Why won't the iPod Classic 160GB charge with the iPad charger?

Question

I found out the hard way that the iPad's 10w charger which works on my iPad-1 and on my iPhone 4s and on my iPod Touch *does not* work/provide a charge to my 2nd gen iPod Classic.
I had gone to Europe with a full charge on all of my devices, and on the long flight I used both the iPad and the iPod. Once in Spain, I left the iPod in the hotel safe; the phone and iPad were used throughout my vacation, receiving a charge every day via the 10w charger and European duckhead. The night before I left, I plugged in the iPod, knowing that I would need it for the return flight. Imagine my surprise the next day at the airport when I discovered that the iPod was practically dead. Before boarding, I verified that the charger itself (with the European connector) was still working on both the iPad and iPhone, and it was; but as soon as I connected it to the iPod...nothing.
When I eventually got home to the States, I sat down with both the 10w charger and two or three of the many other USB chargers I have accumulated over the years. With the exception of my ancient firewire connector, every single one will charge every single device-----except for the 10w charging the iPod Classic. The iPad will even get a charge from the non-10w charger; it just takes a very long time to get back to 100%. If I could find my 3rd gen Nano, I would try that, but I'm not sure where it is.
What gives here? Why won't the 10w charger work on the iPod Classic??

Answer 1

I finally got curious enough to get a young person's eyes to read the stuff on the bottom of my iPad Mini charger: 5v 1a which equates to 5W using Ohm's law P=IE (E=IR).
So that beng the max charging rate of a normal USB 2.0 port explains (derived from above posts by Bilbo_cheshire and CMJRoss) why that iPad charger does work when some of the larger iPad chargers do not work with the iPod. The current is limited so it does not cause the iPod to sqeal .
scott