It's connected to the house current with a mini usb cable. I have a a mouse and a keyboard attached to the pi but I already tried to plug everything but the dongle out and the result was the same

Hi everyone, I'm Italian so sorry if my english isn't that good. I've bought for a week a new Raspberry Pi B+ and a wifi dongle Edimax ew-7811un. I've configured the /etc/network/interfaces and the /etc/wpa_supplicant/wpa_supplicant.conf files and now the RasPi can connect to the Internet without pr...