Entropy and Refrigerators

On September 16th, 2014, Physics 4B learned about the chaotic entropy and the efficiency of refrigerators and air conditioners.

Entropy:

Entropy is defined as the amount of energy needed to heat up a gram of water by one degree Kelvin. Or Q/T. The units for entropy is Joules over Kelvin. But there is a way to make entropy unit-less and that is dividing the equation by constant k.

We also have listed some examples of change in entropy in real life applications.

Stripling Engine Cycle:

The Stripling Engine's efficiency, although built before, is actually similar to that of a Carnot Engine. It relies on the heat applied on the bottom of the engine and the iced top to move the piston. The two different temperatures on both sides means that the piston movements is relied on the change of pressure. Below are some pictures of the Wanckle engine.

Below is a simulation of the process of a Wanckle engine.

As stated before, the efficiency is pretty high but one would prefer not to have it in their cars because that would mean the engine would have to be enormous.

Refrigerators:

Next, we were asked to find the output heat of a refrigerator.

If one were to look at the packaging of an air conditioner, the efficiency is measured in BTU/hr/W. But what is a BTU in Joules? We were asked to find the conversion factor. 1 BTU is about 1055 Joules.

Keeping that in mind, how many Joules per second are in 12000 BTU per hour? Approximately 3516 Joules per second.

Reversible Engine:

We were then given two blocks of water and ice with different temperatures and were given the task to find the final temperature when both blocks were fused. Assuming the change in entropy in the system was zero, the final temperature of the blocks when fused was found to be 46.1 degrees Celsius.

We then found an equation for work.

And found a new equation for efficiency for a reversible engine.

We were then asked a trivial question: How many flights of stairs would a person be able to climb per minute? We found work to be 4900 J and one horsepower to be about 760 Joules per second. After converting, a human would be able to climb 9.3 flights of stairs per minute. Although, thinking about it realistically, it is not possible even with a burst of energy since humans cannot sustain energy for a long period of time during extensive exercise.

We were then asked to find the coefficient of power for refrigerator running at three-fourths of horsepower.

 The goal was to find how long it would take for a freezer to freeze 4.2 kg of water. Theoretically, if the freezer was to run at full power for 13.7 mins, it would be possible.