Electronics > Beginners

Conventional current vs electron current - elements in different order

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rstofer:

--- Quote from: vk6zgo on October 17, 2019, 02:30:19 pm ---Some people are so protective of Conventional current flow, that the very mention of Electron flow makes them react as if you insulted their mother!

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I don't know if it is 'protective' or just trying to have a common frame of reference.  It's nice to know about electron flow, it may even come up when thinking about  MOSFETs, but the "Conventional Current Flow" allows us to follow the arrows on transistors and diodes and that seems handy.

And, truly, it makes no difference but keeping everybody on the same page seems useful.  Particularly in a Beginners forum.

It isn't necessary but it is useful when dealing with Kirchhoff's Laws to have a uniform approach to writing the loop and node equations.  But even there, nodal equations are often written as though all current are entering a node while knowing full well that there MUST be currents leaving the node.  That works well because a negative numerical result will immediately indicate current leaving the node.  That seems handy to me!  Positive resultant currents are entering the node, negative resultant currents are leaving the node.  Perfect!

I guess we just need to be flexible in our thinking!

Bassman59:

--- Quote from: nForce on October 16, 2019, 06:16:30 pm ---Why is there no difference if we look at the circuit and say, there is no matter in which direction current flows? The elements of the circuit (resistors, capacitors, inductors) are in a different order. If we just look at the RC filter, there is a significant change, if we put first a capacitor and then a resistor, or first a resistor and then a capacitor.

Thanks.

--- End quote ---

You do not consider the return current in your description.

pwlps:

--- Quote from: nForce on October 17, 2019, 12:49:54 pm ---Sorry, but I don't understand. If we see the schematics of the RC filter:



Here if we start at V_in, first we get to the resistor then parallel connected capacitor and finally the V_out. If we change V_in and V_out, then first we get to the capacitor then resistor, and the output.

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This is a legitimate question but with all vague explanations given here I understand you are stuck  ;)
 You forgot one thing here.  To define the gain V_out/V_in of a circuit you need a condition on currents (otherwise there is no enough information, for example here we have 1 equation for voltage, one for current, two unknown voltages and two unknown currents, leaving two free variables, you need to fix one to get the voltage ratio).  Usually this condition is I_out=0 i.e.  unloaded output. If you reverse the circuit the gain will be different because it will now be defined with I_in=0.  This is the ONLY reason for the difference when the circuit used in reverse direction. And as stated already in other answers it has nothing to do with the polarity of the signal itself.

Nerull:
Supply your RC circuit with AC - now current switches direction every half-cycle and there is no single polarity to even argue about, but the gain doesn't change. You don't have to switch all the components around in your model for the negative half of an AC signal - polarity has no effect on where your inputs and outputs are.

nForce:
Oh, I get it now. The problem I made is I changed the input and output of any kind of circuit. I should just switch the polarity of the battery at the INPUT. But why do we have then reverse voltage protective devices like a diode, if there is no matter in which direction the current flows?

 

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