Author Topic: philosophical question  (Read 621 times)

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Offline redloffTopic starter

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philosophical question
« on: January 24, 2013, 04:22:04 AM »
Please bear with me as I'm new to electronics and ideas of current voltage and so on...
There is one question that keeps bothering me for a quite a while now, but it's quite difficult to ask it in one sentence, so let me elaborate.

If I understand correctly, there is a formal rule that current (stream of electrons) flows from (+) to (-), while the real situation is the opposite - electrons flow from (-) to (+).
So if I have a power source (a 9V battery) and make a short circuit on it's poles, real electrons should flow from (-) to (+).

Consider the 50$ robot schematics where there are two separate power sources.
From my experience, I know, that for the robot to function properly I should connect all grounds together, otherwise it might not work. I understand this is due to the fact, that all parts of the circuit (electronics and servos) should have a common understanding of the zero potential, otherwise zero potential from one power source could be slightly higher/lower from the zero potential of the other power source, and in consequence, when currents meet at some point (most likely the uC) they might have a problem.

My question boils down to this: shouldn't we be connecting all cathodes together instead of anodes?
Since the formal current flow is reverted, the zero potential should be (+) while number of electrons cumulated on (-) side of the power source should determine the voltage value.
In other words, we know that the real current flow is reverted, but were are building circuits like if the formal flow was the real one.

Why?

Offline newInRobotics

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Re: philosophical question
« Reply #1 on: January 24, 2013, 08:31:17 AM »
While building something, don't bother Yourself with how electrons actually flow as it will get confusing.

Since the formal current flow is reverted, the zero potential should be (+) while number of electrons cumulated on (-) side of the power source should determine the voltage value.
Voltage value is determined not by the amount of electrons on the negative side, but by the amount of "holes" (available space to fit incoming electrons) on the positive side and internal resistance. Imagine it this way, if You have a bottle (positive/receiving terminal) tightly connected to the water hose (wire), which is connected to the tap (negative/providing terminal), You can fill the bottle only as much as its capacity allows, or actually You can fill it as long as pressure from the tap is higher than pressure in the bottle (difference in potential). Same with battery, positive terminal can take electrons as long as it has empty space to fit them. While receiving electrons, resistance (pressure) of positive terminal increases to the point where it is too high for current to overcome, that's when battery is considered "dead", when pressure from negative terminal is equal resistance (counter pressure) from positive one.

Battery terminals are marked "-" and "+" due to the fact that electron itself is a negative particle, and negative terminal contains materials that consist of negative ions (number of electrons is higher than number of protons) and positive terminal contains materials consisting of positive ions (number of electrons is lower than number of protons). Electrons will try to overcome resistance (if they are able to) and go fill empty spaces in positive ions making then regular atoms. Negative terminal has low potential because it already is full of extra electrons, hence its potential to take more is low. Positive terminal is high potential as it has plenty of space to take potentially incoming electrons. Now, if You compare high and low potentials, low potential relatively is 0 as it cannot take any electrons from the high potential.

Regarding to connecting positive terminals together instead of negative ones, take a look this simulation, left circuit produces signal (can be regarded as microcontroller) ans driven by 5V battery; right circuit is responsible for actuation (providing power to LED in this case) and is driven by 12V battery. When there is no "common" between batteries left circuit produces signal (click "Pulse button") as intended, however right circuit does not see that signal, hence does not work. When negative terminals are connected - both circuits work as expected: "Pulse" button push makes LED to turn off, releasing it turns it on again. When positive terminals are connected - right circuit is always on no matter "Pulse" switch position, which is not the desired behaviour.
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Offline waltr

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Re: philosophical question
« Reply #2 on: January 24, 2013, 08:32:26 PM »
Quote
If I understand correctly, there is a formal rule that current (stream of electrons) flows from (+) to (-),

False, in electronics the typical notation is the current flows from (+) to (-). This has been arbitrarily agreed upon and it fact it does not matter which direction used for the notation of current flow as long as it is consistent.
Quote
electrons flow from (-) to (+).

True- but this fact is not really needed in the study of electronic circuits. This fact is however required in the study of semiconductors.
Quote
we know that the real current flow is reverted, but were are building circuits like if the formal flow was the real one.

Again, Current flow is arbitrarily determined. Electron flow is from the negative source, through the circuit to the positive source. Do not confuse the two as being the same.
Current flow is just a notation whereas electron flow is Physics.
Quote
My question boils down to this: shouldn't we be connecting all cathodes together instead of anodes?

Circuits can be built either way and some circuits have both.
Consider circuits that have a signal that varies above and below the 'common' or zero potential. AC coupled Audio is a common type of signal. For example:
http://www.allaboutcircuits.com/vol_6/chpt_6/10.html
This circuit has a 'common' (Ground) and a two supplies (voltage sources), one that connects to the 'common' and +V, the other to the 'common' and -V. For the top half of the circuit the supply is positive with respect to the 'common' but for the bottom half of the circuit the supply is negative with respect the the 'common'.

Offline redloffTopic starter

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Re: philosophical question
« Reply #3 on: January 25, 2013, 07:12:04 AM »
While building something, don't bother Yourself with how electrons actually flow as it will get confusing.


I have to bother, this is my second nature...  8)

Negative terminal has low potential because it already is full of extra electrons, hence its potential to take more is low. Positive terminal is high potential as it has plenty of space to take potentially incoming electrons. Now, if You compare high and low potentials, low potential relatively is 0 as it cannot take any electrons from the high potential.


Thanks for clearing that up! So I understand we have a situation in which there is excess of electrons on one side and deficit on another, and so in terms of electrons being paired with protons we have:
[negative potential (anode)] <--- zero potential ---> [positive potential (cathode)]
Right?
We can say that negative terminal is low potential, because it can not absorb many more electrons, but on the other hand we can say that positive terminal is low potential because it can not be taken away many more electrons, so the only sensible approach for me is the zero potential being the average of negative and positive potentials.


Regarding to connecting positive terminals together instead of negative ones, take a look this simulation


Unfortunately I don't have any java-enabled browser in sight, but I think I can imagine what is going on from your description. I suppose this "pulse" is emited by "microcontroler" by making short circuit between output pin and (+).
Once again - in theory - this "microcontroler" could be designed to send "pulse" by making short circuit between output pin and (-).
Then we could invert (+) and (-) and consider (+) as "ground", right?

Offline redloffTopic starter

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Re: philosophical question
« Reply #4 on: January 25, 2013, 07:21:11 AM »
I think waltr's last example about AC coupled Audio confirms my predictions and answers my questions  ;D - thanks.

One more thing - the Earth (the Dirt as ancients would say) is considered kind of Master-Ground, right?
It is full of atoms with paired protons and electrons, so it is kind of "common" ground for both negative and positive voltages as in the waltr's audio example, correct?

Offline ErikY

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Re: philosophical question
« Reply #5 on: January 25, 2013, 07:24:13 AM »
Quote
If I understand correctly, there is a formal rule that current (stream of electrons) flows from (+) to (-),

False, in electronics the typical notation is the current flows from (+) to (-). This has been arbitrarily agreed upon and it fact it does not matter which direction used for the notation of current flow as long as it is consistent.
Quote
electrons flow from (-) to (+).

True- but this fact is not really needed in the study of electronic circuits. This fact is however required in the study of semiconductors.
Quote
we know that the real current flow is reverted, but were are building circuits like if the formal flow was the real one.

Again, Current flow is arbitrarily determined. Electron flow is from the negative source, through the circuit to the positive source. Do not confuse the two as being the same.
Current flow is just a notation whereas electron flow is Physics.
Quote
My question boils down to this: shouldn't we be connecting all cathodes together instead of anodes?

Circuits can be built either way and some circuits have both.
Consider circuits that have a signal that varies above and below the 'common' or zero potential. AC coupled Audio is a common type of signal. For example:
http://www.allaboutcircuits.com/vol_6/chpt_6/10.html
This circuit has a 'common' (Ground) and a two supplies (voltage sources), one that connects to the 'common' and +V, the other to the 'common' and -V. For the top half of the circuit the supply is positive with respect to the 'common' but for the bottom half of the circuit the supply is negative with respect the the 'common'.


This has been something that has been bothering me as well, ever since reading allaboutcircuits.

I have to say, I have searched for an explanation or understanding of this, and by far this is the absolute best response I have seen.

This is really helpful, and really clears up  lot for me!

Offline jkerns

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Re: philosophical question
« Reply #6 on: January 25, 2013, 09:14:39 AM »
My question boils down to this: shouldn't we be connecting all cathodes together instead of anodes?
Since the formal current flow is reverted, the zero potential should be (+) while number of electrons cumulated on (-) side of the power source should determine the voltage value.


Stick to driving English cars from the '60s and older and you won't have to worry about this since they used positive for the ground.

What really matters is that there is a path all the way around the circut from source through the load and back to the source. It doesn't matter what point in the circuit you call "ground" but using one end or the other of the source as ground is convienent. And, picking the negitive end is even more convienent since that is how a lot of equipment happens to be wired.

Just try findinga radio for a vehicle with positive ground... ;)
I get paid to play with robots - can't beat that with a stick.

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