Ok, like I said, I was clinging to some linear ideas. I appreciate all the examples.

I'll be back with more silly questions.

You are not alone -- it is a common trap. Unfortunately, there isn't a single common way to break out of it. You have (had) misconceptions based on some wrong viewpoint of how things work and until you encounter an example or explanation that sheds light on that misperception, you will naturally continue to try to view all of the examples through that viewpoint and, hence, have a hard time correcting it.

Perhaps this might help (your or others down the road).

I think part of the misconception is due to the use of a water analogy, which is very useful but can't be pushed too far.

Let's say that we have a high pressure source of water, perhaps 3000 psi, and we want to run it through a filter. The filter can handle 200 psi across it's ports and, at that pressure differential, flows 10 gpm (gallons/minute), so we put a restrictor in the line that drops 2800 psi at 10 gpm. So the question is whether it makes a difference which order the two components go in and, in this case, it does. If we put the filter second, then the inlet of the filter is at 200 psi and the outlet is at 0 psi and everything is fine. But if we put the filter first, then the inlet of the filter is at 3000 psi and the outlet is at 2800 psi and there is a good chance that the filter will burst. However, the big difference here is that we have a physically meaningful zero pressure reference, namely the air outside the filter, that the filter is exposed to and interacts with. We can't arbitrarily declare the output of the pump to be 0 psi unless we also recognize the impact of the air outside the filter being at -3000 psi.

The case with electrical components is very different in that, up to a limit, they don't have some meaningful external zero voltage reference -- in the filter analogy it would be the same as building a filter that is so strong that the pressure at or in the filter relative to the air around it has no significance.

But we can use this idea to get a better feel for what is going on, too. In point of fact, that resistor IS sensitive to the voltage difference between it and it's surroundings (in addition to the voltage across its terminals). If the voltage at either terminal gets too high relative to what is surrounding it, then we will get an arc between the two. This can and does happen. But in the vast majority of situations the voltage (relative to the surroundings) needed to cause this is so high, hundreds of thousands or even millions of volts, that we can ignore it. It would be like working with a filter capable of withstanding 30,000,000 psi. If our system pressure never exceeds 3,000 psi, we don't even need to think about it and only care about the pressure difference across the filter.

So, when working with a mental image of a water analogy, assume that the components are so strong that they are never at risk of bursting to the outside air. Hopefully, by the time youi get to the point that you are working with voltages where this assumption isn't valid, you will have enough experience that you won't think in terms of a water analogy -- in fact, at some point you may start thinking about physical systems using electrical analogies.

 

It helps if you view the water analogy as a closed loop pipe with circulating water, not open to air.

 

It helps if you view the water analogy as a closed loop pipe with circulating water, not open to air.

I've never seen a water analogy to electrical circuits where that wasn't the case. But that doesn't help here. The problem that he was struggling with was thinking in terms of the "absolute" voltage at one end of the LED and being concerned that it was too much and that it needed the resistor first in order to drop the voltage before getting to the LED. That's completely analogous to needing to drop the pressure in a closed hydraulic system before getting to a series component that can't handle the pressure -- the order of the components matters on a very intuitive level in such a system and that intuition tends to carryover inappropriately to the electrical circuit case.

 

Here is something I haven't asked in all of this.

You have: battery > resistor > led, then back to battery. What's the current that's on it's way back to the battery (asuming a standard 20ma LED...one of those 5V / 20ma ones)?

If I understood you correctly (the closed loop idea did help), if it's battery>led>resistor (back to battery), it should be the same as above, correct? Because it IS a closed loop.

 

If I understood you correctly (the closed loop idea did help), if it's battery>led>resistor (back to battery), it should be the same as above, correct? Because it IS a closed loop.

Absolutely.
A fundamental principal of electricity is that the current is the same in all parts of a series loop.
The voltage across the various components can be different (all adding up to the source voltage) but the current cannot change.

 

That's completely analogous to needing to drop the pressure in a closed hydraulic system before getting to a series component that can't handle the pressure -- the order of the components matters on a very intuitive level in such a system

But the whole point is, that intuition is wrong.
The order of the components doesn't matter in such a closed-loop hydraulic system, any more than it does in the electrical loop.

 

Here is something I haven't asked in all of this.

You have: battery > resistor > led, then back to battery. What's the current that's on it's way back to the battery (asuming a standard 20ma LED...one of those 5V / 20ma ones)?

If I understood you correctly (the closed loop idea did help), if it's battery>led>resistor (back to battery), it should be the same as above, correct? Because it IS a closed loop.

What you are now encountering is known as Kirchhoff's Laws (pronounced kerkoff).

#1 - Kirchhoff's Current Law - conservation of electric charge - the total algebraic sum of all currents at a single node is zero.

Example: the resistor is connected to an LED. This constitutes a node.

The current entering the node + ( -current leaving the node) = 0.
Note that we have to be consistent with the sign of the current. Current leaving the node must have opposite sign of current entering the node.

It would therefore follow that the current around the loop: battery - resistor - LED, back to battery must be the same.

#2 - Kirchhoff's Voltage Law - conservation of energy - the total algebraic sum of all voltages around a loop must be zero.

Vbattery + Vresistor + Vled = 0

Hmm, this does't seem right, does it?
Again we must pay attention to the sign of the voltage drop

This is the correct sum:
Vbattery - Vresistor -Vled = 0

 

But the whole point is, that intuition is wrong.
The order of the components doesn't matter in such a closed-loop hydraulic system, any more than it does in the electrical loop.

Ah, yes it does. As someone that used to work on the hydraulic systems of jet fighters, I can most assuredly attest that it does. I gave an example of where it very much does. If put a return line filter into the pressure line, there is a very real likelihood that that filter housing is going to fail catastrophically for precisely the reason I described -- the component is not only aware of the pressure differential across it, but also of the pressure differential between each port and the surrounding air.

 

but also of the pressure differential between each port and the surrounding air.

But then it's not a closed system.

 

But then it's not a closed system.

Huh?

Then there is no such thing as a closed hydraulic system.

 

I suppose there could be if the system in questions is contained within an entirely pressurized vessel, compartmentalized and specific to each "node" of the system?

 

Huh?

Then there is no such thing as a closed hydraulic system.

Why not?
You just build the system and seal it.
I would assume all rocket hydraulic systems that operate in space work that way.

Anyway we are talking about a theoretical hydraulic system as an analogy for an electric circuit.
It doesn't have to be buildable.

 

I see though, now, why the flow doesn't matter, so long as you are aware of it. Whether you've 'slowed' things upwind or downwind it's all going to affect the system eventually.

But, is the LED more endangered by current or voltage?

 

ThirtyWest......what are you studying as a student?
And where are you....if I may ask?

 

But, is the LED more endangered by current or voltage?

You can't really separate one from the other.
An increase in voltage across the LED will cause an increase in current.
But LEDs are low impedance devices so it takes only a small increase in voltage across it to cause a large (and damaging) increase in current.
That's why you must use a resistor or other current-limiting circuitry to regulate the current through an LED.

 

Why not?
You just build the system and seal it.

You need to make up your mind. First you claim that a component that can burst because the pressure inside exceeds its maximum pressure capacity means that it isn't a closed system, and then you insist that all you have to do to have a closed system is seal it up. Which is it?

I would assume all rocket hydraulic systems that operate in space work that way.

And do you honestly think that there is a single component in that hydraulic system that is not sensitive to the absolute pressure inside of it as well as the pressure differential across it?

Anyway we are talking about a theoretical hydraulic system as an analogy for an electric circuit.
It doesn't have to be buildable.

No, we are not talking about some theoretical, unbuildable hydraulic system. The water analogy entered the discussion expressly because I was exploring the possibility that one reason why people often have trouble understanding that voltage at a point is largely irrelevant in the vast majority of electric circuits is because when they try to relate it to a water analogy they usually have an intuitive sense that in a fluid system it is not just the pressure differential across a component that matters, but it is also the pressure within the component relative to a non-arbitrary external reference.

Consider the following very simple scenario.



According to you, these two hydraulic circuits are effectively the same since the only difference is swapping the order of two series components. In both cases, the pressure reducer has 2950 psi across it and the sight glass has 50 psi across it. But you put that second one together and you are going to blow that sight glass to smithereens.

In a similar fashion, you could use a garden hose for the tubing connecting the two components on the left but if you did that on the right, no more garden hose.

As an extremely real world example that the order of series components matters in a hydraulic system, the Stratopower hydraulic pumps on an F-15's utility system put out 57 gpm at 3000 psi and there are two of them in parallel (with a 250 psi check valve in series with one of them, but we can ignore that). The pump uses controlled leakage past the pistons and into the case to lubricate the pump. The fluid from the pump case is vented through a check valve (12 psi??) to the return line, which is maintained at 45 psi, and then back to the 14 gallon reservoir. The check valve works very much just like a diode and if it is put in backwards it acts like an open switch. If the order didn't matter, then this would just result in no fluid flowing and everything would be fine (other than the lack of lubrication). But my supervisor (not the world's brightest bulb) couldn't grasp that this valve really did need to be put in the right way. So he sent a pump manifold out to the line with it in backwards. They put it on the jet, fired it up, and because the order DOES matter, the pump case not only had no flow, but rapidly built up to case pressure of 3000 psi, blowing apart long before it got there. The other pump was more than happy to empty the reservoir in about 15 seconds, which then starved that pump and seized it up. So there go two $25,000 (in 1986 dollars) pumps. Thinking that the pump that failed must have been bad, they put two new pumps on it and promptly blew those apart. So after $100k worth of pumps destroyed in less than an hour, they finally looked at the manifold and found the check valve in backwards. If the check valve had been on the other side of the case (between the case and the controlled leak) that wouldn't have happened; unfortunately that wasn't an option since the leak is an integral part of the pump's piston/cylinder tolerances (plus, the check valve is located in the manifold).

Oh, and about a month later he did the exact same thing with the same result -- not the brightest bulb in the box. The rest of us got in the habit of sneaking a peek at his manifolds before they were picked up and, as often as not, putting the valve in the right way.

 

I see though, now, why the flow doesn't matter, so long as you are aware of it. Whether you've 'slowed' things upwind or downwind it's all going to affect the system eventually.

But, is the LED more endangered by current or voltage?

Since the two have a pretty rigid relationship, it's somewhat of a meaningless question. However, the voltage across a forward biased LED changes only slowly with current, so the power dissipated is usually thought of being a largely linear function of the current and it is the current we almost always try to limit. Most of the failure modes are also dictated by the current.

 

First you claim that a component that can burst because the pressure inside exceeds its maximum pressure capacity means that it isn't a closed system, and then you insist that all you have to do to have a closed system is seal it up. Which is it?

I made no such claim.

According to you, these two hydraulic circuits are effectively the same since the only difference is swapping the order of two series components. In both cases, the pressure reducer has 2950 psi across it and the sight glass has 50 psi across it. But you put that second one together and you are going to blow that sight glass to smithereens.

Again, this is not a closed system. It has the pressure to the outside world as an additional variable.
In an analogy for an electric circuit it would only be the pressure across the sight glass, that is of interest, not the pressure to the outside world.
You keep confusing the issue by introducing real world effects that don't apply to a simple theoretical analogy.
I'm not sure why you insist on doing that as it really muddies the discussion (and the simplicity of the analogy), but I see that you need to insist that you are correct, so I will comment no further on this.

 

I made no such claim.

The perhaps you might read your own words in Post #29

Again, this is not a closed system. It has the pressure to the outside world as an additional variable.

Again, make up your mind. You just got done saying that a sealed hydraulic system on a rocket is a closed system and now you are saying that it isn't. Which is it?

In an analogy for an electric circuit it would only be the pressure across the sight glass, that is of interest, not the pressure to the outside world.

And the person that is using the analogy to the electric circuit and knows that the site glass will fail or not fail depending on which order they are in can carry that expectation across to the electric circuit -- which is why I pointed out that no analogy can be carried too far and went on to explain a possible way in which people my let themselves get mislead by doing so.

You keep confusing the issue by introducing real world effects that don't apply to a simple theoretical analogy.
I'm not sure why you insist on doing that as it really muddies the discussion (and the simplicity of the analogy), but I see that you need to insist that you are correct, so I will comment no further on this.

Then perhaps you might read my words in Post #21 to see why the real world effects are germane to the discussion regarding one possible reason why so many people might suffer the same misconceptions as the TS.

 

The perhaps you might read your own words in Post #29

By closed system I meant one that is completely contained unto itself, so the relative pressure to anything outside the pipe is of no consequence.

And the person that is using the analogy to the electric circuit and knows that the site glass will fail or not fail depending on which order they are in can carry that expectation across to the electric circuit

But there is no electrical analogy to a sight glass breaking in the electric circuit (unless you are talking about something like corona discharge or arcing), so that is a red herring.