Hi everyone. I don't understand why all the circuits I read about in books have voltage drops occurring starting with the positive voltage source. Shouldn't it be the other direction, from negative? Please help me understand this concept so I can move on to learning more important things.
Thanks

 

It is common problem. Try our e-book section http://www.allaboutcircuits.com/vol_1/chpt_1/7.html for a thorough explanation.

Dave

 

Hi everyone. I don't understand why all the circuits I read about in books have voltage drops occurring starting with the positive voltage source. Shouldn't it be the other direction, from negative? Please help me understand this concept so I can move on to learning more important things.
Thanks

In the schematics we use the conventional flow of current (current flows from positive terminal to the negative of a battery). In reality electrons flow from the negative to the positive terminal but it doesnt matter because instead of saying current is negative particles flowing from negative to positive, we say current is positive particles flowing from positive to negative which is essentially the same thing.

 

The confusion is historic - our forefathers made the assumption that current was movement of positive particles (intuitively) and hence the concept of conventional current flow was born; however, the reality was the opposite. By which time conventional current flow was embedded, and electron current flow became a new paradigm. If only electrons were "classified" as positive as opposed to negative then this whole conversation would be superfluous.

Dave

 

Personally I don't ever thing in terms of conventional current flow. Reality is complex enough without adding artificial concepts. But you have to be aware that the other concepts are out there.

 

Personally I don't ever thing in terms of conventional current flow. Reality is complex enough without adding artificial concepts. But you have to be aware that the other concepts are out there.

I agree, but then I typically approached electronics from a physics perspective, so that's my excuse. The notion of conventional current flow is a nice simplification for (some) circuit analysis, but I don't find any advantages of it thereafter.

Dave

 

Guys both ways are correct as far as you know what are you doing.

 

Which ever way you choose to go, the main thing is to be consistent.

hgmjr

 

Agreed, keep it consistent. If it works, then it works. Positive and negative are only constructs indicating direction - negative is to positive as positive is to negative.

It is still important to emphasise what is actually going on - the physical mechanism in action.

Dave

 

Thanks for all your help. I am definitely a bigger fan of the electron flow perspective, although I have not been reading schematics for that long. Are there situations where it's important to know which way the current is actually flowing in order to place components properly?

 

As diodes allow current flow in only one direction, it's real helpful to get them oriented correctly. Same for transistors.

 

Actually, yes. I deal in gold wire bonders, which bond 1 mil wires to the chip dies that are the components. During the process they use a small electric arc to melt the tip of the gold wire into a ball, which is critical to the process. It was discovered (after the fact) that if the electrons jump from the electrode to the wire the ions in the arc carry some of metal of the electrode. The gold wire is 99.99% pure, the resultant alloy is flawed. By reversing the direction of the flow the ions go the other way, gold goes to the electrode, problem solved.

The original scheme was called Electronic Flame Off (because before the electronic method was invented a hydrogen torch was used), now it is negative electronic flame off.

 

Conventional Flow v. Electron Flow= current polarity

Conventional flow is for PNP circuits only because they use a negative supply

If the voltage drop is a negative voltage then the current is flowing conventional

A negative power supply has current flowing in a negative direction this is conventional flow , to prove this try measuring with your DVM meter , set it to current and the red and black probes in the right polarity would be in conventional flow

Its the "current polarity" when your do R&D design is the different when dealing with conventional flow vs electron flow

 

Perhaps in future we will be using positrons?

Current isn't flow of electrons only, In electrolytic solutions both charges flow.

 

Which ever way you choose to go, the main thing is to be consistent.

This is the most important statement in these replies.

This is because the movement of electric charge (current flow) is also tied up with other physical effects, such as magnetism.
The physics of these effects has its own conventions. These days science has taken the trouble to mesh the various conventions together, it wasn't always so.

If you progress in the subject you will come to discover that you have the simplified version of Ampere's law. The complete version includes another term, called the displacement current which is the current not carried by electrons, ions etc and is the current for instance in a capacitor or one way electricity can propagate across free space.

 

Perhaps in future we will be using positrons?

Perhaps, but not the way your thinking. Positrons are actually a form of antimatter (the light helf of it), good old E=MC^2 applies. I believe their being used for remote scanners and some medical technologies at the moment, some radioactives emit them during their decay (I think).

 

This is the most important statement in these replies.

I'll disagree a little bit with that statement, sometimes, as in the gold bonder example, it matters a lot. It is important to have the details down straight to get a process to work correctly.

But then, when people are learning radio we use the classic AM examples, which are pretty far off the truth, but are easy to teach and let beginners get a handle on it before they are taught about sidebands. So I wouldn't argue the point too heavily.

One of the things I LOVE about electronics is sometimes how you look at a circuit can change how you use it entirely. Same parts, same arrangement, different concept.

 

The convention you use to calculate something has nought to do with real polarity of the terminals. You can call them positive and negative, Michael and Beelzebub or swallows and amazons they are still the same.

And if you wire them back to front the circuit will not work properly and may self destruct. This is as true of transistors as welding.

 

No, but what the transistors are controlling may care a lot, as in the case of EFO. Names may not be important, but having the facts straight is. Physics and electronics are aspects of the same thing.

Opps, and I said I wasn't going to argue the point.

It won't be the first time we have to change the text books, nor the last.

 

@studiot: I agree with you completely. The Electron / Hole concept may help to explain semiconductor physics better, but for the novice, it only muddies up the water.

AFAIR, there are two types of current flow in any circuit:

1 ) Electron flow, which travels from negative to positive.

-and-

2 ) Conventional current flow, or "Hole" flow, which moves from positive to negative.

What phenomina is actually being described is the ionic state of the atom after the electron has left the atom and migrated to the next atom. (it has a "hole" whre the electron was, so to speak.)

"If you envision a tube large enough to hold a golf ball, and fill the tube with golf balls, pushing a golf ball into the left end of the tube will make a golf ball come out of the right side of the tube."

That was the analogy I was taught to use when describing electron current. i.e. electrons migrate from atom to atom, traveling from the most negative end to the positive most end of the conductor.

Conventional current travels in the opposite direction of electron current. The analogy was a row of shopping carts at the checkout line in the supermarket, with each shopping cart being an electron. As the right-most shopping cart moved up, it leaves a "hole" behind it; the shopping cart then moves up from behind, filling the hole. The hole appears to be travelling from right to left as the shopping carts (electrons) move from left to right.

That's how I was taught to understand Electron / hole current. Perhaps the people who invented hole current were not comfortable with the notion of current flowing up from ground (common) into the positive terminal of their power supply (which, btw, is actually what's happening)

Interestingly enough, lightning current flows from the ground up to the clouds, although it appears to strike the ground from the clouds.

Perhaps that's an example of electron / hole current at work in nature?

:Flubbo.