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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?
• 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.
• At , 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?
• 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.
• In regards to , 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?
• They travel in straight lines until they collide with another particle or the walls of the container.
• i didn't quite understand how it is' logically 'possible that celsius and fahrenheit scales can have a common temp..? please explain .
• u have the Fahrenheit-Celsius equation.
equate it with F=C.
There'll be one solution and that's -40.
• Is absolute zero the lowest possible temperature?
• 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.
• Is temperature in kelvin scale also known as the absolute temperature ??
• 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
• 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..
• The question is little off track...but can someone tell me why gases do not have free electrons?
• 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.
• At - , 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.