----- Original Message ----- From: "Stephen A. Lawrence" <[email protected]> Date: Saturday, September 26, 2009 10:21 am Subject: Re: [Vo]:The Electric Field Outside a Stationary Resistive Wire Carrying a Constant Current
> > > Harry Veeder wrote: > > Stephen A. Lawrence wrote: > >> > >> Harry Veeder wrote: > >>> > >>> If an electric field exists outside and parallel to the current > >> carrying> wire, and the wire is a loop it implies the electric > >> field lines would > >>> form a closed loop. However, this is not suppose to possible. > >> Certainly it is. It's only possible, however, if there's a > changing>> magnetic field in the loop. Curl(E) = -dB/dt. > > > > Ok but it concerns a constant current so the magnetic field is > constant too. > > > >> But in any case, exactly *how* would you arrange to have a current > >> carrying resistive wire carry a current in a closed loop? > Where's the > >> EMF coming from? Answer that and you'll see how the field > outside the > >> wire plays out. > > > > I mean the emf together with the wire form a loop, i.e. a closed > circuit.> I can see how the construction of the electromotive force > (such as a > > battery) might prevent an electric field inside the wire from > forming a > > loop, if the > > emf does not harbor an electric field. > > But it does -- that's exactly what causes the EMF. The EMF is realised by a suitable configuration of matter. If electrical fields have some right to be called real then they are an effect of the EMF rather than a cause of the EMF. > > > However if there is an electric > > field outside the wire then shouldn't the field go around the emf to > > make a closed loop? > > No. Try to draw a picture of what you're thinking of and I think > you'llsee the problem. > > First, a battery is just a fancy capacitor with a major chemical boost > to the energy storage capability, so for our purposes we can > replace it > with a capacitor. Now, let's draw the thing (horrible Ascii graphics; > unit width font, please, or it won't look like anything): > > > <-- > ------------------------------------- > / \ > | | | ^ > | | | | > V | | | > | - plate <-- + plate | > \ | | / > -----------------| <-- |------------- > --> | | --> > <-- > > > I've shown a roughly square wire loop, with a capacitor in the bottom > "leg" of the loop, and I've shown arrows next to the wire > indicating the > direction of the E field at all points. The capacitor plates are > labeled "+ plate" and "- plate". Around the capacitor, note that > the E > field points the *other* *way* from the field near the wires. If the electrons follow the direction of the electric field around the loop, but the electric field between the plates points in the opposite direction, how can the current keep flowing? The logic of electric fields implies the current should cease. > In fact the E field *never* forms closed loops except when there's a > changing magnetic field contained inside the loop. Otherwise the E > field starts and ends at charges -- it's anchored to charged particles > at both ends. That's as true inside a wire as it is outside a wire. > > > > > >> In other words, you have, essentially, hypothesized a closed > loop of > >> wire with an E field pointing along the loop all the way around, > and>> then asked how there can be an E field in the *air* going all > the way > >> around the loop. Well, how can there be such a field inside the > >> wire to > >> start with? > > > > > > In Weber's electrodynamics the electric field is a mathematical > fiction,> a mere calculating device, so this "paradox" is not an > issue. > There is no "paradox" here. > > As to the field being a fiction, yes, I am aware that you can treat it > as such, but it works equally well to treat it as real. And > treating it > as real has the advantage that radio waves are nice, intuitive > objects,made of fields; if we assume that there's no field there it > gets more > confusing. I have been reading more about Weber's electrodynamics and work after 1950 has extended his theory to explain EM radiation through the introduction of "retarded time". harry
