Application of Thermodynamics II Law in the Use of Presto Pan Tool

         Termodinamika yaitu ilmu yang mempelajari perubahan antar kalor dan bentuk-bentuk energi yang lain. Dalam termodinamika, kita mempelajari perubahan-perubahan dalam keadaan sistem, yang didefinisikan sebagai nilai-nilai semua sifat makroskopis yang relevan, seperti susunan, energi, suhu, tekanan, dan volume. energi, suhu, tekanan, dan volume dikatakan sebagai fungsi keadaan (state function) , sifat-sifat yang ditentukan oleh keadaan sistem, terlepas bagaimana keadaan tersebut dicapai. Dengan kata lain, ketika keadaan suatu sistem berubah, besar perubahan dalam setiap fungsi keadaan hanya bergantung pada keadaan awal dan keadaan akhir sistem dan tidak bergantung pada bagaimana perubahan itu dilakukan. (chang, Raymond, 2005)

        Ideal gas is a perfect gas state, has certain properties so that it can be applied to the kinetic theory of gas. Ideal gas is a gas be produced by human thought. Certainly the ideal gas is not find on the surface of the earth. The equation used for an ideal gas is PV = nRT and other gas laws at all temperatures and pressures. The properties of an ideal gas are as follows :

1. Gas consists of particles in large numbers, which always move in an arbitrary direction and spread evenly in a small space.

2. The distance between gas particles is greater than size of the particle, so the size of the gas particles can be ignored.

3. The collision between the gas particles and between the particle and its wall is perfectly elastic.

4. Newton's laws of motion apply.

Hukum II berpusat pada masalah entropi, hukum II termodinamika bisa dinyatakan sebagai berikut : “entropi dapat diciptakan tetapi tidak dapat dimusnahkan”. Hubungan tekanan temperature terhadap tingkat wujud suatu zat dapat dilihat dengan titik didih suatu zat cair yang dipengaruhi oleh tekanan udara, artinya makin besar tekanan udara makin besar pula titik didih zat cair tersebut. Pada tekanan dan temperatur udara standar (76 cmHg, 25°C) titik didih air sebesar 100°C (Anonim, 2011).

Hubungan antara entropi dan kespontanan reaksi dinyatakan oleh hukum kedua termodinamika : entropi semesta (universe) akan meningkat dalam proses spontan dan tidak berubah dalam proses kesetimbangan. Karena semesta terdiri atas system dan lingkungan (surrounding), perubahan entropi dalam semesta untuk proses apa saja adalah jumlah perubahan entropi dalam sistem dan perubahan entropi lingkungan. Sehingga untuk proses spontan hukum kedua menyatakan bahwa perubahan entropi semesta harus lebih besar dari nol. (Chang, Raymond, 2005). 

Thermodynamics is very closely related to the use of kitchen utensils. Even in the human body thermodynamic processes also occur. Lots of thermodynamic events that occur in everyday life. For example the use of presto pan. In cooking using presto pan, milkfish spines or other food ingredients can be soft for a short time without damaging the structure of the food ingredients. This is because in the presto pan follows a closed system, which is a heat exchange but there is no change in mass. Presto pan is made of thick and strong stainless and has a tight lid. When cooking with a presto pan the heat from the environment (stove) enters the system (presto pan) and produces water vapor. Water vapor produced during the boiling process is impossible to come out and only collected in a presto pan, so that water vapor enters the food to produce a heat transfer effect. When the pan is tightly closed and heated at high temperatures, the pressure of the water vapor will increase and the temperature will be rise. Under high pressure conditions, the fiber in the food will be soft and the taste of the food will seep quickly. Based on this, the cooking process is faster and can maintain food humidity.

In thermodynamics, it is like an ideal gas equation, where PV = n.R.T or PV = CT, where: P = pressure; V = volume; T = temperature; C = gas equilibrium constant; the equation is also well know called the Gay-Lussac law. So it can also be interpreted the relationship between P and T that the addition of pressure will affect the boiling point and melting point.

As a safety, the presto pan has a safety valve located on the lid of the pan and has the function to release vapor pressure when excessive. This is because the pressure cooker uses a tight lid so water vapor cannot come out. When the pan reaches the threshold or the standard cooking time calculation starts, a hissing sound will be marked because water vapor passes through the safety valve.

It can be concluded that the workings of the Presto pan are in accordance with the laws of thermodynamics II and the ideal gas equation (Gay-Lussac's Law) is the heat absorb by the pan and trapped in a closed system and makes the reaction spontaneous because the heat absorbed does work that is with the pressure. Then the entropy of the system will rise to steam and steam will become vapor pressure for food so that the boiling point of the system rises and this is directly proportional to the temperature rise.