What is the First Law of Thermodynamics?

First Law of Thermodynamics
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First Law of Thermodynamics


The first law of thermodynamics relates energy, work, and heat. We can write the equation like this. What this is, is basically saying that the change in a system’s internal energy (this is change in internal energy) is equal to the energy transfer to or from the system by heat (this is energy transfered to or from the system by heat) minus the energy transferred to or from the system by work (the energy transferred to or from the system by work, so by heat or by work).


Those are set equal to each other. Think about, for example, when you compress the cylinder of a bicycle pump. You were doing work on the system, because you’re applying a force on the system and moving it a distance. Now, your work transfers energy to the air molecules inside and increases their internal kinetic energy.


It’s increasing their temperature, but you’re doing that by work. The energy does not appear from nowhere. It is transferred there by work. We can also call the first law of thermodynamics the “law of conservation of energy”, because energy is neither created or destroyed, but it can be transferred from one form, for example, like chemical energy, to another form, like heat or light.


Notice here, like I said, the energy is not being destroyed here. The chemical energy is not going anywhere, but the heat or light over here isn’t being created either. It’s just being transferred from one form to another. That’s why we can also call the first law of thermodynamics the “law of conservation of energy”.



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Last updated: 10/15/2018

 

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