Simple Nature - Light and Matter

field in her region of space has been changing, possibly because thatbar magnet over there has been getting farther away. She observesthat a changing magnetic field creates a curly electric field.We therefore conclude that induction effects must exist based onthe fact that motion is relative. If we didn’t want to admit inductioneffects, we would have to outlaw flea 2’s frame of reference, butthe whole idea of relative motion is that all frames of reference arecreated equal, and there is no way to determine which one is reallyat rest.This whole line of reasoning was not available to Faraday andhis contemporaries, since they thought the relative nature of motiononly applied to matter, not to electric and magnetic fields. 10But with the advantage of modern hindsight, we can understandin fundamental terms the facts that Faraday had to take simply asmysterious experimental observations. For example, the geometricrelationship shown in figure d follows directly from the direction ofthe current we deduced in the story of the two fleas.j / A new version of figure iwith a tiny loop. The point of viewis above the plane of the loop.In the frame of reference wherethe magnetic field is constant, theloop is moving to the right.11.5.3 Faraday’s lawWe can also answer the other questions posed on page 688. Thedivide-and-conquer approach should be familiar by now. We firstdetermine the circulation Γ E in the case where the wire loop is verytiny, j. Then we can break down any big loop into a grid of smallones; we’ve already seen that when we make this kind of grid, thecirculations add together. Although we’ll continue to talk about aphysical loop of wire, as in figure i, the tiny loop can really be justlike the edges of an Ampèrian surface: a mathematical constructthat doesn’t necessarily correspond to a real object.In the close-up view shown in figure j, the field looks simpler.Just as a tiny part of a curve looks straight, a tiny part of thismagnetic field looks like the field vectors are just getting shorter bythe same amount with each step to the right. Writing dx for thewidth of the loop, we therefore haveB(x + dx) − B(x) = ∂B∂x dxfor the difference in the strength of the field between the left andright sides. In the frame of reference where the loop is moving, acharge q moving along with the loop at velocity v will experiencea magnetic force F B = qvBŷ. In the frame moving along withthe loop, this is interpreted as an electrical force, F E = qEŷ. Observersin the two frames agree on how much force there is, so inthe loop’s frame, we have an electric field E = vBŷ. This field is10 They can’t be blamed too much for this. As a consequence of Faraday’swork, it soon became apparent that light was an electromagnetic wave, and toreconcile this with the relative nature of motion requires Einstein’s version ofrelativity, with all its subversive ideas how space and time are not absolute.690 Chapter 11 Electromagnetism