The Laws of Circuit - you can learn and practice by just reading
copyright. Charles Kim 2006
The reason is like this: The two things you have to
know and carry all the way to your excellent-pay job are
that (1) the voltages at the inverting input (read as node
n) and the voltage at the non-inverting input (read as
node p) are the same, and (2) there are no currents flowing at the inverting and the non-inverting inputs
toward the Op Amp. I wonder you missed one important
thing above. We said the 'voltage at node n' or
'voltage at node p', as we expressed Vn and Vp,
respectively. So now you know why the node voltage method
is the method for circuits with Op Amps. This leads to
another discussion. Vn is then the voltage between n
and GND. Vp is the voltage between p and GND.
Similarly, Vo, at the output is the voltage between o
and GND. When one end of a source is connected to the
GND, it's easy to figure the voltage at n or p,
but when a source is not connected between a node and
GND, you have to give a little more thought to find out
the voltage at n or p.