ubuntu 12.04 Recognize one of my cores

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http://askubuntu.com – I have a mid-2010 Mac book 7.1. I installed Ubuntu 12.04 on it, after a while I realized that sometimes one of my core isn't recognized by Ubuntu. Today I also realized that when I plug in the charger this wouldn't happen, but when my laptop is on battery this would happen. is it a bug? (HowTos)