When we permanently delete a file from a PC, how does it vanish?? Will it mix in the air?

I don’t think the first law of thermodynamics applies to this situation.

files are recorded on the hard drive as little marks. When you delete these files the marks are smoothed

A marker is placed, (it’s all about file systems, allocation tables, inodes etc) – if the marker indicates that the file is ‘deleted’ then that part of the HDD can be written over – they’re not really ‘deleted’ (that’s how data recovery works!)
If you want to permenantly erase your HDD and make it insanely difficult for someone to retrieve previously stored data then check out dban.

or, if you have a mac you can tell it to formate a hard drive 128 times, meaning a .001% chance of recovery. I think you can even tell the trash bin to do that.

its hard to actually delete all the data of a file. That is y computer forensics is possible.

dban does a damn-good job, run that overnight on repeat and even the NSA will have trouble getting your data.

LOL!!! The other option is to tear to HDD apart and, believe it or not, lick it. There is enough acid in saliva to destroy and eradicate the data. Basicaly you are licking off all of the magnetic film that is needed to accesses a hard drive. Then crack open your car battery and toss your HDD in there. That should do a good job of killing any data on the thing.