SR and a lightbulb
in [Relativity]
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From: dk on 29 Dec 2009 21:39 Hi to all, Consider the following scenario: In frame S thare are two points, x1 and x2, a distance L apart. There are wires running from these two points such that x1 and x2 are connected to the '+' and '-' terminals of a battery in S. Now, image a lightbulb device with straight segments of wire extending from each of its terminals. The lightbulb device, in frame S', is moving with velocity v parallel and very close to the x axis of frame S. An observer in S notices that the lightbulb momentarily flashes as it passes by the x1-x2 region. This S observer correctly concludes that the length of the wire segments of the lightbulb device added up to exactly length L, since the lightbulb flashed, implying that the endpoints of those wires simultaneously coincided with x1 and x2, respectively, in S---Otherwise the circuit would not be complete and the lightbulb would not flash. However, since the observer in S will measure a length L for the lightbulb device, an observer at rest in S' will have to measure a proper length L' that is greater than L. Therefore, in frame S', the endpoints of the lightbulb device never coincide with x1 and x2 and the circuit never completes. How does the observer in S' explain the objective fact of the lightbulb flashing? thanks to all who might respond dk
From: dk on 29 Dec 2009 21:46 I should have mentioned that it is assumed that the wires on the lightbulb device are insulated and only at their endpoints is the conductor exposed so as to make contact with x1 and x2 in frame S.
From: dlzc on 29 Dec 2009 22:17 On Dec 29, 7:46 pm, dk <dakalami...(a)sci.ccny.cuny.edu> wrote: > I should have mentioned that it is assumed > that the wires on the lightbulb device are > insulated and only at their endpoints is the > conductor exposed so as to make contact with > x1 and x2 in frame S. Draw a diagram. How are the wires oriented wrt the x axis? Next, wires have inductance. Only in a fantasy world does the light "flash". David A. Smith
From: Inertial on 29 Dec 2009 22:25 "dk" <dakalamidas(a)sci.ccny.cuny.edu> wrote in message news:85ff994e-85f8-463e-89d5-7ee79f922abc(a)c3g2000yqd.googlegroups.com... > Hi to all, > > Consider the following scenario: OK... > In frame S thare are two points, x1 and x2, a distance L apart. L x1 x2 > There > are wires running from these two points such that x1 and x2 are > connected to the '+' and '-' terminals of a battery in S. L x1 x2 | | | | | '-| |-' +| - > Now, > image a lightbulb device with straight segments of wire extending from > each of its terminals. The lightbulb device, in frame S', is moving > with velocity v parallel and very close to the x axis of frame S. S' .-Bulb-. ->v | | | L' | L ==========x1===x2============= | | | | | '-| |-' +| - > An > observer in S notices that the lightbulb momentarily flashes as it > passes by the x1-x2 region. So the wires from the bulb and from the battery must have some significant thickness, so that at speed v they are in contact long enough to make a current flow throughout the wire and for the bulb to glow > This S observer correctly concludes that > the length of the wire segments of the lightbulb device added up to > exactly length L, since the lightbulb flashed, implying that the > endpoints of those wires simultaneously coincided with x1 and x2, > respectively, in S---Otherwise the circuit would not be complete and > the lightbulb would not flash. However, since the observer in S will > measure a length L for the lightbulb device, an observer at rest in S' > will have to measure a proper length L' that is greater than L. Yes > Therefore, in frame S', the endpoints of the lightbulb device never > coincide with x1 and x2 and the circuit never completes. No .. that would be nonsense. What happens is that the two set of wires do not connect at the same time in S' and do not disconnect at the same time in S'. If the bulb is to light, they would need to both stay connected long enough for a current to flow. > How does the > observer in S' explain the objective fact of the lightbulb flashing? > > thanks to all who might respond > dk
From: Cocoon on 30 Dec 2009 07:11
On Dec 30, 3:39 am, dk <dakalami...(a)sci.ccny.cuny.edu> wrote: > Hi to all, > .... > How does the > observer in S' explain the objective fact of the lightbulb flashing? > > thanks to all who might respond > dk There is symmetry in your system, can't you see In one frame the batt conn contracts, meanwhile in the other the bulb conn contracts Good bye |