# Turn On Time.

Discussion in 'Physics' started by BR-549, Jun 8, 2015.

1. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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I came across a most intriguing question the other day, and I would like to see what members here think.

Setup for circuit.

Perfect 12 VDC car battery. No internal resistance, constant voltage, any load.

Perfect switch at battery.

Perfect ammeters at terminals.

Two perfect conductors, 1 light-year long.

Perfect light bulb, resistive, 12 ohms, will not light until 1 amp.

The light bulb can be slid any where around the 2 light year loop.

Questions for light bulb at 1 light year(center of loop).

1. After closing switch, when will bulb lite? Why?

2. After closing switch, when does current start to flow and when does it reach maximum? Why?

3. Are the currents equal at the same time?

Now move bulb to negative terminal of battery.

Repeat same 3 questions.

Any thoughts?

Slide the bulb out 1/2 light year on either conductor.

Same questions. What do you surmise?

Does voltage have to be established clear around the circuit before current flows.....or only across the circuit for current to flow?

Last edited: Jun 8, 2015
2. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Interesting scenario... I'm assuming that by "perfect" you mean superconducting components?

3. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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No, because we don't know all the properties of superconductors.

By perfect I mean without resistance.

Go ahead and assume no z too. In other words, assume the voltage can propagate at c.

Last edited: Jun 8, 2015
4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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c used as a constant to represent the speed of light is customarily assumed to be "in vacuum". the speed of propagation through a metallic conductor, even one with no resistance to current flow, will not be the same speed as light in a vacuum. restate the question with a speed of propagation as a given so an answer can be formulated

5. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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I believe I have. Electricity can move at c thru these conductors. No Z or the Z of space, ok?

6. ### ian field AAC Fanatic!

Oct 27, 2012
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I'll throw a guess - the free electrons are where they are in the conductors, its rather different to turning on a tap into an empty hose and waiting for the water to arrive at the other end.

However; there will be inertia - you shove a load of new electrons in one end and they have to repel the ones already there to make room.

7. ### cmartinez AAC Fanatic!

Jan 17, 2007
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For starts... it it were perfect, as the OP is stating, the signal would travel at the speed of light. But then there would be a huge inductive load due to the wire's length, whose voltage wouldn't even begin to rise until after 2 years!

8. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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Sorry....Z means impedance.

9. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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There are no light year length conductors that I know of.

I am pretending.

10. ### ian field AAC Fanatic!

Oct 27, 2012
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Borg transwarp conduits.

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11. ### cmartinez AAC Fanatic!

Jan 17, 2007
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but in this case... resistance is NOT futile... all we want is your willful cooperation

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12. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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In the first case, you are asking about the time between the activation of a switch and the subsequent illumination of a bulb 1 light year away. You do realize the the question of Simultaneity comes into play here, no? In fact, the timing of any two events will always be in question unless the two events occur at exactly the same position in 3d space.

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13. ### ian field AAC Fanatic!

Oct 27, 2012
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Forming the conductors into a non-inductive configuration opens the possibility of folding it back so the bulb isn't far away.

14. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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your constraints have rendered the question to be identical to the speed of photon propagation in empty space, the answer also becoming the same as such...the electrons, battery, lamp and conductor are no longer relevant points of difference to the exercise.

15. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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the one I remember wrestling with was about a set of scissors. they are made of a rigid metal that cannot flex. they stretch from the earth to the sun and the blade end is four times as long as the handle end measured from the swivel point. if they are opened with velocity of 'c' measured at the handle end, what would the velocity of opening be at the other end?

16. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Yes and no.... because although photons do not have mass... electrons do... and are therefore forbidden from traveling at exactly the speed of light... although they will get very close.

17. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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i didnt say that .... he said that we should consider the velocity to be the same as 'c' in a follow up posting #5 in this thread... therefore it becomes identical to photon propagation.

18. ### BR-549 Thread Starter Distinguished Member

Sep 22, 2013
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hey guys.......it's just a DC circuit. Nothing more.

19. ### cmartinez AAC Fanatic!

Jan 17, 2007
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So not only do we have perfect conductors and components... but also perfect electrons with no mass...
maybe we should also include Alice and the Mad Hatter into this scenario?

20. ### joeyd999 AAC Fanatic!

Jun 6, 2011
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Don't forget the fruity-flavored rainbow-colored unicorn droppings.