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I was just about to ask the same question as I've mixed up the chargers that came with n8 and c7. Relieved to see that they can be used for either device.
Just bear in mind AC-15 for N8 & C7 being a fast charger has output of 800mA, so ones from older ones will take considerably longer to achieve the same result.
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!