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This video covers the general balance equation, and it applies for any conserved quantity
mass, energy, momentum, and ect. The way we write it is that the input plus the generation
minus the output minus the consumption equals accumulation in the system. In material and
energy balances we start off my using this in the mass or material balances. So lets
take a simple processes here. We have our input, which goes into the system. Our output
of the system, which comes out of the system and inside is were we get our generation our
consumption and our accumulation. So there are some simplification that we can make.
Like for example if the system is at steady -state then our accumulation equals zero,
If its stead-state and non-reactive, then we have that input equals output. Why doesn't
we look at an example. So we have water that enters a tank at some mass flow rate that
we will call m dot. It exits the tank at a flow rate that equals 2 tomes m dot. Our tank
is initially half full, and we want to know the general balance for this process. So we
know that we have water that is entering. So we know that we have input. We know that
we have water that is exiting. So we know that we have an output, but because it is
exiting at a rate twice as what it is going in. We are going to end up with a consumption
term and this is going to equal 0. So this is not at steady-state because eventually
the tank is going to end up empty. Another way you could look at it instead of having
a consumption term you could write it as, input minus output equals accumulation. Where
the accumulation is actually a negative, but physically it makes more sense to us the consumption
term.