Heat Engine

This is a summary of what my lab group and I did in Physics 4B on September 9th, 2014.

Chimney Effect:

The first demonstration was that of a candle that was lit inside of a graduated cylinder. My lab group and I thought the fire will stretch upward attempting to gain more oxygen gas to burn but the fire actually extinguished pretty quickly. This was because not enough oxygen was flowing down into the graduated cylinder. 

As you can see in the clip below, the fire was almost immediately extinguished as it entered the graduated cylinder.

But what if we inserted a small PVC pipe and place it right above the flame?

My lab group and I predicted that the PVC pipe will create an air current allowing oxygen to flow into the cylinder through one side and the CO2 through the pipe. Our prediction was correct.

As the "chimney" is put in place, the fire continues to burn.

And the flame became dimmer as the "chimney" began to grow farther and farther away from the flame.

Next, we were asked what would happen to the candle's flame if it was put inside a gallon jug with a rubber stopper as the cork and in free fall. My lab group and I thought because it was a closed system, the flame would be unaffected; however, we were wrong. The flame actually became dimmer because convection no longer played a role in the system and the flame was fueled by diffusion instead.

We were told to give a real life example of negative work, which means that there is no heat involved in the system. We came up with the compressing of a spring. Because delta internal energy is equal to negative work in this system, the internal energy is increasing as the potential energy goes up in the system.

State Variables and Ideal Gas Law:

As a class, we visited a site that explained Isobaric, Isovolumetric, and Isothermal processes. We answered six questions regarding the processes; three of which we were told to draw the graph of the different processes. And then, with given values, we found various components of the gas (pressure and volume) using the definitions of the three processes and the Ideal Gas Laws.

Then, we were asked to identify what processes the four graphs represented on a PV-plane. Graph A was isobaric because pressure was constant. Graph B was isochoric because volume was constant. Graph C was speculated to be adiabatic because the slope was a lot more steeper than graph D. Graph D was speculated to be isothermal because the slope was less steep than graph C.

Heat Engine:

Below is a demonstration of what a heat piston does. When the heat was applied, the syringe acted as a piston and began to emulate that of a piston by going up and down. In this situation, there was no work being done but heat was being applied.

We were given a question regarding a water tank. The first part of the question was to identify the work being down by the gas on the tank. After some derivation, we determined the equation for the work done by the gas to be the pressure of the air times the volume of the tank times the natural log of 4 since the water only rose to fill three-fourths of the tank leaving one-fourth of the tank of air.

Next, we were asked to identify the work being done by the pump. Here, the equation was the change in mechanical energy plus the absolute value of the work done by gas subtracted by the work done by the air.

This apparatus measured gauge pressure, volume of the cylinder, and the temperature within the cylinder. Professor Mason changed the volume of the cylinder from 50 cc to 30 cc and then asked us how much work he did on the system. The answer has to be small because it did not take him a lot of energy to change the volume of the apparatus.

After identifying the knowns, we determined the equation of the isothermal work to be initial pressure times initial volume times the natural log of final volume over initial volume.

Efficiency:

Below is a cycle that my lab group and I created to describe the process of a rubber band engine.

The Greek letter for efficiency is eta (or e) and that is equal to the work done divided by the energy inputted to the system. If we recall one of our work equations, it was energy input minus energy output. If we substitute that into our efficiency equation, we get efficiency equal to one minus energy output over energy input. The perfect efficiency of a system would be when either energy output or energy input is equal to zero but that is almost impossible to achieve.

Analyzing the Cycle:

Below is the answer to the questions scribed in our lab manual. First, we drew the PV diagram of a cycle, which turned out to be a rectangle because temperature was kept constant.in parts A and C of the graph, work was being done by the gas and on the gas respectively. Also, in part A of the graph, the heat energy is transferred to the gas from a reservoir and in part C of the graph, the heat energy is being transferred from the gas to the reservoir.

This system turns out to be 6% efficient, which is not very efficient.