Question

Guys i need help on this, Thanks in advance!
Explain why energy balance cannot be applied directly on the conduction and convection heat transfer problems

Answer 1

I wish I knew thermodynamics, sorry. Are you talking about conservation of energy? My only guess is that it is because heat will always dissipate. You try to look a a closed system, but the system can't be fully closed with heat. It gets everywhere :P

Answer 2

And so the energy in your system before wouldn't be the same as after, as some energy has left the system.

Answer 3

Its actually a question in Heat Transfer. :S

Answer 4

btw, thansk for trying to help. :)

Answer 5

i don't quite get your question.. can u give a specific example? like where you mean to say energy balance is not applied?

Answer 6

In heat transfer, there are three mode of transfering, which are conduction, convection and radiation, and they have their own equations. The question simply asking why energy balance cannot be used directly for heat transfer

Answer 7

they have their own equations because the three modes of transfer are very different from each other

take conduction and convection for example, in conduction there is mass moving around , but in convection there is mass moving around.
Energy is definitely conserved .. but i still don't get your exact question sorry :P

Answer 8

i meant in conduction there is NO mass moving around

Answer 9

I think the answer lies in the term " transfer". While energy balance may be applied globally to a situation (energy at the start is equal to the energy at the end) there is a problem when you try to apply it locally as to a well defined finite volume. Heat flows from higher temperature to lower temperature which necessarily implies a lack of homogeneity in the energy distribution. This distribution is constantly changing with time ( and this is key) and position which is determined by conditions distal to the current observation point. Energy IS conserved in a specified volume ie energy entering it is equal to energy leaving which is determined by the thermal gradient which is external to the volume of interest. So the energy at a given point is continually changing. This seems a little clumsy but I think it is at the heart of the issue.

Answer 10

I do not see what the word "balance" can mean referring to the concept of energy.

Answer 11

Hmmm... If your question hasn't been answered, maybe you could explain what you mean by energy balance!

Do you mean look at the thermal energy in one body and the thermal energy in another and use that to find what the temperature will be at when thermal equilibrium is reached? When the temperature is balanced? Just a guess!

And you're welcome! :)

Answer 12

To keep this thread going, assume balance refers to conservation of energy. in Thermodynamics energy refers to internal energy and not just thermal energy or heat. Internal energy equals heat plus work done. Unlike internal energy the change in heat content of system depends on how it was produced i.e., its path and not just the state at the beginning and the state at the end. Since one can get from one state to another differently by doing different amounts of work the heat content is not exactly defined like the internal energy since depending on the "path" how the state develops the work done may be different. heat transfer processes only consider heat energy flow. So wouldn't that suggest that only processes in which no work is done can you apply conservation of energy?