Arduino input and output

Using Arduino General Electronics
1

Will my arduino be damaged if I define a digital pin on it as OUTPUT and then provide an INPUT of 5V to it. Similarly, will I damage an op-amp if I provide 5V to its output pin?

2

Yes, very likely it would be destroyed. What is important, a current, if you apply 5V via resistor limiting current below 20mA , than it's o'k.

3

sreedevk:
Will my arduino be damaged if I define a digital pin on it as OUTPUT and then provide an INPUT of 5V to it.

Depends on whether the output is set HIGH or LOW.

If the output is HIGH then nothing happens. If it's LOW then any more than 40mA flowing is in the red zone.