I do a lot of travelling back & forth as an electrical engineer so: The US and Canada work on 110 Volts 60 Hz, as opposed to the UK/Euro norm of 230 Volt 50 Hz. It depends on the charger unit itself - if you look at the detail plate on the back it should say somewhere, something like "Input Voltage", or maybe just "Input". If the figure next to that is "100 - 240V 50/60Hz" or something similar then a simple plug adaptor, usually available from an airport shop, is all you need. If it says "230/240V 50Hz", then I'm afraid you'll need to buy a US spec charger. (Transformers are available but these usually work out more expensive and only convert the voltage).
Most modern devices work on the full range of voltages and frequencies, makes manufacturing cheaper!
