Wands and Use Magic Device?

this wording "Activation: Wands use the spell trigger activation method, so casting a spell from a wand is usually a standard action that doesn’t provoke attacks of opportunity. (If the spell being cast has a longer casting time than 1 action, however, it takes that long to cast the spell from a wand.) To activate a wand, a character must hold it in hand (or whatever passes for a hand, for non-humanoid creatures) and point it in the general direction of the target or area. A wand may be used while grappling or while swallowed whole." and this one "Spell Trigger: Spell trigger activation is similar to spell completion, but it’s even simpler. No gestures or spell finishing is needed, just a special knowledge of spellcasting that an appropriate character would know, and a single word that must be spoken. Spell trigger items can be used by anyone whose class can cast the corresponding spell. This is the case even for a character who can’t actually cast spells, such as a 3rd-level paladin. The user must still determine what spell is stored in the item before she can activate it. Activating a spell trigger item is a standard action and does not provoke attacks of opportunity." seem to mean that it is an automatic activation if you know what is the spell you are casting, or if you find a wand with a spell that you have on your spell list, if you just find one that you have no idea what it is THEN you need to roll. but i would appreciate to read where it is explained that you need to roll even with wands you have bought

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