MadSci Network: Physics |
Hello, George! You asked: "Displacement current seems to be more of a mathematical current rather than the movement of electrons, q/t.Is displacement current the precursor for eletromagnetic radiation?" Charges in motion give rise to the effect, but it is not the same as the magnetic field around a wire with a direct current flowing in it. The term "displacement current" was invented by Maxwell and I suppose it fit into his thinking patterns. Like you, it doesn't fit too well with mine. What it really refers to is the creation of a magnetic field due to a changing electric field. This magnetic field "looks like" a current were present causing it (one magnetic field looking more or less like another....) and so the fictional current was named. My old textbook (Berkeley Physics Course, vol. 2, Electricity and Magnetism) didn't like the term either (I quote "...the name has stuck although it no longer seems very appropriate."), and neither did a number of web sites. By the way, if this little note isn't to you liking, simply go to any good search engine and seach on the key phrase "displacement current" and you will find many, many explanations written by university professors trying to make the matter clear to their students. Perhaps one of their expositions will resonate better with you than what I have to say. Here are two URL's to get started: http://farside.ph.utexas.edu/~rfitzp/teaching/em1/lectures/node41.html http://scienceworld.wolfram.com/physics/DisplacementCurrent.html Let's take the classic example of the capacitor. Electrons flow onto and off of the plates with alternating current, but none of them (assuming the capacitor doesn't leak) actually cross from one plate to the other. However, it is certainly true that the value of the electric field in the interplate gap is constantly changing. This changing field is certainly caused by the charges in motion on the plates (spreading out from and then gathering back together where the wire connects) and like any changing electric field, creates a magnetic field. To Maxwell, this behaved just as if a current were present. So he added the term 1 dE --- ---- c dt into his equation 4 (pi) J 1 dE Curl{B} = ------------ + --- ---- c c dt and gave it the name of "displacement current." The term is extremely important. In the absence of all charge or current, Maxwell's equations reduce to: 1 dE Curl{B} = --- ---- Div{E} = Div{B} = 0 c dt 1 dB Curl{E} = - --- ---- c dt Note the symmetry! Note also that it allows a changing electric field to create a changing magnetic field which in turn makes a changing electric field and so on...propagating along at the speed of light. That is, basically, a description of the photon.
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