How would fantasy magic work if it followed the laws of thermodynamics?

Some questions to consider:<br><br># What is the nature of magic power (mana, willpower,WithName of God, etc)?<br><br># What is its thermal equivalent?<br><br># Is it endothermic or exothermic (heat absorbing or heat producing)?<br><br># Is efficiency of conversion a limiting factor?<br><br># What is the heat capacity of the surroundings?<br><br># Is mages' heat tolerance a limiting factor?<br><br># What is the environmental impact of magic?