Why? The question he is asking is about Kirchoff's law.

 

Wanted to follow up on this by saying I wasn't trying to be rude when I suggested that discussions about particular voltage/current characteristics of LEDs should be avoided. What I was getting at is that the fellow is obviously trying to understand the fundamentals, and fundamentals are about the "rules and laws". Certain devices "seem" to defy these rules and create exceptions that require detailed explanation. I believe it is helpful to understand the rules first, then tackle all the exceptions. It is, to me, backwards and confusing to have all the exceptions dumped upon you along with the rules and then try to sort it all out at once.

Kirchoff's laws are as fundamental as Ohm's law. Once Ohm's law is understood, more complex series/parallel circuits can be studied using Kirchoff's laws. Then it follows that Thevenin's and Norton's theorems can be studied once Ohm's and Kirchoff's laws are understood. At least this is the way I ALWAYS see it in textbooks, ie the "building block" approach.

Perhaps some of you may learn things by a different method, but that would be the "exception" to the "rule"...(couldn't resist)

 

the resistor is basicall to limit the current, in your example you can see it limitting, but you cant achive a certain voltage only to be blocked between a ressitor for different voltages.

the definition for Ohm may give you some idea.
the value of a resistor required to establish 1A (you can say in charge flow Q/S) when connected across a 1V source is 1 Ohm.

resistor is to control the current not the voltage.

 

Do you expect a kid to understand thevenin's theorem and Kirchoff's law ???

 

I think you guys are making it hard on the guy. IMO, he should put the LED aside and understand Ohm's law and how it works in simple circuits containing only resistors. He should also understand how resistors are connected (series and parallel) and how that divides voltage and current.

OP, in your original circuit, if there is no LED, and you only connected the 9v battery to the 3300 ohm resistor, a current of I = V/R = 9/3300 = 0.002727 amps will flow in the circuit(Assuming the battery is exactly 9v). The voltage drop on the resistor will be 9v since it is the only component in the circuit, Or it can also calculated by the same ohm's law. Vr = Ir = .002727*3300 = 9v where I is the total current.

Now, if there is another R connected to the previous 3300 ohm resistor in series like this:

http://mysite.mweb.co.za/residents/v...h/r_series.jpg

Then you will have to first calculate the total resistance which is basically the sum of the two resistors(in this case) before you can calculate the total current and assuming the second resistor is 1000 ohms then I = V/R = 9/(3300+1000) = .00209 amps

In this case the voltage will be divided and it will be

Vr1 = I*r1 = .00209 * 3300 = 6.9 v

Vr2 = I*r2 = .00209 * 1000 = 2.9 v

If this same second R is connected like this

http://image.tutorvista.com/content/...d-battery.jpeg

which means it is connected in parallel then the total R is calculated differently and the law for that is this

http://www.radio-electronics.com/inf...n_parallel.gif

which when we apply, it will give a value of 767.44 ohms which will give a current of V/Rt = 9/767.44 = .011727 amps. This is the total current, and in this case the current is the one that is going to be divided between the two resistors. Some of it will go to the first R and some will go to the second R in a way so that both R's have the same voltage.

==> Ir1 = V/r1 = 9 / 3300 = .002727 amps

and Ir2 = V/r2 = 9 / 1000 = 0.009 amps


This is how it works. You should understand it first with resistors, then go to the more complicated stuff.

 

Kirchoff's Law, certainly.

If he can't, we might as well quit commenting on this thread because that is exactly what we are trying to explain. What we call it is irrelevant. It might as well be called Bugs Bunny's Law.

And I would certainly expect him to understand Kirchoff's Law and how it applies to a very simple, two or three resistor series circuit before I would a circuit combining components that behave in mathematically different ways.

So my question to you is, do you really expect a child to understand the behavior of PN a junction in a light emitting diode and how it behaves in a series circuit BEFORE he understands how two resistors behave in a series circuit?

 

Thank you.

 

To further emphasize my point, I would say that is actually NOT an easy example. You should start by understanding circuits with only resisters first. The LED is basically a diode:

Diode - Wikipedia, the free encyclopedia

And it can get a bit confusing with a diode. I wouldn't explain ohm's law to a starter with a diode involved !

 

Wow, this thread deteriorated rapidly. Why can't we keep to simple ohms law and get him understanding that before going on to more complex stuff.

Looks to me like everyone wants to boast how much they know rather than help the original poster.

Just stick to ohms law and let the guy learn.

Mike.

 

I totally agree, keep it simple.
Ohm's Law is simple.
Actually, Kirchoff's law is even more simple, i.e. it is simple addition, kindergarten stuff (1+1=2)

Stick to those two principals first. Glad we agree on something.

Edit: I apologize, I do not mean to come across as combative. I just VERY STRONGLY feel that learning electronics is a building block process and should be approached as such to avoid confusion....especially with a child.