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The iPad, if you want to charge it with a PC, must be connected to a high-power USB 2.0 port. If not, it will show "Not Charging" and it won't be charged.
Instead, I recommend you to connect it to the power adapter, so the charge will be much faster than charging it with the PC. See > http://support.apple.com/kb/HT4060
I had the same question and I bought a "high-power" adapter as suggested.
But, I accidentally discovered that iPad can be charged with a regular adapter, though it takes longer than with "high-power" one.
If you don't have any other apple device (such as iphone) than iPad, you don't need to purchase an extra USB connector, unless you want to charge iPad quickly.
Just leave iPad connected to PC while you're not using it.
I found it out while I was trying to sync the data in iPad using iTunes in my PC, and battery level was slowly going up.
I also found out some other altertives while I was surfing through the internet.
Please check out the following website.