This page deals with the Chapters 8 and 9: waves, electricity and magnetism, light. The main intent is to try to assist you in using the book more effectively, in particular to stimulate an active approach: try to answer the questions in the book; think about them; return to them, until you have some understanding. (Within a reason.) Of course, some may simply turn out to be too difficult, or unrelated to what we are covering; please use your best judgement – and/or communicate with me. For some general notes on how this page should be understood and used, as well as some general comments on the material in the book, please refer to the similar page, posted previously, related to the midterm preparation. (If in any doubt, please contact me.)
Oscillations are ubiquitous in physics, and present one of the its cornerstones. Many systems exhibit such behavior, and many more can be studied resorting to it; most commonly, one refers to periodic processes when talking about waves. Electricity and magnetism had been discovered and initially investigated by studying electric and magnetic forces, as separate entities; these are really two aspects of the electromagnetic interaction, one of the four fundamental interactions. Along with gravity, it is responsible for most of what we can, and do, directly experience. These phenomena are normally discussed using the far-reaching concept of the field; an electric charge, apart from being a particle with a certain mass, is also borne with an electric field extending throughout, carrying its energy. Changes in this field will generate a magnetic field, whose changes in turn generate an electric field ... such mutual changes of the fields thus propagate through space, and these waves are called electromagnetic radiation. They are initially generated by moving charges (“sources”), and it is said that they ‘emit radiation;’ the changes in the fields themselves then sustain each other, propagating as waves. (They can also form ‘standing waves’ when tightly constrained to a certain region of space.) This is precisely what light is: electromagnetic waves, within a specific range of frequencies – the visible part of the full ‘spectrum’ of electromagnetic radiation. (Our understanding of the electromagnetic field gets much more rounded and deepened at the level of quantum physics, where its dual nature is revealed; descriptions of its aspects then directly relate to many topics of modern physics.) These issues, mostly at the classical level (no quantum physics), are discussed in Chapters 8 and 9; below is a loose breakdown of sections, with a selection of useful problems. Again, for a comment on this refer to the page posted for the midterm reviewing.
Practically all material in this chapter is elementary and should be understood. The opening sections, 8.1 and 8.2, deal with waves and typical wave phenomena (interference); up to this point, only mechanical waves (on a string or water) are discussed. Section 8.3 poses the ages old question of the nature of light; it deepens the discussion of interference and extends the consideration of waves, from mechanical to ones related to light. Sections 8.4 and 8.5 introduce electricity and magnetism; the offered motivation is the quest of what light is. All questions throughout the text ("Concept Check") are rather basic and mostly worth clearing up. (Another good resource for some of this material is your lab manual.) The closing sections (8.6 and 8.7) present a nice, general discussion of a good atomic model; please bear in mind what we have been hinting at in this regard: the model described here predates quantum understanding of the structure of matter. (In other words: this is a good model, but the modern view is rather more involved.)
Concept Check: all of them.
Review Questions:
1, 2, (3-4), 5; (6-7), 8; (9-11); (12-14), 15, 16; (17), 18, (19), 20, (21);
(Many of the questions asking for 'evidence' are really worth thinking through
– even if you cannot identify such 'evidence.')
Conceptual Exercises:
1-4, (5); (8 – do think about it!), 9;
10-11, (12), 13, (14), 15, (16-17);
18, 19, (20 – recall or look up an in-class animation),
21 (your lab!); (27), (28 – a nice question!), 29, (31), 32-33,
(34-35);
Sections 9.1–9.3 round up the line of the development of the previous chapter, by identifying light as an electromagnetic wave; in the process, the concept of the field is introduced and discussed. Finally, section 9.4 discusses the complete spectrum of electromagnetic radiation. Much of this material is absolutely elementary, as well as critical, for appreciating some of the basic concepts of modern physics. (For the test, you do not have to memorize the names of laws or people, or any details of the discussion of the full spectrum.) Section 9.5 sets the stage for sections 9.6 and 9.7, discussing how we affect the environment. None of the (many) details given here will be in the exam, but this is material that everybody should be very well aware of, and I strongly suggest a thorough reading, at some point. (Then make your neighbors and friends appreciate it. As well as others.) A basic awareness of these issues is sufficient for the test.
Concept Check:
1, 2 (p227), (3, 4, p232);
(“MAKING
ESTIMATES,”
p232, is generally useful), 5, 6; (7),
8 (remember thermodynamics?), (9 – lab); (10),
(11 : this reaches into our discussions of ‘atomic transitions’
and quantum physics)
[ Comment : understand
a difference between sound and electromagnetic waves – specially
when one talks about radios. ]
Review Questions:
1-3; 4-6, (8); (9-11, 13, 14 : useful to think through);
Conceptual Exercises:
1 (a “measurement” may be a bit much to ask for), 2-5;
7-10, (13 : interesting to know);
(15-17 : these have to do with how radiation at different frequencies
in the full spectrum interacts with various media);
Critical Thinking Questions: (1, 4, 5);