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Instruite, the op amp has extremely high gain (typically 175,000). In order to drive the MOSFET gate to, say, 5 volts, it would only need the noninverting input to be 29 microvolts greater than the inverting input (using Av=175,000). This is the principle behind all op amp negative feedback circuits. If we followed your reasoning, op amps would be worthless, except maybe as comparators.If I assume for drawing sake the resistors and other elements are not shown then also there is one problrm with the circuit
When Voltage due to current (at negative terminal of opamp) is equal to Vin, which is the desired state, the output of opamp will be 0V which will switch off the load.