.2 (6) The power radiated by the particle The power radiated by the particle into per solid angle The total power radiated is Lienard`s generalization of the Larmor formula
11.2 (7) Example 11.3 Solution:
11.2 (9) The total power radiated Same radiation result.
11.2 (10) producting radiation (or photon) (bremsstrahlung) (annilation) Prob. 11.16 synchrotron radiation
