Direction of current flow on a circuit? And through NPN and PNP transistor?

[Nepaliman]

I can apply and can calculate each ohms law in a circuit and also can bias a simpler circuits But sometime I feel I am missing huge fumndamemtle. Actually tiny fundamental. I have two questions on this attachment please (in simple word)! Thank you.

 

[Ratchit]

Nepaliman,

You are making the same mistake that many nubies do. You are trying to assign the direction of current according to the physical flow of the charge carriers. Don't do that. When calculating current, always assume that positive charges flow from the positive terminal of the voltage source, through the circuit, and on to the negative terminal. This is a mathematical convention that is followed when doing calculations. Semiconductors and ammeters follow this convention. Hook up a positive voltage to the arrow of a diode and the negative to the bar, and the diode will conduct. Same with a transistor. A ammeter will indicate a forward current when the positive terminal is connected to a positive voltage and the negative terminal to a negative voltage. If the charge carriers are positive like a semiconductor hole or an electrochemical ion, then the physical carrier direcxtion is the same as the calculaton. If the charge carrier is a electron like in a metal wire, then the physical current is opposite to what the positive assumption is. Ask if you have any questions about this.

Ratch

 

[unclejed613]

electron flow is from negative to positive. electrons are negatively charged, and are attracted to positively charged atoms (meaning they are missing electrons). the earliest proof of this is found in the operation of vacuum tubes. for convenience in analyzing solid state operation, the use of "hole flow" (the migration of positive charges) in a semiconductor crystal allows some symmetrical thinking on what goes on in the semiconductor, but the truth is, it's still the flow of electrons that is the active element. so you can analyze semiconductors using "conventional current flow (the migration of positive charges)" as long as you understand it's actually electrons that are flowing in the opposite direction.

to borrow from the water analogy.... let's use a system where there is also air in the system.... the water is the electrons, and the air bubbles are the absence of electrons. the air moves up against gravity, and the water moves down simultaneously.