00:01
For this problem, we're told that a gas undergoes a process moving from point a to point c in the figure.
00:12
In doing so, it experiences a change in internal energy, excuse me, delta u equal to positive 800 joules.
00:27
We're told that the work done on the gas is equal to negative 500 joules, which means that the work done by the gas will be a positive 500 joules.
00:45
In part a, we want to know how much energy must be added by heat in order for the gas to move from point a through b to c.
00:56
We can use the first law of thermodynamics that says the change in internal energy is equal to the energy added as heat minus the work done by the gas.
01:13
And we can go ahead and solve for q because that's what we're interested in here.
01:20
So q will be equal to delta u plus w.
01:24
We can plug in the values given in the problem.
01:28
Have 800 joules plus 500 joules for a total of 1 ,300 joules.
01:40
For part b, we're told that the pressure at point a is equal to five times the pressure at point c.
01:54
And we want to know then what is the work done on the system and going from c to d.
02:00
So one thing that we can note is that the pressure, or sorry, the work done in the system from going from a to b is the work that we saw in part one or in part a, which would be the negative 500 joules.
02:31
But we could have calculated this by multiplying the pressure times the change in volume.
02:39
So this would be equal to the pressure at point a times the change in volume.
02:49
If we're looking at moving from point c to point d, the first thing that we note is that we're going in the opposite direction.
03:00
So instead of being a negative pressure, it should be a positive work.
03:05
It should be a positive work.
03:08
And then we need.
03:09
To note that we can multiply again the pressure times the change in volume.
03:22
The change in volume is going to be the same in both of these cases, but we know the relationship between pressure a and the pressure at point c...