Thanks for the answer. I understand it except for the current part. I can't really visualize how there can be current without voltage :?

This is the fundamental basis of all negative feedback.

A basic opamp has very large open loop game, often on the order of 100,000 and more to 1. So 1 mV of difference between the + and - input pins will cause the output to swing volts in the direction of the input difference.

When we connect input and (negative) feedback resistors, these wild large voltage swings are tamed by a closed feedback loop. This has the practical effect of making the - input follow the + input very closely, give or take a very small error voltage equal to the output change divided by the open loop gain, often mV or less.

If the + input is grounded as in your example the practical effect of the negative feedback resistor trying to hold the - input close to the + input is that the - input becomes a “virtual earth” and any current flowing in the input resistor toward or away from the - input will be canceled by an equal but opposite current from the feedback resistor as the output swings volts in response to very tiny error voltages at the opamp's input pins.

If you were to look at that - input with your scope gain turned all the way up you might see a very tiny version of the input/output waveform.

This basic relationship in the virtual earth topology accommodates simple gain calculation from the ratio of input to feedback resistors. Since the opamp works to hold output current equal and opposite to input leg current scaling the resistance up or down scales the voltage to same amount.

This is a very powerful concept and the basis of much modern circuit design.

JR