1) Can I just plug ANTMINER U3 to my computer and let it mine while I use my computer for normal use?
Yes, but keep in mind they can be notoriously temperamental and may need babysitting:
https://bitcointalk.org/index.php?topic=827356.new;topicseen#new 2) Will it be profitable with the current difficulty if it doesn't rise?
You mentioned that you have free electricity, so the answer is generally always going to be yes (as long as it doesn't die on you) and the question is only one of "how long will it take".
3) How long is the return of investment on it with it's 63GH/S ?
Somebody's gonna come complain about your use of the term ROI here;
https://bitcointalksearch.org/topic/roi-is-not-payback-period-1113417Let's assume you meant break-even
Assuming...
1. the difficulty stays the same
2. hash rate is 63Gh/s (good luck)
3. price is
BTC0.10453 ($30 at $287/
BTC - adjust as necessary for 2nd hand units, shipping, etc.)
4. you only care about the
BTC figures (saves arguments on exchange rates, whether it'd be better to buy
BTC, etc.)
5. 0% pool fee
6. difficulty of 52,278,304,846
7. block reward of
BTC25
...then it's about mid-way November; google 'bitcoin mining calculator', make sure you plug in all the various values.
4) How long it will take to mine 1 BTC with current difficulty?
https://en.bitcoin.it/wiki/Difficulty#How_soon_might_I_expect_to_generate_a_block.3F( using the simplified formula )
time = difficulty * 2**32 / hashrate
time = 52,278,304,846 * 2^32 / 63,000,000,000 = [whole bunch of seconds] or roughly 114 years for a block, or ~4 years for
BTC1
Note that next year (2016) the block reward will be halved from
BTC25 to
BTC12.5 as well, which means it will take a good bit longer than that even if the difficulty were to stay the same.
5) How much I could earn daily with it in terms of mining bitcoin for 24/7 ?
With the same assumptions as before, roughly
BTC0.0006061/day
Can't help you with Q6 - doesn't appear to be an middle-eastern section and the
Iran thread is pretty quiet, but apparently there is an Iran-specific forum where you might try asking as well:
http://btctalk.com/forum.php