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### First Law of Thermodynamics Definition

Posted: Sun Feb 09, 2020 6:48 pm
What is the official definition of the First Law of Thermodynamics that we should know for this class?

I know my notes simply say that "Internal energy of an isolated system is constant. Universe is an isolated system; therefore, energy of the universe is constant."

Does the first law of thermodynamics only apply to isolated systems?

### Re: First Law of Thermodynamics Definition

Posted: Sun Feb 09, 2020 6:56 pm
The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. Just because the universe is technically an isolated system, doesn't mean that the systems within it are isolated as well; regardless, energy cannot be created nor destroyed, upholding the law. In 14B, it's simply to allow us to calculate certain quantities without taking in to account the "loss of energy" to the surroundings.

### Re: First Law of Thermodynamics Definition

Posted: Sun Feb 09, 2020 7:51 pm
Technically, it applies to all systems because energy is never lost its just transferred to "the universe", or the greater system that every isolated system is technically part of.

### Re: First Law of Thermodynamics Definition

Posted: Sun Feb 09, 2020 9:04 pm
If you define a system, and it loses energy to its surroundings, which you don't count as part of the system, then the energy WITHIN the system isn't conserved, but the overall energy of the system+surroundings is still constant.

### Re: First Law of Thermodynamics Definition

Posted: Sun Feb 09, 2020 9:08 pm
The first law of thermodynamics is represented as an equation is delta U = q - w. Often simplified to "the energy of the universe is constant." Since U is equal to internal energy, there also must be an energy of the surroundings. Any change in internal energy has the opposite change on the energy of the surroundings. If heat is put into a system, it is being lost by the surroundings. Likewise, if a system is doing work on its surroundings, that energy transfers to the surroundings and is lost by the system.