2nd Law of thermodynamics - Principles of Refrigeration

so in the previous lesson we took a look at this basketball and we were wondering why we never see a ball spontaneously start bouncing from the ground the curious part about this problem is that it doesn't violate any known laws of physics so what laws of physics could it possibly violate if it doesn't violate Newton's three laws of motion the conservation of momentum or the conservation of energy we never see it happen so there must be something that prevents it from happening and that something is called the second law of thermodynamics now we said there are two consequences of this law the first consequence was there is a theoretical limit to the efficiency of a heat engine the second consequence of the second law of thermodynamics is that heat will never spontaneously move from cold places to warm places work is required to pump heat from cold places to warm places so imagine we have two rooms both rooms are the same temperature hot and we want to pump heat from room one into room two making room one cold and room two really hot how could we do this well the second law of thermodynamics says that work is required but if we're willing to put in the work it is possible so how could we create something to actually pump heat from room one into room two well we're going to use this principle of compressing and decompressing gases so if we have a piston filled with gas and we compress it it will heat up then if we allow that heat to escape and release our hand the gas will expand decreasing its internal energy and cooling down it's this principle that we're going to use to build our refrigeration device so here we have room one and room two and we have a container filled with gas which I will just call a refrigerant now this is how it works if we move the container to room to compress it so it heats up and allow that heat to escape and then move the container back to room one and allow it to decompress then the heat from room one will move into the container if we repeated this process over and over over again eventually the heat from room 1 will be pumped to room 2 now if we look at all the components into making this happen we see that there are only really three essential pieces the first piece is some sort of transport medium so this is the container that is filled with the gas or the refrigerant the second important component is some compressor in this example that hand that compresses the piston plays the role of the compressor and the third important component is the D compressor in this case we have a piston that as the gas expands and lifts up that piston if we have these three components we can build some sort of device that transports heat from one room to the other now we have two rooms and we build in a compressor that simply plays the role of compressing gasses we also build an addict compressor that does the very same thing except the opposite if we connect both of these rooms to a set of tubes carrying some sort of refrigerant like a gas or a liquid that becomes a gas then we have all three necessary components as this gas is pumped through the compressor it will heat up and as it k-y-t's up that hot gas will travel through these long s curves and cool down over time and when the gas goes through the D compressor it's going to rapidly cool down so on the other side of the D compressor we have this very cold gas that slowly warms up as it goes around the curves so you can imagine this gas being pumped through these wires through the compressor then the D compressor again and again and again and eventually heat is going to be pumped from this room over to the second room so we're going to find one room that gets really cold and then the other room that gets really hot and this is exactly how a refrigerator works so here is the back of a refrigerator right here we see this motor that plays both the role of the compressor and the D compressor and then we see the cooling coils on one side of the refrigerator inside it is an identical set of cooling coils so hot compressed gas moves through these outer coils and cools down and then it hits the D compressor and cold gas flows through the inside cooling everything on the inside the refrigerator down now if we build a diagram of a refrigeration unit it would take something from a cold reservoir and pump heat from the cold reservoir to the hot reservoir but the second law of thermodynamics says that this process does not happen spontaneously but we need to input a certain amount to work so the work that goes in actually produces heat this means that the heat that comes out of the cold reservoir does not equal the amount of heat going into the hot reservoir but we see that there's more heat going into the hot reservoir than coming out of the cold reservoir the process of pumping heat from one place to another produces more heat now this is good news and bad news the bad news is this means you can't really cool your room down using a refrigerator see if you open a refrigerator door then you're going to draw in heat from the outside and as more heat is drawn from the outside you're going to kick on that pump which produces more heat and that heat is dumped back to the outside world this means if you don't close your refrigerator door eventually the refrigerator will make your room a much hotter than it would have been had you just kept it close to begin with the good news is we can design some sort of heat pump that takes heat in from the cold outside and pumps it into the inside on days when it's kind of cold so imagine now we can pair these two different types of heaters an electric heater and a heat pump on the right is a heat pump and on the left is electric heater now an electric heater works by warming up wires by passing electrons through these wires heat pumps work on the process of reverse refrigeration they pump heat from the cold outside into the warm inside now if we were to compare the amount of electrical units it takes to run electric heater and the amount of heat each one of these heaters produces we will find something very interesting an electric heater may take plus 1 units of energy to run likewise a heat pump may take +1 units but the heat that we get out of an electric heater can only get to +1 units of heat this is because energy is conserved the amount of energy that goes into the electric heater must be equal to the amount of energy that comes out but the heat pump works by different principles it can actually get more heat out than electric energy is put in how is this possible is this a violation of the conservation of energy while actually no because the heat that comes out of the heat pump comes from outside so we get a bonus we get heat that's drawn in from cold air and we get the bonus heat that's produced in the process of making this happen so we find that heat pumps are much more efficient at producing heat than an electric heater and something that should always be considered when designing an energy-efficient home

Channel: SeeTheChangeUSA

Views: 122,504

Keywords: Heat, Pump, Refrigerator, Entropy

Id: dDQgOvmSXCE

Length: 7min 41sec (461 seconds)

Published: Fri Apr 22 2016