i am waiting for your replies.
Sorry I have nothing to do all day but respond.
how much is power dissipation of the 1n4001 in this situation(reverse biased)?
how much current can the diode withstand in reverse bias before failing?
The part is rated for most of one watt. With 50 volts you can find how much current that is.
When you use parts out side their normal rating there is no way to know what will happen.
If the part 'breaks down' nicely it can handle 1W but the part may only break down in a very small part of the die and that small part can not take 1 watt. So it could over heat with a very small amount of power. This is not tested. No one knows.
what is main cause of failure of electronic components?overvoltage overcurrent or power dissipation?
Your teacher will say.....what he wants you to answer.
.....over current leads to over power.
.....over voltage (probably) will lead to over power. The over power might be in only a very small corner of the part.
.........over dissipation will kill the part.
.....Tell your teacher a hammer works every time.
does capacitor have watt parameter?(why wasn't capacitor failed?)
The capacitor might fail in 1nS or 1mS or 1S or 1 week or any time. The next cap you test will be different. There is no knowing. The part will work from -50V to +50V and more than that why knows. There is no rating when it will fail just where it will not fail.
Most capacitors do not have a power rating. Very high current caps do but that is very complicated to explain.