Wednesday, 5 June 2019

thermodynamics - Why is heat a scalar quantity?


After looking at several definitions,Vectors are quantities having both magnitude and direction.Heat has a magnitude and also a specific direction(from higher temperature to lower temperature)...so why is it considered as a scalar but not as a vector?



Answer



Heat, by definition, does not have a direction. It is just the amount of energy transferred some thermal process. The quantity you're asking about is the heat flux, which is a vector. (Note that heat does not have to travel from higher to lower temperature; it can and does go the opposite way!)



Similarly, energy does not have a direction. If you have an electromagnetic wave traveling in some direction, it carries energy with it. That energy still doesn't have a direction. Instead, there's a new quantity, the Poynting vector, that is a vector and describes the direction of energy transfer. That is the way it is with heat. Even when heat is moving, we don't say the heat has a direction. Instead we define a new quantity for that.


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