Standard USB is 500 mA. Wall sockets chargers can provide whatever they want, but there's no "standard" for wall socket chargers. Some provide 1A, because the manufacturer designed them that way... or 2A, or 750 mA, or anything really
The device needs to have more than just charge to turn on. It's extremely likely that a device with a 7" screen and the processing power of the N7 will require more power than a 500 mA charger can provide during normal operation. So the device will probably take a trickle charge and eventually charge up if you have the screen off and the device is idle, but while in use, you're likely to still be draining the battery with a 0.5A charge. It'll discharge slower than when not plugged in, but it'll still discharge.
My TouchPad does this. If I plug it into the computer and use it, it will drain - slower than when not plugged in, but it'll still drain. If I shut the screen off and am not doing any file transfers or whatever, it'll very slowly charge up.