I have noticed that the phone charges more faster with Adapter ratehre than USB from PC. measured the voltage and both of them from PC and adappter are same...why is the charging time difference then. As far as i think...The current is same as the load is the phone itself in both case ..so like kind of constant load...!! Any ideas ladies and gentlemen!

 

USB ports are current limited.

 

You assume the current is the same, but it probably isn't. I just designed a device that charges a battery and I reduce the charge current when USB is the power source so as not to exceed the 500mA USB limit, and increase the charge current when an external adapter is present.

 

...when an external adapter is present.

Apple probably recognizes it´s own adaptors, so the phone can tell that it can use more current.

 

There is often a special pull-up resistor in those high-current USB chargers for phones and other mobile devices, the value of which signals the current capability of the charger (either 1 A or 1.5 A, usually).