When Eragon uses magic for the first time he goes unconscious and brom gets his arm slashed by a urgal, Eragon breaking his arm is later in the story.
Eragon breaks his arm from tripping, he then finds a urgal track and goes back to brom/saphira, urgals catch up, he tosses 8 of them with magic. Brom then rides Saphira to hunt down the urgals one by one.