Ch 18: Heat and TemperatureWorksheetSee all chapters
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Ch 01: Units & Vectors
Ch 02: 1D Motion (Kinematics)
Ch 03: 2D Motion (Projectile Motion)
Ch 04: Intro to Forces (Dynamics)
Ch 05: Friction, Inclines, Systems
Ch 06: Centripetal Forces & Gravitation
Ch 07: Work & Energy
Ch 08: Conservation of Energy
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Ch 10: Rotational Kinematics
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Ch 13: Rotational Equilibrium
Ch 14: Angular Momentum
Ch 15: Periodic Motion (NEW)
Ch 15: Periodic Motion (Oscillations)
Ch 16: Waves & Sound
Ch 17: Fluid Mechanics
Ch 18: Heat and Temperature
Ch 19: Kinetic Theory of Ideal Gasses
Ch 20: The First Law of Thermodynamics
Ch 21: The Second Law of Thermodynamics
Ch 22: Electric Force & Field; Gauss' Law
Ch 23: Electric Potential
Ch 24: Capacitors & Dielectrics
Ch 25: Resistors & DC Circuits
Ch 26: Magnetic Fields and Forces
Ch 27: Sources of Magnetic Field
Ch 28: Induction and Inductance
Ch 29: Alternating Current
Ch 30: Electromagnetic Waves
Ch 31: Geometric Optics
Ch 32: Wave Optics
Ch 34: Special Relativity
Ch 35: Particle-Wave Duality
Ch 36: Atomic Structure
Ch 37: Nuclear Physics
Ch 38: Quantum Mechanics

Concept #1: Temperature

Transcript

Hey guys in this video we're going to start our discussion on temperature now we all have a definition of temperature or an understanding of temperature based on our interactions with the physical world but what we need to do in physics is to define these things that we may have sort of an understanding and intuitive understanding of we have to define them in terms of actual physical quantities so that's what we're starting with here let's get to it. Now what temperature is a measurement of or what the measurement is associated with is how hot or how cold a substances is and we know this, this is exactly how we interpret temperature how hot or how cold something is right off the bat I want to say that temperature is related to a quantity known as heat we have terms like heating up to mean increasing the temperature the word hot itself comes from the word heat however temperature and heat are two completely distinct terms they don't have the same physical meaning they don't have the same units they are absolutely distinct but they are related through physical processes so it's going to be important to have good definitions of both alright now whatever heat is exchanged between two substances those substances will usually change their temperature. Imagine having a hot objects touching a cold object that hot object sends some heat whatever that is we don't know what it is yet to the cold object as that hot object descending this heat its temperature is dropping and as the cold object is receiving the heat its temperature is rising one thing we can absolutely say about heat right off the bat is that heat is a type of energy related to temperature so that hot object is exchanging energy with that cold object it's giving some energy which we call heat to that cold object in order to raise the temperature of the cold object alright and this is something that we know hot objects touching cold objects get colder, cold objects touching hot objects get warmer that's why you add ice to a warm drink to make it cold . What this means is that clearly if you have a change in the temperature there has to be some sort of associated change in energy heat exchange changes the temperature heat is an energy so a change in temperature has to be related to some sort of change in energy and this is going to be very very important in all of our study of thermodynamics this concept now just a quick question because we are so naturally OK With temperature we naturally have sort of this understanding of temperature what does it actually mean to be hot? Right when we say an object is hot, What does that mean to us what does it mean to be cold? These are sensations that are felt by nerves in our skin when we come into contact with objects of a certain temperature. The nerves in your skin react to a heat exchange with the subject in contact with you such as the air it could be the air which is always around you or it could be a hot surface or it could be a cold thing like an ice cube but whenever you're in contact with a different temperature you always have something like this you have that heat exchange between a hot object and a cold object if the air is hotter or the surface that you are touching like a cold surface or an ice cube or whatever the ice cube tends to be colder but if there is hotter energy enters the nerves and this is detected as hot or a high temperature if the air is colder energy leaves the nerves and this is detected as cold or a low temperature hot and cold are relative terms they mean hot compared to your body temperature or cold compared to your body temperature or more precisely in this case your skin temperature since we're talking about the nerves in your skin, now What we want to do is we want to define what temperature is and what we're going to start is something that we would call a naive definition a naive definition is a definition that is purposefully not complete. But gives just enough information to get us started it's something that we're going to build on later and naive definitions you see throughout the sciences you see them constantly in chemistry and physics and biology where in one semester you'll define something as this and another semester your professor come back and say hey you remember that thing well it's not actually quite that there's a little bit more going on here that original thing you learned was a naive definition will naively define temperature as a measure of the average kinetic energy of the particles making up the substance I cannot emphasize this enough imagine a block of metal, that block of metal can be moving and that block of metal has a kinetic energy due to its motion but that block of metal is also made up of a whole bunch of atoms inside of them the temperature is not related to the kinetic energy of the block that macroscopic kinetic energy the temperature is related to the kinetic energy of the movements of the individual particles that make up that block which would be called internal kinetic energies or microscopic kinetic energies, temperature related to the average kinetic energy of the particles making up the substance not of the substance itself the faster those particles are moving the higher that kinetic energy the greater the temperature and this is our naive definition of temperature this is where we're going to start alright so let's do an example a hydrogen gas with the mass of 2U, U being the atomic mass unit moves with an average particle speed of 6 meters per second and a nitrogen gas 28U moves with an average particle speed of 2 meters per second which gas is at a higher temperature. Remember temperatures were measurements of the average kinetic energy of the particles in a substance the higher that average kinetic energy the higher the temperature so we have these two separate gases each of a different mass each of a different speed and we know that kinetic energy depends upon both. That's our equation for kinetic energy let's look at the hydrogen gas for the hydrogen gas that kinetic energy is just one half times the mass which we're told is 2U, we don't actually need numbers we just need to know which is at a higher temperature the one that's going to be at a higher temperature is the one that's going to have a higher kinetic energy. So as long as we use consistent units we don't actually need to find numbers we just need to compare the results times this average speed is 6 squared so that 2 goes away and 6 squared is 36 I'm not even going to bother with the U let's just say 36 , what about the nitrogen gas well for the nitrogen gas we have K is one half times the mass of nitrogen is 28U this 28 has the same units as the 2 here that's very important we have to keep the same units if we want to compare our numbers and they move with an average speed of 2 so 28 becomes 14, 14 times 4 turns out to be 56 so which one is at a higher temperature. The 56 this is the hot gas this is the cold gas now notice that because the gases were at different masses we couldn't simply say the faster moving gas was at a higher temperature in fact the slower moving gas was at a higher temperature remember that temperature is a measurement of the average kinetic energy of the particles not of the speed right the nitrogen is moving slower but because it was so much more massive than the hydrogen it carried more kinetic energy and therefore had more temperature alright guys that wraps up this introduction to temperature. Thanks for watching.

Concept #2: Temperature Scales

Transcript

Hey guys, in this video we're going to talk about temperature scales which are the different units that are used to measure temperature alright let's get to it. Temperature is measured with three common units so there are three temperature scales we have Kelvin, we have Celsius and we have Fahrenheit. In the US the standard unit of temperature is degrees Fahrenheit. Which we put I forgot to put the symbol here ok degrees Fahrenheit in most countries though that use the metric system the standard unit of temperature for the metric system is the Celsius the degree Celsius I forgot to put the symbol here so Fahrenheit is the unit for what would call the imperial system or the British system in other countries which use the metric system Celsius is the standard unit for the metric system and finally that SI unit which is what we're going to use in physics for temperature is the Kelvin. Just to be technical incase you notice this in your books some books make a big stink about it some professors make a big stink about it you have degrees Celsius and degrees Fahrenheit but you do not have to degrees Kelvin it's just Kelvin it's just a convention now you can easily convert between any of these units of temperature with this table that I've put here going across we have whatever your temperature is given in going down we have whatever temperature whatever unit you want to convert it right so if you're in Fahrenheit and you want to convert it to Celsius this is your equation Fahrenheit to Celsius is a complicated relationship, Fahrenheit to Kelvin is also a complicated relationship and vice versa for both but Kelvin to Celsius is actually a very very simple relationship they're just off by a constant Kelvin is always 273 units larger than Celsius so 0 degrees Celsius is actually 273 Kelvin all you do is add 273 to it and 0 Kelvin is -273 degrees Celsius so those are very simple the relationships with Fahrenheit are a lot more complicated now Kelvin is known as the absolute temperature or the absolute temperature scale there's a very physical reason why Kelvin is called this and Kelvin was chosen to be the SI unit but you guys don't really need to understand this, this is a lot of history and a lot more complicated thermodynamics So what we're going to say is that it's called absolute because the bottom of the Kelvin scale 0 Kelvin is the lowest possible theoretical temperature and this is known as absolute zero. That's all you need to understand about why Kelvin is an absolute scale because the bottom of it is the absolutely lowest possible temperature theoretically possible. It turns out that zero Kelvin is not actually possible to achieve in real life this is sometimes called the third Law of thermodynamics that you cannot achieve zero Kelvin in a finite number of processes like if you take something that's at 20 degrees Celsius and then you cool it down to 0 degrees Celsius and then you cool down to 10 degrees Celsius -10 and a -20 you can never do a finite number of those cooling to get to -273 degrees Celsius which is zero Kelvin the closest that scientists have ever gotten as of 2015 this was the most recent number I could find were scientists at MIT Massachusetts Institute of Technology that got all the way down to 500 nano Kelvin Nano is 10 to the -9 that's a really really really small temperature just to give you a bit of scale space is about 3 Kelvin so the coldest we've ever achieved in a laboratory is 500 nano Kelvin and space is 3 Kelvin that's about 10 what 10 million times larger in space and one of the reasons why you can reach 0 Kelvin is because there's no such thing as a true vacuum you can't actually have a vacuum that has no particles in it and there are reasons due to quantum mechanics why this isn't true that we don't need to get into but space is not a true vacuum, space actually has a lot of crap in it which is called interstellar medium because it's not a vacuum it's not 0 Kelvin it has to have some temperature because that interstellar medium has some kinetic energy. Something important to note it's really really important to note this is that a change in temperature in units of Kelvin is equivalent to a change in temperature in units of Celsius this is because they just differ by a constant. You always have to remember this, this is absolutely not true for temperature whenever you're dealing with temperature you always need to use Kelvin because Kelvin is the SI unit in your equations 99% of the time it's just safer to use Kelvin in any equation because then it guarantees you won't get it wrong but whenever you're dealing with equations that use changes in temperature differences in temperature then technically the difference in temperature in Kelvin is the same as a difference in temperature in Celsius the size of one unit of Kelvin that size of one unit of Kelvin is the same as 1 unit of Celsius. That's why their differences are equal to each other alright. Let's do an example liquid nitrogen evaporates at a temperature of -196 degrees Celsius What is the temperature in Kelvin? What is it in Fahrenheit? Kelvin is easy all we have to do is take Celsius and add 270 to it the temperature in Kelvin is the temperature in Celsius plus 273 Sorry I said 270 I meant 273 so this is -196 plus 273 which according to my notes is just 77 Kelvin that one's really easy Fahrenheit is going to be a little bit more complicated. The temperature in Fahrenheit is going to be nine fifth's times the temperature in Celsius plus 32. So it's going to be nine fifths times -196 plus 32 is -321 degrees Fahrenheit and once again just to be super technical in case your professor harps on this these are degrees Fahrenheit there is no degree Kelvin. Alright guys that wraps up our discussion on temperature Scales. Thanks for watching.

Practice: For liquid hydrogen, H2, we can say the temperature is given by 3 2 π‘˜π΅π‘‡ = πΎπ‘Žπ‘£, where πΎπ‘Žπ‘£ is the average kinetic energy and π‘˜π΅ = 1.38 Γ— 10βˆ’23𝐽/𝐾 is the Boltzmann constant. What is the temperature of liquid hydrogen in Β°C if the average speed of each molecule is 509 m/s. Note that the mass of atomic hydrogen is 1.6 Γ— 10βˆ’27π‘˜π‘”.