questionsare all "usb wall charger" plugs the same?

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Here is a good answer ( http://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone ):

Can I blow up my USB device?

There is a huge variance, then, between normal USB 2.0 ports rated at 500mA and dedicated charging ports which range all the way up to 2100mA. This leads to a rather important question: If you take a smartphone which came with a 900mA wall charger, and plug it into a 2100mA iPad charger, will it blow up?

In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will blow up — and in fact, using a more powerful charger should speed up battery charging.

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The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. In 2007, the USB Implementers Forum released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your PC’s USB ports, or by using a wall charger. Shortly thereafter, USB devices that implemented this spec started to arrive.

If you have a modern USB device — really, almost any smartphone, tablet, e-book reader, or camera — you should be able to plug into a high-amperage USB port and enjoy faster charging. If you have an older device, however, it probably won’t work with USB ports that employ the Battery Charging Specification; it might only work with old school, original (500mA) USB 1.0 and 2.0 PC ports. In some (older) cases, USB devices can only be charged by computers with specific drivers installed.
Appendix (Updated)

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Finally, a quick word about a few foibles of USB charging. For a start, while PCs can have two kinds of USB port — standard downstream or charging downstream — OEMs rarely seem to label them as such. As a result, you might have a device that charges from one port on your laptop, but not from the other. This might be a trait of older computers, as there doesn’t seem to be a reason why standard downstream ports would be used, when high-amperage charging ports are available.

In a similar vein, some external devices — hard drives and optical drives, most notably — require more power than a USB port can provide, which is why they include a two-USB-port Y-cable, or an external AC power adapter.

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Shorter answer: In general, you can use (almost) any USB charger with (almost) any USB device, but charging speeds may vary, and some devices may not charge at all if the amperage is too low. But the risk to your equipment is minimal.

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Let me get to the bottom line. The USB standard is 5V, 500mA. There are many devices out now, even though they can be plugged into a standard USB plug, want far more than that. The most notable one is the iPad. The charger that comes with it actually puts out slightly more than the USB standard 5 volts (5.1) and wants 2 amps (2000 mA, or 4 times the USB standard). If you plug in your iPad to the generic USB charger it either will take a long time to charge, or may not charge at all.

In short, know what devices you want to charge, and how much current they want to eat.