a question regarding NODE equation..

http://img185.imageshack.us/my.php?image=img8857hj0.jpg

i watched an MIT lecture
and he presents there an example
in which he constructs node e2 equation in a very weird way
some how he decides the directions of the current
i dont know how
???

i think i understand the system of how he does the
he takes out e2 and subtracts by the voltage from the line
and divides by it resistor
but why the I1 is negative

i dont know why he does this equation like this
and not the normal KCL method
????

 

I think it's easiest to see if you transfer I1 to the other side of the equation,
and then regard I1 as the current going INTO the node e2 and all the other as going out. And according to KCL they must be equal.

So, I1 goes in and equals: IR4+IR5+IR3 which all go out (as was indicated in the picture)

Hope that helps.

 

but there is not I variables
if KCL we suppose to have each current represented by I

how did he substitute that?

 

There actually are current terms contained in the expression. Don't forget ohms law states that current = volts divided by ohms. There are several terms that are volts divided by ohms. Each of these V/R terms has the units of Amps.

hgmjr

 

i cant understand the reason why he subtracts e2-V0??

its always e2 - (the other voltage)

why is that??