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Simple Nature - Light and Matter

Simple Nature - Light and Matter

Simple Nature - Light and Matter

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Discussion question A.Discussion question B.Discussion questions C <strong>and</strong>D.a single thing traveling in a single direction, whereas no such assumptionwas made in arguing for the E × B form. For instance, iftwo light beams of equal strength are traveling through one another,going in opposite directions, their total momentum is zero, which isconsistent with the E × B form, but not with U/c.Some examples were given in chapter 3 of situations where itactually matters that light has momentum. Figure q shows the firstconfirmation of this fact in the laboratory.Angular momentum of light wavesFor completeness, we note that since light carries momentum, itmust also be possible for it to have angular momentum. If you’vestudied chemistry, here’s an example of why this can be important.You know that electrons in atoms can exist in states labeled s, p,d, f, <strong>and</strong> so on. What you might not have realized is that theseare angular momentum labels. The s state, for example, has zeroangular momentum. If light didn’t have angular momentum, then,for example, it wouldn’t be possible for a hydrogen atom in a p stateto change to the lower-energy s state by emitting light. Conservationof angular momentum requires that the light wave carry away allthe angular momentum originally possessed by the electron in thep state, since in the s state it has none.Discussion QuestionsA Positive charges 1 <strong>and</strong> 2 are moving as shown. What electric <strong>and</strong>magnetic forces do they exert on each other? What does this imply forconservation of momentum?B 1. The figure shows a line of charges moving to the right, creatinga current I. An Ampèrian surface in the form of a disk has been superimposed.Use Maxwell’s equations to find the field B at point P.2. A tiny gap is chopped out of the line of charge. What happens whenthis gap is directly underneath the point P?C The diagram shows an electric field pattern frozen at one moment intime. Let’s imagine that it’s the electric part of an electromagnetic wave.Consider four possible directions in which it could be propagating: left,right, up, <strong>and</strong> down. Determine whether each of these is consistent withMaxwell’s equations. If so, infer the direction of the magnetic field.D Discuss what happens to the wave pattern shown in the diagram ifwe switch to a frame of reference moving along with the wave.706 Chapter 11 Electromagnetism

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