What is the real every moment exact situation of an a.c source?
This diagram may better help you to understand how a full-wave bridge rectifier works:



The + &amp; - signs indicate voltage potential difference (i.e. one end of the wire is greater V than the other end of the wire).  Since AC switches between + &amp; - phases 50 or 60 times per second (depending on your main power source), current flows through a different diode pair with each AC phase transition.

re "...what is the amount of the negative end of the source?!"
For current to flow through a diode, the actual V difference just has to be enough to cause forward biasing to occur (~.7V for silicon diodes).  That means the anode (+) end of the diode must be .7V "more positive" than the cathode (-) side of the diode.  In the USA, during the AC positive (+) phase, the anode will be subjected to a sinusoidal shaped V (which starts at 0V, peaks @ ~170Vac, &amp; then returns back to 0V).  During the AC negative (-) phase, the cathode will be subjected to a sinusoidal shaped V (which starts at 0V, peaks @ ~-170Vac, &amp; then returns back to 0V).
Note: Whenever the anode (+) reaches its forward bias V condition, it will conduct current--even when the anode is 0V &amp; the cathode V is -.7V or less).

Take note of the output from the bridge.  Both arrows indicate the output current is flowing in that direction as positive (+) sinusoidal waves.
Also, if the input is 60 Hz, then the output will be 120 Hz ripple dc.