Embarassingly simple question from a total newbie

[TBBucs]

Ok, so today was my first day learning electronics. I picked up the Tab Electronics Guide to Understanding Electricity and Electronics (I hope I remembered that correctly) and started reading through until I got to the very basic circuit schematics. I had a question about this one (sorry, this one's hand drawn in GIMP because I haven't yet installed a schematics program):

**broken link removed**

I mean, this is as basic as it gets. The thing that I'm not sure about is the battery (DC current). Current runs from negative to positive (electrons want to leave negative ions in order to fill the outer layer of a positive ion, yada yada yada), but the schematic shows the negative lead of the battery going downwards, which means the current would get to the LED before the resistor. What the heck is the point of the resistor, then? Am I misinterpreting the battery symbol? The shorter line is negative, right?

Any help is greatly appreciated. And please don't rip me apart too bad

 

[Nigel Goodwin]

The resistor is to limit the current, nothing else - without it the LED would be destroyed.

Don't get confused by which way current flows (it makes no difference), conventional current flows from positive to negative, and electron flow from negative to positive.

Personally I never think about it either way, I just consider current flows from top to bottom.

 

[Willbe]

the current would get to the LED before the resistor.

Think of the electrons in a circuit as peas in a soda straw; they're always there. It's just that when voltage is applied, they start moving.

When you push in a new pea [voltage is applied], another leaves the other end of the straw.

It seems to happen instantly, but that's because the pea that leaves the straw is not the same one that just entered.

 

[RadioRon]

Yes, this seems a bit confusing but the fact is that everyone else in the world has agreed to say that current flows from positive to negative, and this is what we mean by "conventional current". You are correct in your observation that the charge carriers in metallic conductors are free electrons and these flow from negative to positive, but electron flow is not the only kind of current that exists. For example, in the electrolyte of a battery two types of current carriers flow and in opposite directions inside the battery. Negatively charged chlorine ions move toward the negative terminal and positive hydrogen ions move toward the positive terminal.

"When we are working with pencil and paper circuit diagrams it would be very helpful if we could agree on a standard conventional direction for electric current. This is essential if we are to calculate the algebraic sum of the currents at a junction point in an electric circuit. For such a chore, the physical nature of the actual flow of the current carriers is not important" This selection of direction was done in the days of Benjamin Franklin and Michael Faraday, when they had no knowledge of atomic structure. Their choice of direction was made based on observable effects in their experiments. For example the result of observable current flow through Faraday's electrolytic cell suggested that current was a flow of positively charged particles.

Therefore, all the laws and rules that we use are based on a direction from the positive terminal of the source of EMF, through the external circuit, to the negative terminal. Academics who need to distinguish this from electrons use the word "current" to indicate this positive to negative flow, and "electron flow" to refer to the physical process of electron conduction.

Now, as for your concern that the electron flow never gets to the resistor, you must recall that the amount of current flow in a series circuit is the same through each element of that circuit. In such a circuit, each element uses up some of the voltage, not the current.

edit: one trick that they used on me in my school days was to say that, as Willbe describes, the electrons are being pushed into one end of the circuit and so pushed out of the other end at the same time. As each electron along the line jumps a position it leaves a "hole" (or rather a place where an electron might feel comfortable) behind into which the next electron down the line can jump into. So, they would say that "conventional current" is the flow of the holes!

Note: much of the text above is excerpted from "Introduction to Electric Circuits" by Herbert W Jackson.

 

[rezer]

The resistor is in the circuit to limit the current flow through the LED. Without it, the LED would draw excess current and burn up (assuming your battery voltage was high enough to turn on the LED). It doesn't matter how the components are hooked up. The resistor could go to the negative terminal of the battery. Since it is a series circuit, the same current that flows through the LED also flows through the resistor.
An LED operates much the same way as a regular diode. It has a P-N junction with a depletion region that collapes when the diode is forward biased (i.e. when a negative potential is applied to the cathode with respect to the anode). This is what causes current to flow through the LED. If the LED and resistor were swapped, current would still flow because the anode would be positive with respect to the cathode, which is the forward bias state.

As far as the direction of current flow goes:
Engineering schools teach conventional current flow which is positive to negative. ET programs (and electronics techs) think of electron current flow. In the grand scheme of things, it just doesn't matter and won't help you one iota in troubleshooting pcbs. So don't worry too much about it. Just know the difference so you get the question right on the test.

Good luck with your studies!

 

[TBBucs]

Fantastic responses. Let's see if I'm understanding this correctly.

1.) Current is comparable to a line of, say, beads. The beads are situated all the way around the circuit (not moving) before voltage is applied. Then, once voltage is applied, those beads start to move. So it isn't a sequence of events (current comes out of the negative terminal, flows through the resistor, then reaches the LED), but rather a cycle of "beads" that are already situated around the entire circuit.

2.) The location of a resistor (or any other component) is irrelevant, because, in the case of a resistor, if one section of the "chain of beads" is slowed down (the current is resisted), the entire chain slows down.

Assuming the above statements are correct, I think I understand this now. I'm used to thinking of things sequentially (I'm a programmer, after all), but that doesn't appear to be the case with electronics.

Am I on the right track?

 

[RadioRon]

Fantastic responses. Let's see if I'm understanding this correctly.

1.) Current is comparable to a line of, say, beads. The beads are situated all the way around the circuit (not moving) before voltage is applied. Then, once voltage is applied, those beads start to move. So it isn't a sequence of events (current comes out of the negative terminal, flows through the resistor, then reaches the LED), but rather a cycle of "beads" that are already situated around the entire circuit.

2.) The location of a resistor (or any other component) is irrelevant, because, in the case of a resistor, if one section of the "chain of beads" is slowed down (the current is resisted), the entire chain slows down.

Assuming the above statements are correct, I think I understand this now. I'm used to thinking of things sequentially (I'm a programmer, after all), but that doesn't appear to be the case with electronics.

Am I on the right track?

Yes, you have the right idea