Another Law Zero
Somewhere in the mists of time, or possibly confused memory, your humble author recalls the following definition of Law Zero:
If two bodies touch, heat will flow from the hotter to the colder.
Not Earth shattering, but not really stated anywhere in the first Three Laws. and actually very important in understanding Thermodynamics. Left off because its so obvious, everyone already knows it. Or do they? Our perception is that "cold" is a thing, not the absence of heat. It appears that cold flows every bit as much as heat, but not so. Its a one way flow.
On could go on to say "until the temperatures are equal", which is true. But its enough to just say the first part, because once the temperatures are equal, heat stops flowing because neither is hotter.
In its favor, this Law Zero does actually say something true, something unique to Temperature, and something not stated by the FIRST three laws.
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Law Zero
Somewhere along the way, we got a forth law of thermodynamics. Its usually stated in the most obscure possible way. And like the other three laws discussed the next could of pages, you get your choice of several Law Zero's.
Why isn't it Law 4? Possibly scientists cannot count. Don't have the answer to that.
What is Law Zero? From site that rhymes with Encyclopedia:
Law 0: If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.
Now that is pretty obscure. What s this in English? Well "Thermal Equilibrium" has a definite numerical measurement, commonly known as Temperature. So in English, this means two systems are the same temperature as a third, they are the same temperature. as each other.
Now then. What is a system? The usual definition is a complex thing with multiple parts and multiple internal states and so on, like a heat engine could be a system. In other words, by any precise definition of "system" it does not have a single "Temperature" at all. In general "systems" are not at Thermal Equilibrium at all. A system could even contain Vacuum, in which Temperature has no value, and arguably no meaning. For something to have temperature, it must have substance. This Law Zero says nothing useful about what happens inside systems either, which is kind of the point of Thermodynamics. To explain systems. Not to confuse systems with matter.
To say it differently, temperature is a property of a "body", in Clausius's terminology. The "body" can be matter in any state. A general system does not have a temperature, because it is made up of many bodies at different temperatures. An Airplane is a system. It has 2000+ degree temperature in its engines, comfy room temperature in the cabin, and parts are freezing cold due to the air temperature at high altitudes. A Hurricane is a system. a Television is a system. Each body within any system has a potentially different specific temperature, which may not even be constant.
Stating the Law in "Obscure speak" is in no sense stating the Law precisely.
So to simplify further,
Law 0:
If two numbers are the same as a third number they are the same number.
Which is Mathematics. Equality is transitive. This is true of things that are the same height or weight or numbers of sides or numbers of atoms or numbers of stars. Not a property of Thermodynamics at all. A property of any number, including any measurement expressed as a number.
So if this is Law Zero, it is pointless. Perhaps one could define the measurement of Thermal Equilibrium as Temperature At least it is less pointless than wrapping transitive mathematics in obscure shrouds and calling it physics.
Perhaps they really meant "system", and by "thermal equilibrium" they meant the net amount of heat flowing between the two systems is zero. Then, this is just false. Everything is in thermal equilibrium with the moon by that definition, and but not necessarily with each other. Even requiring the systems touch, this is not true. A and B are in equilibrium with C, by the "heat flow" definition, A can still give heat to B. Systems do not have a single temperature, so there is no transitive property.
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