I’m not exactly sure what you mean by “charge propagation,” but (as I’m sure you’re aware) the average drift velocity for the electrons in a wire is really low at most typical currents. Something like 1 mm/s if I recall correctly.
The velocity of the energy from source to load, OTOH, is a significant percentage of c. The energy is not in the wires, nor is it in the electrons. The energy in the electric and magnetic fields around the wires, and we use Poynting Vectors to describe them.
The example I like to use, because I worked it out once for a project for middle-schoolers and it happens to come to round numbers, is that a foot of copper wire hooked up to a 1.5 volt battery like a C or D cell will have electrons in it moving at 1 cm/s.
I then used that velocity as an example of “relativistic speed”, which can be demonstrated by wrapping the foot of copper wire around an iron nail and using it to pick up paperclips.
I had to switch from physics convention to engineering convention. When diodes & transistors start talking about cathode & anode I still get tripped up.
Propagation delay is important in digital work - delay in a single line doesn’t usually matter, it is making sure you have the same delay in all the wires in a bus that is important. There is also this thing called a delay line where you do fun math tricks by making part of a signal lag behind by a clock cycle or two.
Back to the light-year wire question, normally you switch to current & voltage or em waves instead of thinking about electrons. The signal travels in the wave very fast, the electrons just wiggle back and forth in one place. Playing crack the whip with a length of rope is the example I remember. This is also important because you can have multiple signals traveling down a wire at different speeds (frequency dispersion). Uh, tldr; nanosecond differences definitely matter, pico not so much, yes it would take at least a year to zap you.
I heard really scary stories from the older folks, but my respect for electricity came when I saw what happens if you leave out the center conductor in coax, then apply a few kW. Lotta magic smoke that day…
Use X<sup>2</sup> for superscript X2 and X<sub>2</sub> for subscript X2.
To display the examples like I did use backslash \ in front of the angle brackets to “escape” = neuter them as format control characters and make them simply display as whatever character they are. To display a backslash as a backslash you need to escape it too with a second backslash.
How’s this: V*(1-e-t/RC)
You can quote my post in your edit box and look at what I typed to make that look like that.
Would you elaborate? What happened to .8 c? (We got .8 C in coax in an undergraduate lab.)
Bits of digression:
Dad and I were building a power supply, and I got the electrolytic in backwards. It sounded like a gun went off, and the house was festooned in a long roll of mylar.
There is a story of a guy in military electronics school pushing volt meter probes into his thumbs with such force that he pierced his skin and got 25mA through his heart with fatal results.
There’s a difference (big difference) between the drift speed of the electrons and the speed of the energy.
The average drift speed of the electrons in the wires is very slow, around 1 cm/s. The speed of the energy (from source to load) is very fast, usually somewhere between 0.5c to 0.8c. As I noted above, this energy is in the electric and magnetic fields around the wires. See Poynting Vector for more info.
To bring back the analogy of a narrow tube full of peas. The drift velocity is how fast you’re pushing the next pea into the tube. The signal speed (e.g. .8c) is how quickly this results in a pea falling out at the other end. (The signal speed in the tube of peas wont be .8c)
But it will be about 0.8 of the speed of sound in peas.
The “speed of light” is really the speed of charge propagation.
The “speed of sound” is really the speed of physical pressure propagation.
Both measured in the relevant medium of transmission.
Ah, hole theory. I’m getting electronics school PTSD just thinking about it. Even knowing that 12VDC is insufficient breakdown voltage, touching car battery electrodes still makes me nervous and I’ve been zapped by far higher voltages (300V through the hand during an exam in my tube theory class…). Didn’t have feeling in it for a little while.
Keep in mind that British cars had positive grounds and they worked in more or less the same way as N.American cars so it’s less about which way the charge moves and more about how you design the circuit.
Don’t do this. Kids in wet weather can be actually killed by a car battery, so you’d be setting a bad example. Kids are smaller, and typically their skin is more conductive, and being wet gives you better conduction through the skin.
In normal circumstances you won’t even notice grabbing both terminals of a car battery: the significant health-and-safety risk is dropping it on your foot. Handle weights and acids carefully, keep conductors away from the terminals, don’t touch the terminals when wet, and keep it away from kids.
Wow, I don’t like the idea of experimenting around with car batteries to see what will shock me. I generally don’t worry about 12 volts, but if I got curious how well I could feel it with clean sweaty skin, I’d put a forearm across it. I wouldn’t conduct the current across my chest, and wouldn’t conduct it through the muscles I’d have to use to move myself off it.
I’ve tested many 9 volt batteries with my tongue, and know there’s a decently strong shock sensation involved. And I also know I’m not going to freeze in that position, and my tongue will be fine afterwards. The point is that it doesn’t take that many volts to get a significant reaction.
I don’t think kids should be playing around car batteries, but I’m a little skeptical about this.
Wet skin is going to drop it’s effective resistance to maybe a couple thousand ohms (a very significant drop from the tens to hundreds of millions of ohms it would be dry, to be sure). If we figure a worst case of maybe a thousand ohms, that’s going to result in a current of 12 mA. Most safety standards are built around 5 mA as the maximum “safe” current, and yes, 12 is greater than 5, but you are just barely above the safe rating at that point.
Does anyone have a cite for a kid (or anyone else) ever being electrocuted by a car battery? Seems to me like it’s one of those things that is technically possible, but would be so rare that it might not have ever happened.
Mine is 470K Ohms.
Leave it to the Brits to do things the weird way around. US railroads – and almost everybody else – use brakes with positive air pressure to stop the train. The UK went with a vacuum system instead. Among other disadvantages, this means the maximum pressure differential is 15 psi instead of 90 or 110 (freight vs. passenger).
OTOH, vacuum systems fail safe. Pressure systems fail unsafe. There’s always a tradeoff.
In the 2020s failures in well-engineered pressure / vacuum systems are rare.
In the 1820s, not so much.
Eh? I don’t know about rail, but on heavy trucks air pressure is required before the service brakes will disengage. If the truck loses air pressure on the road, the service brakes engage, stopping the truck. it may not be the safest stop, but it’s better than a truck with no brakes.
Nope.
But if you look at the simple numbers, 110V is safe – which is why it is safe, people touch onto 110V all the time and aren’t hurt at all. Yet a small number of people are killed by 110V, even when not standing in a puddle of salt water and holding on with both hands. There is no way 110V on 500K can get you anywhere close to 100mA, but deaths happen.
As with adults, the main danger to kids from batteries is being underneath it when it falls. There is a reason batteries are normally stored on the floor or just above it. Even in shops, normally on the low shelves. Small wet kids come inside the envelope of shock grabbing danger (10mA), but a lot more kids put mains power sockets in their mouth, and even that doesn’t normally kill them straight up – there has to be something else going on.
++ PUTTING A SOCKET IN YOUR MOUTH IS DANGEROUS. IT MAY CAUSE IRREVERSIBLE BURN DAMAGE
On a Westinghouse Train Brake system, a reservoir of compressed air is maintained on every carriage, so that if the supply is lost, broken or disconnected, the air pressure automatically applies the brakes. These local reservoirs characteristically bleed out over time, so the driver MUST apply the standing brakes if turning off the engine and leaving the train. Failure to do so continues to cause accidents: we had a mining train off the rails in the last couple of years because the driver didn’t do so.
In the 'Vacuum" system, the brakes are applied by a steel spring. If the local reservoir bleeds out, the brakes are applied.
Nobody uses vacuum systems anymore, because of the disadvantages. Systems derived from the Westinghouse brake are used, because they are generally better/safer than the vacuum system, even including the fact that they are directly implicated in a continuing series of accidents.
Trucks? When I was a kid, loosing pressure in the service reservoir was a known problem that mostly only happened when you had other air services as well as engine failure. It doesn’t seem to be a problem now: Apart from not having air-pressure operated doors and windshield wipers on the same system, I don’t know what improvements were made.
Here (wiki) is a 2013 Canadian railway disaster caused by bleed down. It was (in)famous throughout North America when it happened; I don’t know how much play it got Down Under.
Ultimately the key difference between trucks & trains is nobody expects to move a truck trailer without hitching it to a live truck with a live air supply. So in the absence of air supply the brakes lock firmly and anchor the trailer in place.
Conversely, longstanding railway practice is that loose railcars may be moved by gravity or a tow rope or a dozen beefy guys shoving or …
And so each railcar must be left with the powered brake circuit disengaged. The iconic big steel wheel on the upper ends of each boxcar is precisely so the otherwise released car brakes can be hand-cranked to a (weakly) engaged state.
I was told in chemistry class that in physics, the direction of current was the direction the net positive charge moved, but in chemistry, the direction of the current was the direction the electrons moved. I haven’t taken any reasonably high-level physics class to confirm this, but this thread seems to indicate that what I was told was true for engineering, but not physics. Or maybe physics changed their convention to match chemistry to make science consistent but making EE inconsistent with physics. Or maybe my teacher had only studied electromagnetic physics from an engineering perspective.