1. Chapter 11 Radiation
11.1 Dipole Radiation
11.2 Point Charges

2. 11.1 Dipole Radiation 11.1.1 What is Radiation?
Electric Dipole Radiation

3. What is Radiation?
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,
Static field does not radiate.

4. 11.1.1 (2) But, Jefimenko’s equations: In the and terms,
These terms are responsible for EM radiation

5. 11.1.2 Electric Dipole Radiation
approximation 1:

6. (2)
approximation 2:
approximation 3:

7. 11.1.2 (3) Keep only zeroth order of [ approximation 1:

8. (4)
[ approximation 3]

9. (5)

10. 11.1.2 (6) The energy radiated by an oscillating electric dipole is
determined by the Poynting vector:
The total power radiated is

11. 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 .

12. 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.

13. 11.2 Point Charges The fields of a point charge q in arbitrary motion
Velocity field
Acceleration field
Radiation field
The Poynting vector

14. 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.

15. 11.2 (3)

16. 11.2 (4)
The total power radiated
Larmor formula

17. 11.2 (5) if
Rate of gun shoot
Rate of strike on the target

18. 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

19. 11.2 (7)
Example 11.3
Solution:

20. 11.2 (8)

21. 11.2 (9)
The total power radiated
Same radiation result.

22. 11.2 (10) producting radiation (or photon) (bremsstrahlung)
(annilation)
Prob
synchrotron radiation