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 sort of infrared frequency radiation
with hotter objects radiating higher amounts 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.