This circuit is an oscillator that generates a square wave. The op-amp starts with its two inputs in an unknown state; let's say it starts with + slightly higher than –. The op-amp greatly amplifies this difference, bringing its output to the op-amp's positive power supply voltage, its maximum output (15 V in this case). The two 100k resistors act as a voltage divider which put the + input at half the output voltage, or 7.5 V. The – input is at ground, lower than the + input, so the op-amp output stays at 15 V.
Current flows from the op-amp output to ground through the capacitor, charging it. As soon as it charges to slightly more than 7.5 V, the – input is now higher than the +, and so the output flips to -15 V. This brings the + input to -7.5 V.
Now current flows in the other direction, discharging the capacitor and reversing its polarity until it reaches -7.5 V. Then the cycle repeats.
To hear the waveform, increase the simulation speed and press the Play button.
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