Which bulb lights first?
Which bulb lights first?
(OP)
I do a little lecturing on a temporary basis in a local college. Some time ago,one young, eager lad was keen to know the answer to this question: A bulb is placed at the remote end of a circuit of 10m length. There is a switch to operate the bulb. Now a similar circuit is 10miles in length, if both switches are operated at the same time, would there be any difference in the time it takes current to flow through the element, regardless of light output.
After some deliberation and chin rubbing, I offered my opinion. This seemed to asuage the lads thirst for a while. However, on throwing the question up in conversation during tea break in the staff room, I was amazed to see significant differences of opinion amongst the other lecturing staff!
Would you care to offer your opinion?
After some deliberation and chin rubbing, I offered my opinion. This seemed to asuage the lads thirst for a while. However, on throwing the question up in conversation during tea break in the staff room, I was amazed to see significant differences of opinion amongst the other lecturing staff!
Would you care to offer your opinion?
Regards,
Lyledunn






RE: Which bulb lights first?
So now you can calculate the time difference! (if any).
(this of course ignores the voltage drop!).
RE: Which bulb lights first?
RE: Which bulb lights first?
Since the switches are placed at the bulb end, both bulbs will light at the same time. Voltage is present at the bulb end of the wire even with the switches off.
RE: Which bulb lights first?
RE: Which bulb lights first?
Did you ever give them this problem?
Ole takes his light bulbs from Minnesota back to Norway where the voltage is 220 volts. He doesn't want to waste them so he wires up a couple of sockets in series and screws in a 60 and a 100 watt 110 volt bulbs and plugs them into the 220 volt outlet. How many watts does the circuit draw?
or
There is a metallic conduit going underneath a wide river with 7 wires (same
color and size) through it. Your job is to identify the wires and lable #'s
1-7 at each end. You are supplied with a boat w/ motor, wirenuts, locknuts,
continuity tester, wire tags, tape. You may use any basic electrical tools
you think necessary.
Here's the catch... You are only allowed to go across the river once and
then back. How would you do it?
RE: Which bulb lights first?
1. 150W
2. Mark one wire #1, ground it. Mark another #2, leave it open. Splice and isolate two more, and then splice and isolate the last three.
Go to the other side, mark the grounded wire #1, mark the two wires that test continuous #3 & #4(it doesn't matter which), mark the three wires that test continuous #5, #6 & #7(again, it doesn't matter which). Mark the wire which tests open #2. Connect the wires you marked #3 and #5 to ground. Connect wire #6 to wire #2.
Go back to the other side. Open the splice with the two wires, the grounded one is #3, the other is #4. Open the splice with the three wires, the grounded one is #5, the one continuous to #2 is #6, and the last one is #7.
RE: Which bulb lights first?
Both bulbs would energize at the same time if the circuit was ideal, no inductance or capacitance. The electrons are already in the conductors so they start moving instantaneously.
Imagine a water hose with perfectly rigid walls and an infinite source available. One hose is 10m long, the other 10 miles. If the hoses are initially full of water (electrons), the moment you open the valve water runs out of both the short and long hose ends simultaneously.
RE: Which bulb lights first?
RE: Which bulb lights first?
RE: Which bulb lights first?
Try the light bulbs again.
RE: Which bulb lights first?
Imagine your full water hose doesn't have a constant pressure at the valve (like dc voltage), but has a vibrating membrane (like ac voltage). Now when the membrane starts to vibrate, will the pressure changes occur instantly at the other end of the hose? No, what you are doing is transmitting sound and it will travel at the speed of sound in water.
Similarly, the ac electrical current will not start to flow instantaneously at the end of the 10 mile wire. "The velocity of propogation of an electromagnetic disturbance in air is the same as that of light, namely 2.998x1010. The only difference for transmission lines is that the conductor provides a guide. In terms of the constants of the line, this velocity is equal to 1/sqrt(C·L)."1 For most cables, this is about 1000 ft/µs.
1. Westinghouse T&D Book.
RE: Which bulb lights first?
RE: Which bulb lights first?
http://www.eskimo.com/~billb/miscon/elect.html
RE: Which bulb lights first?
Laplacian's analogy of the water hose is a good one. But he's simplified both the water hose and the electrons just a bit too far. The water will not exit instantaneously. The analogy needs to be taken further:
When the valve is opened, the first water molecule will squeeze into the next one, and that second one will bounce off it and squeeze into the third, and so on. This will proceed as a wave down the hose. The speed of the wave will be equal to, or a percentage of, the speed of sound in water.
Now we got us a tight analogy.
RE: Which bulb lights first?
V(% speed of light) = 1 / sqrt(k)
k = relative permitivity of insulation = 1.0 for air
Also, Eistein's Theory of Special Relativity states that no signal or other conveyence of information can travel in excesss of the speed of light in a vacuum.
RE: Which bulb lights first?
2.998x10E10 is in centimeter/second
2.998x10E8 is in meter/second
2.998x10E5 is in kilometer/second
RE: Which bulb lights first?
The original question is an interesting one. I have to scratch my head a little to try to figure out how to combine the wave and circuit elements of the problem.
Perhaps another refinement in considering the whole question: doesn't the very long wire have a large inductance which will slow the rate of increase of current?
Lyle - I seem to remember you from a long time ago at alt.engineering.electrical. Are you from down under?
RE: Which bulb lights first?
http://www.radioelectronicschool.com/downloads/howfast.pdf
for electricity flow and the speed of electricity flow under different angles/theories
RE: Which bulb lights first?
Crudely, at 1ns/foot, then ~30ns difference.
RE: Which bulb lights first?
//would there be any difference in the time it takes current to flow through the element, regardless of light output.//
It seems like we are only concerned with when the initial voltage wavefront arrives at the lamp.
electripete - I'm not sure about permissivity (permeabiltiy?). Can you elaborate?
RE: Which bulb lights first?
RE: Which bulb lights first?
http://icg.harvard.edu/~phys15c/lectures/Lect08.ppt
for the speed of an electromagnetic wave in a transmission
RE: Which bulb lights first?
RE: Which bulb lights first?
RE: Which bulb lights first?
From J.D. Kraus, "Electromagnetics",
Velocity of propagation in any transmission line is a function of the permeability and permittivity of the surrounding medium. The following can be derived directly from Maxwell's equations:
v2 = 1/ue
where v = velocity, u=permeabiltiy, e=permittivity
or
v = 1/sqrt(ue)
and
vr (the velocity relative to light) = 1/c*sqrt(ue)
where c = velocity of light in a vacuum
In the case of a non-magnetic medium (e.g. air), u app.=u0 and the above reduces to
vr = 1/sqrt(k)
where k = e/e0, the relative permittivity of the medium
For air, e app.=e0, so vr app.= 1, the speed of light
With polystyrene dielectric (e.g. coaxial cable), k = 2.7 and
vr = 1/sqrt(2.7) = 0.61
jbartos and busbar are correct assuming the transimission line is in air. skogsgurra's 60 to 70% is correct of typical solid dielectrics.
RE: Which bulb lights first?
Your light bulbs - I reckon 0 watts.
Maybe 150 watts instantaneously, until the 60 watt lamp
pops!! I don't really want to calculate how long that will take after the switch is thrown !!
KVKEV
RE: Which bulb lights first?
Go to the head of the class. There are sure to be arguement about voltage tollarance etc. But the question is academic. It's just a quiz or homework problem to teach students to follow through and look at all the details of a problem.
RE: Which bulb lights first?