Tangentially: as mentioned above, semiconductors (transistors, electronics) and ionic solutions have both positive and negative charge carriers. And the speed of the positive (holes or ions) is always more-or-less different to the speed of the negative (electrons or ions), and that does matter.
Not at the terminals: you can’t feel any difference (and charge carriers are converted at the interface), but inside a transistor the charge carriers do travel at different speeds, and it does matter. In other situations, (inside you, or long transmission lines), there may be only one kind of carrier, so the relative speed doesn’t matter, but the absolute speed does.
Or something like walking pace for a high-current power transmission line, or, according to my lecturer, even 4m/s (bicycle speed) for an old high-current power transmission line.
Semiconductors don’t have any positive charge carriers. Hole current is really electron current. We just pretend there are positive charge carriers to simplify things.
And yet, the Hall effect in hole-carrier semiconductors shows a flow of positive charges, not of negative ones.
On the aside about train brakes, what’s wrong with having springs to apply the brakes, and a positive-pressure system to hold the springs open when not in use? That’d allow arbitrary pressures and still be fail-safe.
LSLGuy’s comment above seems to say that they want to be able to move railcars “manually” (without a locomotive) so requiring an air supply to disengage the brakes would make that a lot harder.
heavy trucks work just like you said, though. the brake acutators at the wheels are sprung so that the brakes are applied in the absence of air pressure. this works OK because 99.9% of the time (a statistic I just made up) a trailer is being moved while attached to a tractor with air supply.
this is also why heavy trucks don’t need a separate parking brake. I found this out after getting a ride in an air-brake equipped F-750; it had an automatic trans and the gear position indicator only said “R-N-D-M-2-1.” On those when parking you just shift to Neutral and pull a knob to cut pressure to the brake actuators.
Actually, if you want to move a rail car on level track and have no equipment, the usual practice is to jam a pinch bar under a wheel and heave on it, lather, rinse, repeat. If the track is not level or needs to be moved more than a few feet, you’re going to need a gypsy head or tractor.
There is no “power brake circuit.” Freight cars have no electrical connections and everything is done by air pressure alone. With the automatic air brake system used in most of the world, an engineer wanting to release the brakes on a train applies pressure to the train line. The train line runs through flexible couplings down the entire length of the train and the triple valve on each car senses that the train line pressure is higher than the pressure in the auxiliary reservoir, and moves to a) release the air pressure in the brake cylinder and b) connects the auxiliary reservoir to the train line.
This lets the auxiliary reservoirs charge up to 90psi in the US (for freights) and if the train was just assembled or has been sitting a long time the process can take upwards of a half hour. It will be held in place by hand brakes on the locomotive(s) and probably several cars during this time.
When the train is moving and braking is needed, the engineer releases pressure from the train line – say lowers it 5psi for a moderate application. The triple valve senses the train line pressure is now lower than the auxiliary reservoir pressure and moves to release air from the reservoir to the brake cylinder until the aux reservoir matches the train line – 85psi – and the brake cylinder has 5psi in it. Through mechanical linkages this makes the brake shoes press against the treads of the car wheels (discs on passenger) and the train slows.
To release, the engineer raises the train line’s pressure to 90psi again, and the same things happen as when charging the line the first time around. This means that by adjusting a small amount of air in the train line, a great deal of stored air in the train can be used to control the train and also should the train line part, as in a derailment, the pressure in the line drops to zero and 90psi is in the brake cylinder bringing the train to a halt as rapidly as possible.
One disadvantage is that a ham-fisted engineer on a long downgrade can apply, release, apply, release, etc. not giving the auxiliary reservoir enough time to recharge. This is bad juju.
When a car is parked at a siding the air in the brake cylinder will keep it there for a while but inevitably, it will leak away; that is what the handbrake is for. However, a puny human cannot cinch the brake as tightly as the air can. If the brakeman merely cranks on the wheel and walks away, when the air leaks away the linkages go slack and the car might go a-wandering.
Therefore SOP is to pull on a handle at the side of a car by the brake cylinder which releases air from the cylinder, then crank on the wheel until it can’t be tightened any more. Apparently this is easy to overlook which is why wheel chocks and derails were invented.
I would recommend any more posts on train or truck brakes be moved to a new thread as this has zip to do with electrons.
As to the risk of electrocution there is an excellent article on Electrical Injury on Wikipedia, here:
There’s a citation for an industrial electrocution fatality at 42 volts. If somebody in industry, presumably an adult, died at 42 V, I’m not confident about a child at 12 V. But, no, I didn’t find any cites about car batteries electrocuting children.
Thanks for the superb detailed explanation that cleared up a lot for everyone. Including me.
FYI by “powered brake” I meant air-powered as distinct from human-powered. And by “circuit” I meant the long line of connected air hoses from head end loco to last trailing car. Trying to use generic laymen’s terms can make it easier for non-experts to communicate at the expense of misleading the actual experts. Thanks for filling me/us in.
Just to be clear, if I put a light bulb close to the negative battery terminal it will light up faster than if I had put it close to the positive terminal, correct? And if so, don’t issues like that matter when designing things things like cesium clocks?
No. In the case of a simple circuit like a light bulb and a battery it would make no difference; or so small as to be irrelevant. Since there is no current flow until you close the circuit the whole line has the same potential up to the switch. More complex circuits have to deal with inductance and a host of other factors as noted by @engineer_comp_geek.
Light bulbs are normally connected using copper wires. Copper has only negative charge carriers, and they move into one end of the wire at the same speed that they move out of the other end of the wire. It doesn’t matter where in the wire you put the light bulb: light happens when electrons move through the light bulb, and just as many move just as fast at both sides of the battery.
If you had a very long wire – several light-seconds long perhaps – then a charge detector in the far middle of the wire would detect charge movement after it had already happened at the plus and minus sides of the battery. It’s just electrons: they may take a while tp feel the push, but that’s just distance and speed, there aren’t different things on the plus or minus side.
This kind of action happens by electrons pushing on each other. The push is felt much faster than the electrons move.
There are other situations, like inside Transistors, where you are trying to change the Charge Density, the number of electrons (or holes) inside a volume. That depends on the speed of the electrons coming in. Slow charge carriers move slower: fast charge carriers move faster, the volume fills or drains slower or faster. When you are collecting [electrons], more come in one side than go out the other. You may want to arrange the physical structure of the transistor so that some things are near where the electrons come in, and other things are near where electrons go out. You may want to arrange things so that you are using fast electrons or slow electrons. Transistors aren’t light bulbs, they aren’t copper wire, and we are talking about something different than “left side of the battery / right side of the battery”
Holes are not mobile atoms or molecules with a positive charge. Hole current is due to the movement of electrons.
A good analogy is the sliding puzzle game. The tiles are like electrons, while the missing tile is analogous to a hole. The movement of the missing tile is really due to the movement of the tiles.
yeah, this is where I think the usual analogy of comparing an electrical circuit to water flow in a hose breaks down. with electricity the copper wire doesn’t have to “fill” before current starts flowing.
Usually, it doesn’t matter whether you think of a conductor as containing moving positive charges, moving negative charges, or both, and so you can just use whichever convention is most convenient, regardless of whether it describes “reality”. And for certain semiconductors, it’s more convenient to talk of positive “holes” moving, even though the holes are just lack-of-electrons.
Usually. But there are a handful of experiments you can do where it does make a difference, such as the Hall effect. Take a ribbon-shaped conductor, and run current through it. Put a magnetic field perpendicular to the ribbon. The magnetic field will cause the moving charges to deflect to the side, and so you’ll end up with a voltage difference between the left and right sides of the ribbon. And the direction of that voltage difference turns out to depend on the sign of the moving charges.
Well, if you do this experiment with a hole semiconductor, you do, in fact, find that the voltage difference is opposite of what you would get with a metal conductor. In other words, it turns out that it really is positively-charged holes moving. Huh.
I don’t disagree with anything you said. Put another way, semiconductors contain just one type of mobile charge (electrons), and their movement can be modeled as a flow of negative charges or a flow of positive charges. But - when it comes down to it - they’re all really negative charges.
Not so with batteries. In batteries, current is due to mobile ions, not electrons.
Where you place the light bulb, assuming that you are using the action of inserting the light bulb to complete the circuit, would make no difference.
The battery is creating an electric field at all times(assuming it has charge). Completing the circuit just allows electrons to be pushed along by it. Having wires hooked up to the battery, with a gap between them, is essentially making a very weak capacitor.
However, where the light bulb is in relation to a switch would. If you already have the light wired into the circuit, and have switches at different distances, then there would be a longer delay to light up the bulb the further the switch is from it. You would need a really good stopwatch, or a very long wire, to measure this, though.
Designing things like cesium clocks requires a whole lot of precision and math. I’ve never built one myself, but I’d think that precision in the millimeter range would not be nearly good enough.
Just to be clear, mobile ions move inside the battery. Current flow through the wires is still only electrons. Negatively charges ions (ions with an extra electron) migrate to the cathode and can give up the extra electron (if current is being drawn from the battery). Positively charged ions (ions missing an electron) migrate to the anode where they can pick up an electron (again if current is flowing).
I’m an engineer and former Electrical Officer on a nuclear submarine, who also taught general chemistry and physics for seven years—so I’ve seen just about every convention out there.
Anyway, the one place in a general chemistry class that electrical current comes up is the chapter on electrochemical cells, comprised of electrolytic cells, galvanic cells (batteries), and fuel cells. And because most of the focus is on what’s happening inside the electrochemical cell, the flow of electricity in the rest of the circuit is a secondary consideration. For that reason, in a chemistry class I would usually avoid using the terminology of electrical current In the wire outside the cell entirely (thereby sidestepping the whole issue) and instead simply refer to the direction of flow of electrons in the wire. This would be drawn on the chalkboard/whiteboard as a lowercase e– along with an directional arrow next to the wire.
In a physics class, there is generally a whole semester spent on electricity and magnetism, with a chapter or two on circuits. The direction of electrical current in the circuit is a very important consideration. In my experience, the conventional definition of current is used in a physics class (positive flow convention).
Last but not least, I recall being told during my Navy training that decades ago (until the 1960s?), the Navy used a negative flow convention for current (because it was supposedly more correct). Unfortunately, this led to widespread confusion when dealing with people not used to this convention (not to mention the Navy electricians who went to work as civilians upon leaving the service). Eventually the Navy bowed to the fact that the whole rest of the technical world was using the opposite convention, so they finally switched to this as well. This was all ancient history by the time of my Navy training in the 1990s, where we used the positive flow convention for current.
Exactly. (Except that I always tried to avoid saying “current flow” in a chemistry class for the reasons described above, and instead referred to the flow of electrons in the wire.)
The analogy I read (in 1973 when people did this a lot) was a line of cars waiting in line for a gas pump. The lead car departs the pump and one at a time everyone else moves up. The hole they’re moving into isn’t anything physical moving, but it nevertheless propagates backwards from direction the physical entities that were moving.
In the late 60’s, mid 70’s, as part of the same movement that lead to ‘new math’, people wrote textbooks that used electron-flow conventions for electric current. So that students would be taught a ‘deeper understanding’ as the basis for a wider understanding and foundational for further learning.
This was particularly important to the military, because the military was one of the prime movers in demanding that students actually learn math and science – something that school teachers had observed was of no value in their own life and career, and which they had become less inclined to impose on their students.
This particular idea fizzled out, because, as discussed here, it’s actually confusing to talk about the ‘real direction of current flow’ before physics and chemistry.
(Except that I always tried to avoid saying “current flow” in a chemistry class for the reasons described above, and instead referred to the flow of electrons in the wire.)
