Question

3. Griffiths draws a donut on page 448 (Fig 11.4). What is this drawing trying to demonstrate?! He doesn't label the figure, so take your own shot: how would you briefly and clearly explain to a reader what is being shown here? 471 11.1 Dipole Radiation FIGURE 11.4 mutually perpendicular, and transverse; the ratio of their amplitudes is Eo/ Bo c. All of which is precisely what we expect for electromagnetic waves in free space. (These are actually spherical waves, not plane waves, and their amplitude decreases like 1/r as they progress. But for large r, they are approximately plane over small regions-just as the surface of the earth is reasonably flat, locally.) The energy radiated by an oscillating electric dipole is determined by the Poynting vector: sin coso(t -r/c)] 1.20) The intensity is obtained by averaging (in time) over a complete cycle: (11.21) 32r2 Notice that there is no radiation along the axis of the dipole (here sin θ-0); the intensity profile takes the form of a donut, with its maximum in the equatorial plane (Fig. 11.4). The total power radiated is found by integrating 〈S) over a sphere of radius r (11.22) 12 с

Answer 1

Figure 11.4 shows the spatial distribution of the intensity of the radiation produced by an oscillating (oscillation of charge) dipole.

It is for equation 11.21.

If we place the dipole along z axis and slice the figure (11.4) at any value of  $\phi$ (the polar angle), then we will get the 2-D, something like shown below.