Now we can use Kirchhoff's current law and

do nodal analysis on this upper node, let’s call it node 1.

And we can sum the currents into that node or

we can choose to sum the currents out, it's our choice.

So if we do this, first of all looking at the current that

flows through the capacitor, we use our IV characteristic for the capacitor.

That is the current of the capacitor is equal to C dv dt.

And our voltage is our difference between v

sub n and the lowest voltage which is zero volts.

So it's V sub n- 0, dt.

And we also sum more came from the resistor and through the resistor.

It's going to be V out, which is the voltage at the upper node,

minus zero divided by R And

we have one other input to this node and

it's the current coming out of the inverting input of the opamp.

We know using our properties of our ideal opamp that that current is equal to zero.