Difference between "СЕЙЧАС" and "ТЕПЕРЬ"

Talking about words-synonyms, there is always the question - do they have any difference and if there's no any, why do we need two words instead of one? This time - the adverbs "сейчас" [siychas] and "теперь" [tip'er'] - (now).

Well, we use the abverb "сейчас" [siychas] (now) meaning:

1. "Очень скоро" [ochin' skora] - very soon, "вот-вот" [vot-vot] - in a moment (you can notice the composition of a word: "сей час" i.e. "в этот час" - this hour).
Сейчас будет гроза. - Now there will be storm.
Сейчас начнется экзамен. - Now the exam will start.

2. "Стало" [stala] - became.
Сейчас понятно, ты хороший человек. - Now it is (/has become) clear, you are a nice person.

3. "В настоящее время" - at the present time
Сейчас мы учимся в университете. - Now we study at university.

We use the adverb "теперь" [tip'er'] (now) meaning:
1. "В настоящее время" - at the present time.
Теперь совсем тепло. - Now it's quite warm.
Теперь все можно купить через Интернет. - Now we can buy everything on the Internet.

2. "В настоящее время" - at the present time in comparison with the past.
Поработал, пообедал, теперь отдыхаю. - I've worked, had dinner and now I relax.

3. "В настоящее время" - at the present time talking about circumstances which have been changed in length of time.
Раньше я не говорил по-русски, а теперь говорю. - I couldn't speak Russian before, and now I can.

As you can see, the meanings of the words are quite similar. But there is a stylistic difference - the word "сейчас" refers to shorter period of time, and the word "теперь" is a broder concept of time like the concepts "раньше, теперь, потом" [ran'she, tip'er', patom] - before, now, then.

And one more difference: the word "теперь" implies a result.
Он купил велосипед, теперь ездит на нем на работу. - He has bought a bycicle and now he rides it to work.

And "сейчас" implies the meaning "at the present time", when there is a process.
Сейчас она говорит по телефону. - Now she is talking on the phone.

Most often you can use the word "сейчас" instead of "теперь" - but not vice versa.