What is the Zeroth Law of Thermodynamics?

These three cups are in thermal equilibrium. I can measure the temperature in all three
mugs here with a thermometer and you can see they are all come up as being the same temperature. I could also use three different thermometers
and they would still say that the temperature is the same across all three containers. That�s because temperature is a fundamental
and measurable property of matter. The framework that describes all of thermodynamics
can be summed up in the four laws, and these describe heat, energy, and their
interactions with matter. While the first, second and third laws of
thermodynamics are quite well known, there’s a fourth law that underpins all of the other
laws. It was devised after the three laws of thermodynamics, but was so fundamental and provided such a
basis for the other laws, that it was termed the zeroth law of thermodynamics. The zeroth law of thermodynamics is actually
an observation and states that if two thermodynamic systems
are in thermal equilibrium with a third system, they are also in thermal equilibrium with
each other. Or to put it mathematically, if A=C and B=C
then A must also=B. If all of these three glasses are at thermal
equilibrium, then we can say that they’re all at the same temperature. And this is where the significance of the
zeroth law lies. It lets us establish temperature as a measurable
and universal property of matter. If something is measurable, then we can create
scales for it and this is exactly what happened with temperature. The first temperature scale that, in a slightly
altered form, is still in use today was established by Daniel
Gabriel Fahrenheit in 1724. A more common scale, named after Swedish scientist
Anders Celsius, uses the point of water freezing as the 0
point on the scale, and the boiling of water as the 100th degree
on the scale. Rather than using a scale where 0 is defined
arbitrarily, what we really want is to use a scale where
0 is the coldest temperature you could ever get to. Scientists around the world use the Kelvin
scale, where 0 is defined as absolute zero. Absolute zero is something which is defined
by the third law of thermodynamics, but that’s something we’ll get to in another video. It was observed that the properties of matter
could change with temperature. For example, liquids can expand or contract
when heated or cooled, and the conductivity of wires can change as a function of temperature. Some old fashioned thermometers like this
one use the expansion of mercury or other liquids under heat as a measuring tool. Modern electronic thermometers commonly use
thermoresistors, where the resistance in a material changes with changes in temperature. Yet another way of looking at temperature
is with a thermal imaging camera. All objects above absolute zero radiate some
of infrared. This allows us to us thermal imaging to see the world even without any visible light. If two objects at different temperatures are
brought into contact with each other, they will try to reach thermal equilibrium. If the two objects are of sufficiently different
temperatures, the reaction can be rather violent due to a phenomenon called thermal shock. Because the heat gradient makes the glass
want to expand, but the other side is cold. This causes a strain that breaks the glass. The concept of the zeroth law may seem trivial, but it lets us define temperature, and that in turn allows us to quantify the
effects of the other three laws.

76 thoughts on “What is the Zeroth Law of Thermodynamics?”

• kole081 Post author

Is that Lean ?

• CrazyIvanTR Post author

It just seems like the "0th law" is a given. I'm thinking that must have been why they didn't even bother to write it down in the first place. Well, we wouldn't know what they were thinking unless they documented it. So, all I have is speculation.

• Hythloday71 Post author

Temperature isn't a 'fundamental' property of matter. It is an emergent phenomena!

• Hythloday71 Post author

Leonard Susskind has coined the phrase for conservation of information as the 'minus 1'th law' as the most fundamental principle in theoretical physics / thermodynamics – it's a great concept !

• Noriaki Kakyoin Post author

This woman looked like she was gonna break out laughing any second.

• Shawn Hawkins Post author

The law states: "Nobody is as cool as the Fonz"

• EdMcF1 Post author

If you use an invasive thermometer, you change what you are measuring unless the thermometer is at the same temperature as the thing that you are measuring, so you must be approximating when measuring temperature, until equilibrium is reached.

• Edvards Treijs Post author

Celius did not invent celius scale

• rayen ochi Post author

a=b
b=c
then: c=a
…. i can't believe someone had to create a law out of this. .*

• Mr マックラ Post author

I wish we all use Kelvin universally and drop Fahrenheit and Celsius.

• Fransamsterdam Post author

How can you measure the temperature of a system without interfering/disturbing the system, which means changing the temperature?

• Emit Post author

Is there anything at all known to us that doesn't follow the "if A=C and B=C then A=B"?

• notsyort Post author

Temperature isn't a fundamental though. It's a rule-of-thumb classical-scale evaluation of thermal energetic potential. That's how you can get weird circumstances. For example, the one reported on in January 2013, where a cloud of atoms were communicated as having gone below absolute zero. They hadn't, of course. The physics involved simply exposed the inadequacy of classical physics in describing very-low-energy ensembles.
https://youtu.be/yTeBUpR17Rw

• crabshank3 Post author

1:08 Transitivity

Thermodynamics is a fascinating subject!

• Gilbert Kerley Post author

very interesting
very charming

• Attila Asztalos Post author

An overview of why the other laws follow from this one would have been more useful. Also some thoughts on why this isn't self-evident and needs expressed as a "law".

• Al Post author

… isn't there already a mathematical law that says the same thing?

• OKKai Post author

Brilliant woman, talking about science and temperature.
This might be the hottest video on youtube.

• Thomas Drowry Post author

Is that an Australian accent she has ?

• MG1 Post author

what transferred from your glass jars which altered the thermometer? that what you call heat is physical and all physical things have structure and shape. with this clue work it out. when you do you have progressed world and science. MG1

• John Breslin Post author

Temperature does not equal energy.
The 1st, 2nd and 3rd laws of thermodynamics all deal with the energy of systems not their temperatures.
For example the corona of the Sun is much hotter than its surface but has a much lower energy density.

• Someone Post author

the first temperature scale was actually Rømer from 1701

• Herman Hesse Post author

Oh it's impossible to contradict the principles of thermodynamics ? Oh, all right then, let's go back to 1915 and tell Einstein to become a farmer, for Newton's laws of gravitation are absolute and unfalsifiable truths.

• Jjames Anderson Post author

Great video , but in my opinion the best way to explain this subject is in terms of molecular vibration and the Kelvin scale. To me it makes it easy to visualize . Shake slow , cold , shake fast , hot.

• Tiago Seiler Post author

I couldn't place that accent. New Zealand?

• Shawn Hawkins Post author

I like some of the other laws written by the person who came up with the zeroth law:
"heat makes things hot"
"if our team scores more points, they'll win"

• 0rderofTheWhiteLotus Post author

I recommend people update their understanding on scales using veritasiums recent video

• Marc Frank Post author

I think atoms at absolute zero are in the present longer then atoms that are warmer ( you can take their pictures) so are photons the transformation of time into mater?

• Peter Rabitt Post author

Danger Dolan used this music on his videos.

• gyrojomo Post author

Nice and clear. Thanks.

• Nalminji Post author

You could have brought a more interesting case instead of saying A = B = C

• Mr. Peanut Post author

isn't the zeroth law basicly just a restatement of the law of congruence from geometry?

• KalKyuLayTor Post author

Thank you kind lady for the explanation.

• santiago gimenez Post author

Possibly difficulty coast absence reduce skirt diversity significance generous celebration.

• Crys Tal Post author

Justice is not done to this topic by this Ri video. A physics text book like "Resnick and Halliday" explains way better why this zeroth law is fundamental and not so obvious.

• Yukha Dharmeswara Post author

English isn't my mine, can someone give me the point of this video, please :v ?

• tiuschiu Post author

I think to properly motivate this zeroth law you need to start at a more basic level. Maybe explain what exactly thermal equilibrium means, derive the notion of temperature and establish there is no a priori transitivity. Even though this is meant for non-experts, it just doesn’t make sense without properly explaining the underlying model. You can see the resulting discussion in many of the comments.

mLaw (Mikes Law) if a 4th glass is added, and it is not at equilibrium, then N4 != N1-3
Zeroth Law kinda feels like he got an everyone wins medal, unless, there is more I am missing, thus, missing from this video

• Martin D A Post author

Valeska Ting you are lovely.

• johnsonmaje Post author

What would it look like if you used thermal imaging in a 3-D camera technology?

• herrbetto 55 Post author

the girl speaks perfect English with British accent

• Dennis van Gils Post author

@The_Royal_Institution I'm wondering: Does the zeroth law also remain valid if relativistic effects are taken into account, say, if one of the glasses is traveling near the speed of light?

• Ben Larson Post author

Thermydonamics. Now you cannot unhear.

• matt sanderson Post author

a clever kiwi

Go Kiwi!

• another damn tom Post author

All scales of measuring heat are arbitrary and relative.

• Vani Vasil Post author

Quantum mechanic does not say so

• Bigganium Post author

what is the music?

• Pythia Sibyls Post author

Is this not common knowledge? I'm seriously asking.

• Harry D Post author

wait…

1) Isn't this just basic logic? (https://en.wikipedia.org/wiki/Transitive_relation)
(A=B ^ B=C) => (A=C)?

2) Was Celsius into lipstick? not judging here 🙂

• its ok Post author

a = b
b = c
so a = c
why is there a law on this ….. 5 year olds proably know this aready

• D Master Post author

that's a rather boring example. this is a much more interesting example. imagine you're having and menage a trois, a threesome, with two girls and a guy. if the two girls are each in thermal equilibrium with the guy then the two girls will also be in thermal equilibrium with each other 🙂

• Soufiane Tijjini Post author

BUT WHY????? Does every object with a temperature above 0K emit infrared frequency radiotion???!!!

• Prettygreen parrot Post author

Brilliant to see someone who looks so happy and excited to be talking about science.

• Dennis Hudson Post author

this is a clear, concise explanation of the 4 laws. I will use this as a model for my class and i hope you will produce new presentation..

• yogi jarupla Post author

Zeroth lawIf A is a friend of B and B is a friend of C then A is also friend of C 😱😱😱

• Michael Post author

question…is it not wrong for scientists to tell the peoples that time only began when the baby was born in the big bang………………………………..when in fact it is not improbable that our universe is nothing more than a new time zone in a much older universe and that the steady state universe may well exist far beyond our capability to detect whats out there. that the steady state and big bang universe can be unified in theory.

• CogitoErgoCogitoSum Post author

I really tire of people mocking the traditional system with such colorful adjectives. Its called "pretentiousness". The Fahrenheit scale was not decided upon "arbitrarily", so dont use that word to contrast the two. Anyone with half a brain can look up the basis of the scale. You want to talk about idiocy? Mr. Celsius originally defined 100 as freezing and 0 as boiling.

• CogitoErgoCogitoSum Post author

How exactly does "transitivity" make way for measurability? There is a huge leap in logic here. I think an explanation or some elaboration is in order.

• Paul Paul Post author

I think I'm missing the point, but what you said about if A=C and B=C, then A=B… I was just sat here thinking "no shit"… am I missing the point? It just seems obvious

• Jack Conway Post author

Galileo did invent a thermometer, called Galileo's air thermometer (more accurately termed a thermoscope), in or before 1603

• EddieVBlueIsland Post author

You missed the subtle point of the 0th law.  Temperature is a state function based on the nature of heat itself and temperature is its independent measure regardless of it source. So for objects A, B and C (not necessarily identical)  if A is heated by microwaves to temperature T, B heated by chemical reaction to temperature T, and C heated by the sun to temperature T  – the heat in A, B and C is the same in the sense of summed atomic motion  (but not the total amount of heat for different objects)  independent of the heat source. Subtle but necessary for the other laws to work.

• vikas Kumar Yadav Post author

U r very beautiful

• Tomy Lim Post author

She's so cute, it hurts!

• Pablo Ramos Post author

Mr. Celsius actually created the temperature scale with zero as the water boiling point and 100 as freezing!.

• Pockets MacCartney Post author

what temperature do the thermometers start at?

• The Royal Institution Post author

Merci beaucoup! We now have French subtitles for this video. If you too would like to try your hand in translating some science into your native language you can follow the link here – https://www.youtube.com/timedtext_video?ref=wt&v=PE_zpk-EznQ&auto=yes&bl=watch

• Jyotismita Doley Post author

I found a good source to master my basics!!! Thank u😎😎

• free thinker Post author

https://www.youtube.com/watch?v=t8Nm9bnWOTs his track record speak for it self but i am sketish. initially from someone in chemestry and physic ( Caltech) Tolman, Richard C. (1917).
“The Measurable Quantities
of Physics”
.
Phys. Rev
.
9
(3): 237–253 it is mentionned all over the web but couldnt find one place the article of physic review was existing . only reference , no article . there is the other guy dr Robitaille mention but that is 1961 way way later ! would you per chance have data of this or another since if i recall you did invite this guy at the royal whatsnot!( whatsnot beign the exact name of that time since name change!

• free thinker Post author

isnt the thermometer calibrated with gravity in mind? wouldnt that be an issue ( given that gravity laymen exemple of its existance have yet to be born?)

• free thinker Post author

since most things in universe work via delta( no delta = death) its safe to say a lot of assumption are useless in the real world, unless they are known. applying this on unknown? i would ratter assume there is a delta , cause its very rare that something happen without a type of delta of a sort! how i make this work? simple: nothing( infinite) want to be filled ( you can replace the word with something that convey similar intention since there are likely a lot of variant) by something ( finite)! something( finite) want to fill nothing( infinite) simple hey ! applicable to all of periodic table and likely also alchemy table! so this means that you cant ask nothing ( infinite) to fill something( finite) its just impossible! so what does this mean in real world on the periodic table? lets take bismuth and hydrogen! lets use the electron value. they have similar electron count . closer to nothing . lets use the same 2 exemple but this time for weight! hydrogen still is closer to nothing ( want to be filled) but bismuth weight is closer to something ( want to fill nothing) easy right! and no , this isnt from me, nikolas tesla worked via this daily in his electrical experiement . like the a/c motor . he mentionned in some paper and it ended up in a free for all thing called my invention from nikolas tesla . but if you read between the line , he does think that the bigest delta possible is always very close to nothing. his brush motor made use of this initially ( before he came up with the brushless version)

• Robert Shaver Post author

Do the laws of thermodynamics apply to all forms of energy or only energy in the form of heat?
For example, if I raise a one pound weight up one foot in the Earth's gravitational field, it now has one foot-pound more of potential energy than it did before I raised it. Did it get warmer or colder or stay the same temperature? I'm confused as to how thermodynamic principals apply in this case.

• Rinkazhi Rinkashime Post author

I think I have a crush on her.

• بو فارس بو فارس Post author

يالها من إبتسامة
What a smile