# Physics question - or - is electricity like a conga line?

I remember asking this in physics at high school (a scary number of years ago) but the teacher couldn’t answer it.

Imagine you have a circuit with 2 bulbs in series, a DC power source and an on-off switch. When you close the circuit, in high school physics terms, the current has to flow around the circuit from the negative terminal of the power source to the positive.

Does this mean that if you had a fast enough camera, you would see the bulbs light in sequence, the closest one to the power source lighting first? (I know, bulbs take a relatively long time to heat up compared to the speed of electrons. If necessary substitute two incredibly fast measuring devices for the bulbs). If so, why is it that when you break the circuit after the bulbs, the electrons “know” ahead of time that they won’t make it back to the positive terminal and so the bulbs don’t light at all?

I know the notion of all the electrons hurrying through the wires like cars in a tunnel is naive, given quantum mechanics etc. - so what really happens? Is this something to do with the electrons “seeking out all possible paths” ahead of time, a la two slits experiment with photons, so they know not to start on their journey if there’s a break in the circuit?

I have a good layman’s grasp of quantum mechanics (if that’s not an oxymoron) but here it seems like a macroscopic phenomenon.

The electric signal moves through the wire very quickly. Something on the order of 0.5C (1/2 speed of light).

The electrons themselves move very slowly, something like 1 to 5 cm/hour but they do actually move through the circuit in the DC case you described. In the AC case, it’s alittle more complicated.

To answer your question though, YES, one bulb would see the signal before the other. And would light a little sooner.

To the rest of you about to start complaining about my simplistic answer, the wording should match level of the audience (OP) and this should be good enough.

A better analogy might be a train starting to move when all the couplings are slack and the engine pushing. The first coupling in front of the engine slams then the next and next… far faster than the engine or the train is moving.

I think it’s the light closest to the switch that turns on first, not necessarily the one closest to the power source. Using Padeye’s analogy, if one car in the middle of the train had brakes on and suddenly released it, that car would first start to move, then the adjacent cars, etc.

I suspect that the two lamps would light at exactly the same time. It seems to me that it is completely arbitrary to consider the positive end of the power supply as a ‘high’ energy end and the negative as the ‘low’ energy end. You could equally consider the negative terminal to be doing the ‘pulling’ and suggest that the lamp nearer the negative terminal would light first because of this! The power supply causes a potential difference to appear across the system once the switch is closed, and this difference causes the electron drift, but I can’t see how this drift, or flow of current, can be any greater or lesser at any point in the circuit. So for the train analogy, the ‘current’, ie the train, is being both pushed and pulled at the same time, and the train is linked back to itself on a circular track!
So that’s my thoughts.

Thanks for the answers, guys. I’m still having trouble with the notion that, if current is “flowing” from one place to another, when you break the circuit after the bulbs, how does it know not even to start out on its journey? If you broke the circuit between the bulbs, why wouldn’t one light but then the other wouldn’t?

Not only does it work pretty much just as the OP has stated, there are electronic devices that are made just for this effect. The are called delay lines. Many of these delay lines are made with multiple “taps”. Each tap will have a specified amount of delay. The delay will be measured in nano or microseconds. The delay time specifies how long after an electrical signal is applied to the input until it will be available at each tap.

These are frequently used in RADAR and IFF equipment as a very high speed serial to parallel decoder.

Internally, they are just a long piece of thin wire wound up (in such a way that there is minimal inductance) with other wires attatched along the length to make the “taps”.

Delay lines built to decode data are built with the taps spaced so that all the data bits in a signal will line up with tap at one time. (You have to know what the data frame will look like to build a tap for it). The data will usely be “framed” by appropriately named framing bits before and after. When the a signal is detected at both framing bit taps at the same time, a signal will be generated to clock the data off the rest of the taps into some kind fo storage.

Hopefully, this explanation and example make sense.

A more concrete example might be this:

If you laid out a mile of wire with a power source and a switch at one end, and an LED (which can turn on and off very fast) at the other you could do this experiment. You string the wire in a loop so the LED is close to the switch so you can watch it.

You could turn the switch on and off very fast. Let say you turn it on and off once each microsecond, so that it was on for a microsecond and off for another. You do four complete cycles, using 8 microseconds. Then you watch the LED.

You would see nothing for about another 4 microseconds after you finished flipping the switch on and off. Then you would see the LED turn on and off 4 times in exactly the pattern that you used the switch.

It takes light about 12 microseconds to travel a mile in wire. So, after you spent 8 microseconds playing with the switch, you still have to wait 4 microseconds for the first on pulse to reach the LED.

Hope that makes it clear enough.

Your conga line description is pretty decent, think about it this way, if you shut the door that the conga line was going through, everybody stops, even the people who can’t see the door. A circuit is like a circular conga line, break the line and stop one person from moving forward, and everyone has to stop.

Because, as soon as the transients die out the entire wire segment on either side of the break is at the same potential. If the potential difference is zero the electric field is zero, and current would have no reason to flow.

When you close the switch to an open circuit, there will flow some charge into the wire. Like blowing air through a garden hose. As long as the end is open, the air can flow. But even if you close it, you can blow some air into it, though the pressure building up will prevent you from moving as much of air into the hose as if the end was open. Air will flow as long as there is a difference in pressure. Current will flow as long as there is a difference in charge density.

Nothing can propagate faster than the speed of light, including the forces created by electric potential differences that make the electrons move. The conga line model is a good one. But consider that if the person at the front of the line starts running away, the one at the end doesn’t get immediately yanked his arms forward. First, the people at the front get pulled apart. There will propagate a wave of pulled arms through the line. Their yanking of the arms corresponds to the elastic properties of the electrons bouncing off each other’s electric field. The speed this wave moves at is the speed of electricity in that medium.

Now imagine the front man running into a closed door. The running people will only stop when they are stopped by the man in front, and they can be happy if only the length of their arms gets squashed. There will propagate a wave of compressed people through the line. The last one won’t stop at the instant the first one runs into the door, because of the propagation time of this pressure wave.

Likewise, break a circuit, and the charges keep flowing in and out of the energy source until the pressure wave from the now open ends reaches it and equals out the charge pressure to the one of the voltage of the terminals. There will even be some bouncing back and forth of this wave, like the squashed people moving back to arm-length distance.

Erm…mayby abit of a dumb question but, what is a charge? I get the idea it isn’t the electrons moving, izzit?

Electric charge is a basic property of matter. It is a quantity of electricy. It comes in two varieties arbitraritly called negative and positive. One way to define it is by Coulomb’s law which states that two like (point) charges (i.e. both positive or both negative) will repel each other (two unlike charges would attract each other) with a force proportional to the product of the charges and inversely proportional to the square of the distance between them. The unit for charge is the coulomb and an electron has a charge of 1.6022*10[sup]-19[/sup] coulomb, which is the basic unit in which electrical charge appears. I.e. all electrical charges are some whole number multiple of the electron charge.

Perhaps you’re thinking that wires are like hollow pipes? And that a stuff called “current” flows through them?

If so, then you’ll never understand circuits.

First of all, all circuits are always full of electricity because all metals are MADE of particles of negative electricity. But aren’t metals made of atoms? Not exactly. When copper atoms come together to form bulk copper, electrons leave the atoms and form a “fluid” which soaks through the whole metal. This “electron fluid” gives copper it’s silvery-orange look. When pushed, this “fluid” can flow along. And during an electric current, it is this “electron fluid” which flows. A copper wire is like a pre-filled pipe. A battery is like an electron-pump. Batteries do not supply electrons, wires do. Batteries do not supply “current,” since “current” is not a stuff. Instead, batteries CAUSE electric current: they force the electricity of the copper to start flowing forwards.

Second, the name of the “stuff” that flows through metal wires is not “current”, instead it is “negative electricity” or “charge.” Current never flows. Yes, we can have a flow of charge, but we can never have a flow of “current.” Ask yourself what flows in rivers, water or “current?” Now try to imagine a dry riverbed with a flow of “current.” Impossible? Yes, and that’s why people don’t understand circuits. They think that “current” flows, but don’t realize that this “current” is actually a flow of charges which were already in the wires beforehand.

An electric circuit is like a leather belt which passes over some pulleys. Wires are like hollow tubes with a belt inside. If you connect wires in a circle, and if you push on the “leather belt” within, then the entire “belt” starts moving just like you’d expect.

Third: the explanations you encounter in grade school are garbage. Those science textbooks teach us that batteries supply the electricity. This is totally wrong. Wires are the source. They teach us that electricity is a form of energy. Wrong. Negative and positive electricity are components of matter. They teach us that whenever the switch is opened, the electricity vanishes. Nope. Instead, the electricity just stops flowing. It stops right where it is, and there it sits. (Remember that the wires were already full of negative electricity even before the battery was connected, so when you remove the battery, the wires are still full of electricity.)

See:

ELECTRICITY QUESTIONS (FAQ)
http://amasci.com/elect/elefaq.html

Good question!

Charge is electricity. Up until 1910, scientists used the word “electricity”, but today they use the word “charge” instead. It makes sense… CHARGE OF ELECTRICITY gets shortened to the word “charge.”

It’s hard to define “charge” because it is a fundamental entity like mass and time. If we know what charge is, then we can use it to define OTHER entities.

Here’s a list:

Charge is…
…the stuff that flows during an electric current.
…the stuff that appears on a balloon when you rub it on your hair.
…the stuff that comes in two kinds: positive and negative.
…the stuff that causes electrostatic attraction/repulsion forces.
…the positive and negative stuff that forms atoms.
…the stuff that’s carried by electrons, protons, positrons, and other particles.
…the Plus and Minus electrical poles (as opposed to North and South poles of a magnet.)
…the stuff that, when it wiggles fast, creates light.
…the stuff that, when it wiggles slower, creates radio waves.
…the stuff that, when it wiggles very slowly, creates energy in electric circuits.
…the stuff that, when it flows or spins, creates magnetism.
…the substance where all of the flux-lines of electrostatic fields connect to matter.
…the stuff that reflects light and makes objects visible.
…the stuff that makes metals look silvery.
…the stuff that causes electrical attraction and holds everyday objects together.
…the stuff inside of wires that’s movable, almost fluid.
…the stuff inside of nonconductors that is immobile and “frozen” in place.
…the stuff inside of semiconductors which acts like a compressible gas.
…the stuff that is measured in units called Coulombs.
…the stuff that scientists once called “quantity of electricity” and “particles of electricity.”

What is charge?
http://www.amasci.com/elect/elefaq1.html#n

You can’t have a flow of current, because the current is itself the flow: Current is flow of charge. Just as you might measure flow of, say, water in gallons per second, you measure the flow of charge in coulombs per second (also called amps).