The Zeroth Law of Thermodynamics

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  • 0:02 Thermodynamics
  • 1:05 The Zeroth Law
  • 2:37 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After watching this video, you will be able to state the zeroth law of thermodynamics, what it means, and why it is important. A short quiz will follow.


Sometimes when people make arguments, they make the mistake of failing to define their terms. Two people will argue back and forth, and only an hour later will they realize that they're really arguing about the meaning of words. The zeroth law of thermodynamics is kind of like those arguments - it isn't profound or hugely important, but you have to get it out of the way or everything else becomes a waste of time. But before we talk about the zeroth law, we first need to define thermodynamics in the first place.

Thermodynamics is the study of the movement of heat energy. This includes the types of heat transfer (conduction, convection, and radiation), phase (or state) changes, the relationship between energy and work, heat engines, and of course, the laws of thermodynamics, which talk about how and why heat moves.

The laws of thermodynamics are central to thermodynamics as a whole. They're like the supports on which everything else is built. But the first two or three laws of thermodynamics also need their own support - it's no good talking about temperature, for example, if you don't know what temperature is. That foundation is the zeroth law of thermodynamics.

The Zeroth Law

The zeroth law of thermodynamics says that if system (or object) A is in thermal equilibrium with system (or object) B, and is also in thermal equilibrium with system (or object) C, then B and C must also be in thermal equilibrium with each other.

Perhaps that's confusing, so let's break it down. First of all, what is thermal equilibrium? Thermal equilibrium is when two systems or objects have no flow of heat between them despite being connected by a path permeable to heat. When does this happen in real life?

Well, heat always transfers spontaneously from hot places to cold places. That, as it happens, is one way of stating the 1st law of thermodynamics. But this means that for heat not to flow when it can, the two objects or systems must be the same temperature.

So we can restate the zeroth law of thermodynamics like this: If system (or object) A is the same temperature as system (or object) B, and is also the same temperature as system (or object) C, then B and C must also be the same temperature.

Written like that, suddenly the zeroth law becomes really obvious. Of course, if object A is the same temperature as B and C, then B and C are the same temperature as each other. So why are we talking about something so obvious?

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