For all I know there could be a wizard inside that triangle shooting lightning bolts.
Ah that means my previous reply might be confusing..
Think of an opamp as an ideal amplifier. The output voltage of the opamp...
Vout = A * (V
+ - V
-).
In other words it amplifies the difference between the voltage on the V+ terminal and the voltage on the V- terminal. A is a large number called the Open Loop gain and can be as much as 1x10
6.
Because the gain is very large V+ and V- have to be at a similar voltage or the output will be huge (eg limited by the supply rails). So if the amplifier is operating in a linear mode you can normally assume V+ and V- are at roughly the same voltage.
When you analyse a circuit you can normally connect nodes at the same voltage together without effecting it's operation. In the case of the opamp circuit you couldn't actually connect them together in real life but you can to help analyse the problem.
At this point its worth looking at the circuit for an inverting opamp set up. Let's call the input resistor Rs and the feed back resistor Rf.
If V+ is at 0V then for it to be in a linear mode V- must allways be close to 0V as well. Let's assume it is at 0V.
The current through Rs is therefore (Vin-0V)/Rs = Vin/Rs
Likewise the current through Rf is (Vout-0V)/Rf = Vout/Rf
but where do these currents go? The V- input has a very high input impedance so it doesn't go in there. Applying KCL at the V- node you find...
Vin/Rs + Vout/Rf = 0
Rearrange to give
Vout/Vin = -Rf/Rs
This is the closed loop gain.
If Rf was 100K and Rs was 10K the gain would be 100K/10K = 10x