anger magic

[Sayaka]

No: HPFGUIDX 60465

   My apologies... this post is completely unrelated to OoP. I'm 
confused about one thing. All those times when Harry did magic 
without meaning to (when he was staying with the Dursley's), all he 
had to do was get really angry, or something like that. But there are 
other times he's gotten really angry (at Snape, Malfoy, etc.), and 
nothing has happened. Is there an explanation for this? Is it only 
muggles who are effected by this power? And it still counts as 
breaking the rules when he does it over the summer, even though he 
doesn't do it on purpose. Shouldn't there be a class or something at 
Hogwarts that explains this sort of magic and how to effectively use 
and control it? Could this be another genre of magic altogether? 
(Like wand magic, Old Magic, etc.)

-Sayaka

(Sorry if this topic has already been discussed)