hello everybody my name is Iman welcome back to my YouTube channel today we're continuing chapter three for MCAT physics this chapter is about thermodynamics so far we've discussed our zero with law and our first law of thermodynamics we've defined heat we've looked at three means that heat can transfer energy we discuss heat specific heat as well as phase changes as a last point to our third objective before we move into discussing the second law of Thermodynamics we want to talk about thermodynamic processes all right in the last chapter we gave significant consideration to work as a change of energy in a system both as a function of force and displacement and as a function of volume and pressure all right now we will briefly review the latter and its relationship to heat transfer within a system keep in mind that work accomplished by a change in displacement is not likely to be motivated by heat transfer and any heat transfer that does occur is most likely a result of friction dissipating mechanical energy from the system now with that preface during any thermodynamic process a system goes from initial equilibrium state with initial pressure temperature and volume to some other equilibrium state which may be a different final pressure temperature or volume these thermodynamic processes can be represented in graphical form with a volume on the x-axis and a pressure or or temperature on the y-axis all right so we can see an example of this kind of pot right here all right we can see that plot right here now the MCAT focuses on three particular thermodynamic processes as special cases of the first law and we're going to be covering those in each of these cases some some physical property is held constant during the process all right these processes are isothermal in isothermal processes all right we have constant temperature and therefore no change in internal energy in a pressure versus volume graph an isothermal graph is going to have a slight tilt to it all right it looks like that on a pressure versus volume graph all right another thermodynamic process is adiabatic and adiabatic we have no heat exchange whatsoever all right so it'll look something like this on a pressure versus volume graph all right another kind of thermodynamic process is isovolumetric All Right iso volumetric or in other words all right isotoric all right also isovolumetric and isochoric are synonyms so I'm going to write this down all right this happens when we have no change in volume all right so obviously in a pressure versus volume graph if you have no change in volume then you're going to have the same volume throughout the process the only thing that will change is pressure all right so Isobel ISO volumetric or isochoric these are no change in volume okay therefore by the way no work is accomplished all right and then last but not least we have isobaric all right isobaric processes are those that occur at constant pressure all right and so if there's constant pressure then there's no change in pressure and so of course your plot for an isobaric process is going to be a straight line a straight horizontal line for an on a pressure versus volume graph now with that mostly just remember and recall these names and what they relate to that's going to be very important for the MCAT all right and with that we can move into our last and final objective and here we're going to cover the second law of Thermodynamics now the second law of thermodynamics states that objects in thermal contact all right and not in thermal equilibrium they're going to exchange heat such that the object with higher temperature will give off heat energy to the object with the lower energy the temperature until both objects have the same temperature at thermal equilibrium all right as such energy is constantly being dispersed in other words the second law of thermodynamics states that energy spontaneously disperses from being localized to being spread out if it is not hindered from doing so pay attention to this though the usual way of thinking about the second law of Thermodynamics is in regards to the word entropy and most people will tell you that entropy is a is is a a disorder thing all right but this disorder thing term should not be too taken too literally this is a trap that many students fall into all right thinking that entropy is disorder entropy is the measure of the spontaneous dispersal of energy at specific temperature all right it's how much energy is spread out or how widely spread out energy becomes in a process all right so in this in in this discussion earlier all right we consider that one ice melts all right the Freedom Movement of the water molecule is going to increase right when we're talking about phase changes all right ice was an example all right and when we consider that ice melts all right what happens when ice melts well the freedom of movement of the water molecules increases so that it can melt when ice melts all right this is and it has this freedom of movement that increases this is due to the increased number of available microstates microstates refer to the different Arrangements of particles within a system that are possible at a given energy level when you go from solid to liquid for water molecules you have more freedom of movement all right because water is not as rigid as ice and so when ice is melting it's introducing to a freedom of movement that these water molecules increase that increases all right and this is because there are more microstates available now if the water remains at The Melting Point it will have the same average kinetic energy as molecules of ice the difference between the two is just the available microstates that is while both in water both water and ice at zero degrees have the same kinetic energy the energy is dispersed over a larger number of microstates in liquid water liquid water therefore has a higher entropy and by extension it is indeed less organized than ice all right the following equation can be used for calculating change in entropy all right this one right here all right this equation can be used to calculate entropy what we have in this equation is Delta s is changing entropy Q is the heat that is gained or lost in a reversible process and T is the temperature in Kelvin the units of entropy are usually joules per mole times Kelvin all right when energy is distributed into a system at a given temperature its entropy increases and when energy is distributed out of a system at a given temperature it's entropy decreases all right notice all right notice that the second law States that energy will just will spontaneously disperse all right it does not say that energy can never be localized or concentrated however the concentration of energy will not happen spontaneously in a closed system work usually has to be done to concentrate energy for example refrigerators they work against the direction of spontaneous heat flow all right that is they counteract the flow of heat from the warm exterior of the refrigerator to the cool interior thereby concentrating energy outside of the system in the surroundings as a result refrigerators they consume a lot of energy to accomplish this movement of energy against the temperature gradient thereby keeping your temp your your inside of the refrigerator cool all right so second law a discussion about how energy moves all right how energy is dispersed all right and it is also it also talks about entropy the entropy of a natural spontaneous process either increases or remains constant heat flows from a hot body to a cold body all right and an important thing to note which will Define reversible and irreversible in a second all right but reversible processes are ones where Delta s is equal to zero and irreversible processes where Delta s is greater than zero all right now let's let's do a practice problem but I just want to make one more statement about entropy and the second law of Thermodynamics the second law has been described as times Arrow because there is a unidirectional limitation on the movement of energy by which we recognize before and after or new and old for example we instantly recognize whether a video recording of an explosion was running forward or backward another way of understanding that this is to say that energy in a closed system will spontaneously spread out and entropy will increase if it is not hindered from doing so remember that A system can variably diff can be variably defined to include the entire universe in fact the second law ultimately claims that the universe that the entropy of the universe is increasing and because of that we can write the following expression that Delta s of universe which is equal to your Delta s of system plus surroundings is greater than zero because it's always increasing all right now in thermodynamics we also have two important terms we have reversible and irreversible we've defined it using entropy here right Delta s equal to zero reversible process in other words though a reversible process is a process that can be reversed without leaving any trace on the surrounding this means that if the system is returned to in its initial state by reversing the process both the system and the surroundings will be in their original states in contrast we said that Delta s greater than zero signals an irreversible process and an irreversible process is a process that cannot be reversed without leaving some effect on their surroundings such as the dissipation of energy or an increase in entropy all right now in the real world most processes they're irreversible but some processes like phase changes under certain conditions could be considered reversible under highly controlled conditions all right with that being said let's do a practice problem to end this lecture this problem says all right if in a reversible process 6.66 times 10 to the four joules of heat is used to change at 200 gram of block of ice to water at a temperature of 273 Kelvin what is the change in entropy of the system and note that the Hue of the heat effusion of Isis 333 joules per Kelvin beautiful we know that during the phase change the temperature is constant all right and so what we can do is just use our entropy equation Delta s is equal to Q all right reversible over temperature and we're just going to plug in the values that they've given us they've given us how much heat is used 6.66 times 10 to the 4 joules and they've given us the temperature it stays the same 273 Kelvin all right and we can just calculate this we get two four four joules per Kelvin the amount of heat was exactly enough to completely melt the block of ice without changing the temperature of the resulting liquid water all right so we have completed the problem and we have figured out what the change in entropy is and what the heat is as well all right fantastic let me know if you have any questions comments concerns down below other than that we've covered chapter three all right and next video we'll do some practice problems all right other than that good luck happy studying and have a beautiful beautiful day future doctors