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Conventional current. A question about DC current flow. Answered

I just started reading electronics for dummies and it tells me that DC current flows from negative to positive.  It later states that in the early days of electronics it was assumed that current flowed from positive to negative.  Later they discovered that it flows from negative to positive. 

Some schematics will have an arrow for current flow going from positive to negative (conventional flow).   Why do they still do this if they know the current flows from negative to positive?  This is confusing. 




Best Answer 8 years ago

Electron current flow does go from negative to positive. Conventional current flow is assumed to flow from positive to negative. It really doesn't matter at all. However with the circuit representations (diagrams) of diodes, transistors, etc the symbols used represent conventional current flow (direction of arrows, etc). Generally it is easier to understand the conventional current flow ideas then once you've got the hang of it just remind yourself that it is actually backwards (electron flow). Anyway, if you think that current flow is confusing just wait for the difference between EMF (electromotive force) & PD (potential difference). They seen the same but they aren't. Keep studying and, as always, ask questions.

In the early days of electricity, they GUESSED that it flows from positive to negative... however, science discovered years later that the flow was from electrons from negative to positive.  Nobody wanted to REPRINT all the old textbooks of electronics, so only SCIENTISTS work with negative electron flow... but if you are just a technician or electronics person, we keep the idea of voltage flowing from the POSITIVE terminal throughout the circuit.  THAT is how it was explained to me in my electronics classes at college 30 years ago.

Ok, I've been reading further in the book and everything states that current flows from positive to negative.   So which is it?  Does current flow from + to - or from - to +?? 

Also, I don't see how the result can be the same if I put the resistor on either side of the bulb.     Lets assume that anything over half an amp will blow the bulb.  If I place a resistor between the positive and the bulb, the bulb will only get half an amp.   But if I put the resistor between the negative and the bulb, how is that lowering current if the current flows through the bulb BEFORE it flows through the resistor?  

I guess I'll keep reading the book, perhaps it'll explain it later.

One of the best ways, initially, and for the DC case to understand what's going on is to consider a water flow model of a circuit. Imagine a circuit composed of a pump, some connecting pipe, a valve and a couple of thin pipes, thinner than the connecting pipes. Put all the bits in series, out of the pump, into a pipe, out the pipe into a thin pipe etc etc, and return to the pump.

Fill the pipes with water, and start the pump. The  Pressure of the water in the pipes is analogous to voltage, the rate of flow of the water is analogous to current. Suppose you measure the flow rate in the pipes near the pump outlet. What is it ? Now measure it at the pump INLET, what is it ? Rearrange the bits in the loop, what is it now ? 

CURRENT is always conserved. All the current that enters a point HAS TO MATCH exactly the current leaving a point.

Like I said, it matters not how you consider charge carrier flow, whether its "really " from +ve to -ve or vice-versa. Things could/will/are much more complicated than your first book on DC current would indicate. It wasn't known for a very long time that the charge carriers in metals are electrons, and that electrons have -ve charge (in this universe)....

Well, look at it this way: If you cut the wire before the bulb in the circuit, the bulb goes out. If you cut the wire after the bulb instead, the bulb still goes out. The resistor works kind of the same way.

2+3=3+2. It really does not matter which location the resistor is in.

CAVEAT - When you start putting other components in (e.g.- capacitors, transistors) where the output is to directly affect another component is where placement is important.

Yes, but if I build a simple circuit say of a battery, light bulb, and resistor, shouldn't the resistor be between the negative and bulb?  

Lets say I want to use a resistor to lower the current from say 1 amp to .5 amp.   Wouldn't I want to put the resistor between the negative and the bulb?   Would I get the same result if I put the resistor between the positive and the bulb? 

It's called "convention" - as long as everybody uses the same style of schematic, it doesn't matter which way the arrows point.

Don't sweat it, accept it. Its even more complicated than you currently (sorry) understand.