# Transmission lines

Discussion in 'Homework Help' started by screen1988, May 18, 2013.

1. ### screen1988 Thread Starter Member

Mar 7, 2013
310
4

Assuming no warm-up time for the lamp filament, and no resistance along the 372,000 mile length of both wires, the lamp would light up approximately one second after the switch closure. Although the construction and operation of superconducting wires 372,000 miles in length would pose enormous practical problems, it is theoretically possible, and so this thought experiment is valid. When the switch is opened again, the lamp will continue to receive power for one second of time after the switch opens, then it will de-energize.

How about the case when the switch is put right next to the lamp? I think the lamp is no longer receive power after the switch opens, right?

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2. ### ActivePower Active Member

Mar 15, 2012
155
23
The event indicated by the opening of the switch (i.e. the current falling down to zero) would take 1 second to travel to the lamp if the line is 186,000 miles long.
On the other hand, if the switch is placed right next to the lamp, the distance the EM wave needs to travel is almost zero and consequently the lamp will turn off instantaneously.

screen1988 likes this.
3. ### screen1988 Thread Starter Member

Mar 7, 2013
310
4
Can you say it more detail in terms of electrons movement in the case switch open and the switch is placed right next to the lamp?

4. ### ActivePower Active Member

Mar 15, 2012
155
23
When you turn off the switch in the first case, the bulb would still be receiving the electrons from just before the opening of the switch.

Let's say that you open the switch at t = 2 seconds. The electron which got through the switch at t =1.9999 second would make it to the lamp at t = 2.9999 seconds, i.e. almost a second after the switch was closed.

If on the other hand, you put the switch closer by, the electron that left at t = 1.9999 second would reach the lamp at, say, t = 2.000001 seconds i.e. almost instantly. This makes the lamp respond to the turn-off instantaneously.

HTH.

screen1988 likes this.
5. ### nsaspook AAC Fanatic!

Aug 27, 2009
5,143
5,665
I think your first answer was better, what makes the lamp turn on/off quickly is not the lengthwise movement an electron from the switch to the lamp, it's the EM wave the travels around the wire from the switch to the lamp. The electrons in the wire as a whole (current) move very slowly in response to the field but each individual electron (charge) will be accelerated by the electric field from the battery at the switch as the switch closes causing a reactive near-field EM wave coupled to the electrons to move the distance from the switch to the lamp. As the lamp lights and the fields become stable and non-varying the electrons stop being accelerated and move at a constant speed, as this happens there is no longer an EM wave but just ordinary electric and magnetic fields transferring energy from the battery to the lamp. When the switch opens we have the same condition of varying electron movement due to changes in fields giving rise to a collapsing EM that stops the energy transfer to the lamp.

ActivePower likes this.

May 19, 2013
214
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Let's say that the distance between the battery and the lamp is millions and millions of light years.
And the switch(that is near the lamp) has been opened for a long time and now i close it.You say that the lamp turns on instantaneously.
How does the electrons near the switch,near the lamp know that there is a battery at the other side of the universe so that they would start moving instantaneously when the switch is closed thus turning on the lamp instantaneosuly?This information that there is a battery out there would only travel at the speed of light.How come?

7. ### ActivePower Active Member

Mar 15, 2012
155
23
No, I said, the effect of opening of the switch would take a second (or a million years in this case) to get propagated to the lamp if the lamp and switch are placed miles apart. If you were to place the switch close by, the opening of the switch would cut off the supply to the lamp even if the EM wave that left just before the switch was opened would reach the point of open-circuit a second later.

In the case that you propose, the event of closing the switch would propagate to the lamp after a million years. The EM wave would take a million years to travel as you state.

It is the same phenomena due to which we see supernovae explosions years later after they occur.

8. ### LDC3 Active Member

Apr 27, 2013
920
161
Yes it would turn on instantly. Why, because there is a voltage difference between the 2 ends of the switch. The propagation starts and ends at the switch, not at the battery.