INSTRUCTOR: In this screencast,
we talk about energy balances
and the difference between a closed and an open system.
So, for a closed system,
no mass crosses the boundary
between the system and the surroundings.
In an open system, however, mass will cross the boundary.
However, in both of these systems,
energy crosses the boundary
between the system and the surrounding.
So, let's start with the governing equation
for a closed system.
And that is delta U, which is the change in internal energy
plus delta E sub p which is the change in potential energy
plus delta E sub k which is the change in kinetic energy
is equal to the heat Q plus the work.
The first thing we do with this equation is to simplify it.
So, let's say that there is no acceleration.
Therefore, the kinetic energy is going to be equal to zero.
Let's say there's no change in height.
Then the change in potential energy
is gonna be equal to zero.
Now, we have times where the system's temperature
is equal to the temperature of the surroundings.
In that case, delta U is going to be equal to zero,
since internal energy is a function of temperature.
And finally, you might have a system that has insulated.
And another term that we use for it is adiabatic.
And if that's the case, then Q equals zero.
If we have no moving parts
and examples of that are pumps or turbines.
We have no electric currents,
we have no radiation.
Then our work W is gonna be equal to zero.
The governing equation for the open system
is similar to that of a closed system.
However, there are two important differences.
Instead of our delta U,
we have a delta H
and then we have our change in potential energy
or a change in kinetic energy.
And this is gonna be equal to Q plus W.
The other change is that because it's an open system,
so there is flow across the boundaries,
these are all going to be rates.
And therefore, we put a dot on each one of these terms.
Again, we have the same kind of simplifications
that we did in the closed system,
but one important thing to note here
is that the change these deltas for H,
for potential energy, for U, for kinetic energy,
it is the difference
between the final state minus the initial state.
However, when we're dealing with Q and W,
what we're looking at is what happens across the boundary
between the system and the surroundings.
So, why don't we look at an example?
So here we have a cylinder, it has a movable piston
that moves slowly compressing a gas.
We're going to assume
that the change in potential energy is negligible,
and we want to find the energy balance.
So the first thing we ask ourselves,
is this a closed or open system?
And the answer is it's a closed system
because no mass crosses the boundary of the system
where the cylinder is the system.
Is there a change in kinetic energy?
Again, we look at our system, that's the cylinder,
does the cylinder move? No it doesn't.
So, we have no kinetic energy.
And so what we're left with then is that Q plus W
is going to be equal to our delta U.
So now let's say that this is carried out isothermally.
So, if it's isothermal,
that means that our delta U is gonna be equal to zero.
Therefore, Q is equal to minus W.
Let's say now that the system instead is adiabatic.
Well, that means that Q is equal to zero
and now we're just left with that delta U is equal to W.
Now let's look at an example of an open system.
Here we have a liquid stream flows through a heat exchanger
where it is heated.
It's constant diameter, so there's no change in velocity
and it's horizontal
so there's no change in elevation.
Note that we said it was a liquid stream,
so we're going to assume that the density stays constant.
So, what that means is that our potential energy
and our kinetic energy are both equal to zero.
In addition, there are no moving parts.
There is no electrical current, radiation.
So we can also say that W is equal to zero.
So our energy balance
simplifies to that Q is equal to our delta H.
Now, let's say that we add a pump to this system.
So now we do have moving parts
and therefore we do have work done.
So our energy balance is Q plus W
is equal to delta H.
Finally, let's say that the system is adiabatic
or insulated, which means that our Q is equal to zero.
Therefore, our delta H is equal to zero
and H initial
is going to be equal to H final.
Generally, when you do an energy balance problem,
you need to start with using
the proper form of the energy balance,
decide whether it's open or closed,
and then look at the problem itself
and decide based on some of the suggestions
that I've given you,
what terms can be dropped to simplify your equation.