2nd Law of Thermodynamics (Fifth Day)

Spring 2015
10th of March's Class

2nd Law of Thermodynamics

Today we began class by learning of 3 types of conditions for ideal gases. Isobaric [Constant Pressure], Isovolumetric [Constant Volume], and Isothermal [Constant Temperature]. We were asked to make a graph that shows our prediction on how the graph would look like based on the relationship of the other 2 variables that does not change.

We then made calculations based on the 6 questions given to us from the website []. The image below is how we attempt to answer the questions.

We were then given 4 different graphs and was tasked to predict which graphs would correspond to which type of condition. And we have to explain why the 3rd (Isothermal) and the 4th (Adiabatic) seem similar but are different.

Isothermal means that the Temperature remains constant. Therefore its PV = nR delta T, is just PV = nR, this is the resultant equation for the graph's curve.

Adiabatic involves heat, therefore the formula Q= mC delta T comes into play. Singling out the delta T, we have delta T = Q/(mC). Substitute that into the PV = nR delta T formula, and we have PV=nR[Q/(mC)]. Adiabatic type has negligible Q so its Pressure and Volume relationship can be written as PV=nR/(mC).

So when we put them in comparison, Isothermal is PV =nR, while Adiabatic is PV = nR/(mC). Therefore, it can be observed that the curve of the graph for Adiabatic would be steeper as it would be divided by an additional (mC) variable.

The professor showed us an experiment similar to the one on 26th of February. A water bottle would be hung from a rubber band, and then the rubber band would be heated up by  hair dryer. We predicted that the rubber band would expand, and the height of the water battle from the floor would decrease.

However, in reality it was contradictory from our prediction. The rubber band actually shrunk as it is a polymer, and as it was heated up, it curls upward, causing it to shrink in size.

We derived a different form of the efficiency formula.

We then attempted a heat cycle question about an engine. This can be found in the Lab Manual.

At the end, we end up with this table shown in the image right above.

The 2 image above shows the 2 different ways of finding Net Work on the cycle. The first one is by calculating all the components, while the other is by determining the area of the graph.

After that the professor attempted to replicate the experiment in real life. And we were supposed to calculate it in the same way we did the heat cycle question before.

We picked the 3rd point as our base PV = nRT as it has the closest Temperature to room temperature and Pressure in the room.