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## Class 11 Physics (India)

### Course: Class 11 Physics (India) > Unit 15

Lesson 1: Measurement of temperature and pressure# Absolute temperature and the kelvin scale

This video explains temperature as a measure of average kinetic energy in a system. It highlights the Kelvin, Celsius, and Fahrenheit scales, detailing their differences, conversion methods, and applications. The concept of absolute zero, the coldest possible temperature, is also introduced. Created by Ryan Scott Patton.

## Want to join the conversation?

- Why do the Fahrenheit and Celsius scales have a common temp of -40 degrees? Is it possible that they have other equivalence points(Where Fahrenheit = Celsius) further in both directions?(4 votes)
- no.

The equation is:

F=C(9/5)+32=====>Similar to a line equation with slope=9/5 and y-intercept=32

Lets have a Cartesian plane with C and F as x and y axes.

=>For having equivalence point, C=F which is same as x=y.

So, solving the two equations

F=C(9/5)+32

F=C,

I will get only one solution and that's the intersection of these lines @(-40,-40). So, the answer is F=C is only at -40.(9 votes)

- At0:30, it is stated that particles move in rotation and curved paths but the kinetic molecular theory states that particles move only in straight lines. Error?(5 votes)
- In real life, particles can indeed rotate. The kinetic molecular theory is an approximation.

Whether in quantum mechanics or classical mechanics, particles can rotate about some axis. However, if the particles are single atoms, then they are approximated by point masses. Then they don't have rotational kinetic energy. In classical mechanics, this is because rotational kinetic energy depends on moment of inertia, mr^2. But r is zero since points don't have radius. So we can ignore rotational energy for single-atom molecules.

Therefore, kinetic molecular theory only applies to single-atom molecules like helium and neon. For hydrogen and oxygen (two atoms), you must use the equipartition theorem. In fact, for polyatomic atoms, you also must take into account vibrational kinetic energy and potential energy.

But I don’t think particles move in curved paths; we’re neglecting any outside forces like gravity, so they should move in staight lines at constant speed until they hit something.(5 votes)

- In regards to0:30, the kinetic molecular theory states that particles only move in straight lines. The video says they move in rotation and curves. Is this an error?(4 votes)
- They travel in straight lines until they collide with another particle or the walls of the container.(6 votes)

- i didn't quite understand how it is' logically 'possible that celsius and fahrenheit scales can have a common temp..? please explain .(3 votes)
- u have the Fahrenheit-Celsius equation.

equate it with F=C.

There'll be one solution and that's -40.(5 votes)

- Is absolute zero the lowest possible temperature?(3 votes)
- Yes. Kind of.

It is impossible to have temperatures below zero, like it is impossible to be north of the north pole (*not*above the north pole, that doesn't count). Temperature is defined in a way that it cannot be below absolute zero.

But in fact, the "third law of thermodynamics" states that you can't even reach absolute zero in a finite number of processes, let alone below it. No one has succeeded in achieving absolute zero, even though they got*really*close.

If you could achieve absolute zero, that would mean there would be 100% efficient energy generators and we would live happily ever after.(4 votes)

- Is temperature in kelvin scale also known as the absolute temperature ??(3 votes)
- Yes, and also the 0 degrees at kelvin scale known as absolute 0 temperature.(1 vote)

- Why do you use keliven and where did Kelvin come from(2 votes)
- From what I remember its like the most accurate unit for measurement since at 0 K is the temperature in which there is no kinetic motion of atoms whereas units such as Celsius is based on the properties of water, 0 degrees being the temperature at which water freezes..(4 votes)

- The question is little off track...but can someone tell me why gases do not have free electrons?(2 votes)
- They can have free electrons, like for example if you were able to get a metal into the gas phase it would have free electrons and essentially be a plasma.(3 votes)

- At2:40-2:42, I saw online that the freezing and boiling points were actually 273.16 and 373.16 respectively. I know that a half-degree can make all the difference, so which one is correct, .15 or .16?(1 vote)
- I don't see why freezing and boiling points of water would be 273.16 and 373.16. I'm pretty sure they are .15 not .16. Where did you get that information?

However, freezing and boiling points do vary with pressure. They are usually given in atmospheric pressure, so it's possible your data was with another pressure?* Or for a different substance? Or was it referring to the triple point (273.16 K) and critical point (647 K) of water?

*This, however, doesn't make sense either because for water, lowering the pressure raises the freezing point and lowers the boiling point. But raising the pressure lowers the freezing point and raises the boiling point. There is no way to raise both of them, unless they are at different pressures.(3 votes)

- what is the signification of kelvin temperature scale. How is it different from the celsius one?(2 votes)
- During calculation for gases, we sometimes need to get into the negative part of the Celsius scale. To prevent calculation confusion and easy calculations, we use the Kelvin scale. Also, 0 degrees at kelvin, is known as absolute zero as all of the gases are liquid at this temperature.(1 vote)

## Video transcript

Voiceover: All bodies and systems possess a property called temperature. Most commonly temperature is used to refer to how hot or cold something is, but the real sciency
definition of temperature is that it's a measure
of the average kinetic energy of the particles in a system. I've got a system and I'm filling it with little individual particles. If we think about this microscopically each little particle in the system is moving in some way whether in rotation or in a straight line, or curving, or by kind of a
combination of these means. All of these little particles are moving. The energy of motion is
called the kinetic energy. All of these moving particles
have kinetic energy. The faster those little
particles are moving, the greater their kinetic energy. If each of those little
particles in the system has greater kinetic energy, that means the system as a whole has a larger amount of total energy, and we would say that it
has a greater temperature because, again, temperature is a measure of the average kinetic
energy of those particles. Because knowing the amount
of energy in a system can be really useful in
chemistry and in physics, we've developed temperature scales to help us quantify or
measure the amount of this value, this value of energy. The three scales most widely used are the Kelvin scale, the Celsius scale, and the Fahrenheit scale. For all of these scales I'm going to draw a little thermometer, one for Kelvin. Then we have a thermometer for Celsius, and then another thermometer
for the Fahrenheit scale. The two scales used most
in the physical sciences are probably the Celsius
scale and the Kelvin scale. As a point of comparison
here on these thermometers, the freezing point of water occurs at zero degrees Celsius. We have zero degrees Celsius. That's where water freezes. Then the boiling point of water occurs at 100 degrees Celsius. So the boiling point of water occurs at 100 degrees Celsius. That's where water turns into steam. I'm going to write H20 here real quick just so we don't get confused that we're talking about the freezing and boiling point of water. Now when we use the Kelvin scale, we find that water's freezing point is 273.15 Kelvin. Then we find that water
boils at 373.15 Kelvin. They differ fundamentally
in the zero point. The Celsius and Kelvin scales differ in the zero points that they use, but between water's freezing point, and water's boiling point, we have a span of 100 temperature units for both scales. So although they differ in
the zero points that they use. They use the same size unit, or the same magnitude of unit to measure the temperature. Converting then between the two scales only really requires that
we make an adjustment for the two different zero points. This is what I mean. If we want to know the
temperature in Kelvin, all we need to do is take the temperature in Celsius and add 273.15
degree units to it. If we want to know the
temperature in Kelvin for the freezing point of water, we take the temperature in Celsius which would be zero, and we add 273.15 units to it, and that would give us 273.15 Kelvin. Now if we want to flip that, and if we want to find
the temperature in Celsius from Kelvin, all we have to do is
take the Kelvin figure and subtract 273.15 to it, or subtract 273.15 from it, excuse me. We would see that 373.15
Kelvin minus 273.15 would give us 100 degrees Celsius. Just as another example, let's convert 300 Kelvin to Celsius. To start, since we're looking for Celsius, we'll take that Kelvin value, and we'll subtract 273.15 from it. That's going to give us 26.85 Celsius. So 26.85 degrees Celsius is the same thing as 300 Kelvin. I just want to point out really quickly that I'm only using the degree symbol here for Celsius, and I'm doing that intentionally. We don't need this
symbol with Kelvin scale because instead of calling
the temperature units degrees, we just call them Kelvin. The only thing we need is an uppercase K. Now converting between the Celsius and Fahrenheit scales is a
little bit more complicated. You see in Fahrenheit, water freezes at 32 degrees Fahrenheit, and water boils at 212 degrees Fahrenheit. This give us a span
between the freezing point and the boiling point of
water of 180 degree units. We're going to need to consider two different adjustments here; one for degree size because the units have a different magnitude, and the same value, or the same span of temperature is 100 units in Celsius and 180 units in Fahrenheit. We're also going to need to account for the two different zero points, zero degrees Celsius for freezing, and 32 degrees Fahrenheit for the freezing point of water. First we can say that
180 degrees Fahrenheit is equal to 100 degrees Celsius. Again, we can say this
because both of these magnitudes refer to the
same change in total energy. If we write this as a ratio, we have 180 over 100
which just reduces down to nine over five. So the ratio of Fahrenheit to Celsius is nine to five. Now we need to think
about the two different zero points. Because 32 degrees Fahrenheit is equal to zero degrees Celsius, we can find the Celsius temperature if we take the temperature in Fahrenheit and we subtract 32 degrees from it. This makes sense because
32 degrees Fahrenheit minus 32 degrees Fahrenheit would give us zero degrees Celsius. Now we just need to apply the unit ratio, so just like any dimensional
analysis problem, we need to cancel out
the degrees Fahrenheit. If we put the degrees
Fahrenheit on the bottom here, so nine degrees Fahrenheit, we can cancel out the Fahrenheit leaving us with just degrees Celsius. To find the temperature in Celsius, we take the temperature in Fahrenheit subtract 32 from it, and multiply it by a
ratio of five to nine. Then we can also manipulate this formula if we want to start with Celsius. All we have to do is solve for the temperature in Fahrenheit. To start we would divide both sides by five over nine, or that's the same thing as multiplying by the reciprocal. Then to finish it off,
we would just add 32, so plus 32 is equal to the
temperature in Fahrenheit. Now if we want to start
with temperature in Celsius, we can move to temperature in Fahrenheit, or we could start with
temperature in Fahrenheit and move to temperature in Celsius. To practice this let's go from Celsius to Fahrenheit. It turns out that these temperature scales actually cross paths at a temperature which is kind of a fun fact. If we plug in negative 40, let's go from negative 40 degrees Celsius to Fahrenheit. We'll find that TF is equal to negative 40 times nine-fifths plus 32. We can reduce this term here, so five and negative forty
reduces to negative eight. So negative eight times nine plus 32. That's negative 72 plus 32. So the temperature in Fahrenheit would equal negative 40 as well. So negative 40 degrees Celsius is the same thing as saying negative 40 degrees Fahrenheit. That's kind of just a fun fact. And another observation
from this little factoid is that Celsius and Fahrenheit scales can both have negative or positive values. We see that both can be at negative 40. These can both have negative values. That's actually a point where they differ from the Kelvin scale. The Kelvin scale can only
have a positive value. It turns out that the
absolute coldest temperature is zero Kelvin. So zero Kelvin is absolute zero. The reason we can't get any colder is that at this point,
no particles would have any kinetic energy. That means no motion at all. We said that temperature is a measure of the kinetic energy, and the coldest that you can get is no kinetic energy whatsoever. It turns out that the laws of physics specifically the uncertainty principle just don't allow for this. We can get close like within
a billionth of a Kelvin, but we can't get all the way there. Because Kelvin scale always
has a positive value, it becomes a little handier
in various formulas, so to use as the standard,
the SI unit for temperature. I'll show you in future
videos why absolute zero happens at negative
273.15 degrees Celsius, but I'm starting to run
out of time in this one so I'm going to have to save it for later. I'll talk about that with
Charles's Law in the future.