FIRST LAW OF THERMODYNAMICS

Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter. The fundamental principles of thermodynamics are expressed in four laws.

The First Law of Thermodynamics states that heat is a form of energy, and thermodynamic processes are therefore subject to the principle of conservation of energy. This means that heat energy cannot be created or destroyed. It can, however, be transferred from one location to another and converted to and from other forms of energy. 

other explanation;
“The First Law says that "the internal energy of a system has to be equal to the work that is being done on the system, plus or minus the heat that flows in or out of the system and any other work that is done on the system,"

ΔU = Q – W, 

 ΔU is the change in the internal energy,

 Q is the heat added to the system

 W is the work done by the system.

In figure a explains (Q) is the heat emits by the person and W as the work can be perform by the person plus the food as the source of energy that is transform to do work.

In figure b explains that the heat Qin that comes from the sun is entering the flowers body by photosynthesis and can be converted into Qout which is the glucose.  

Sample Problems
1. A gas in a closed container is heated with 10J of energy, causing the lid of the container to rise 2m with 3N of force. What is the total change in energy of the system?


ΔU=Q−W We are not given a value for work, but we can solve for it using the force and distance. Work is the product of force and displacement.

W=FΔx

W=3N∗2m

W=6J

Now that we have the value of work done and the value for heat added, we can solve for the total change in energy.

ΔU=Q−W

ΔU=10J−6J

ΔU=4J


2. A gas in a closed container is heated with 50J of energy, causing the lid of the container to rise. If the change in energy of the system is 30J , how much work was done by the system?


For this problem, use the first law of thermodynamics. The change in energy equals the increase in heat energy minus the work done.
ΔU=Q−W
We are given the total change in energy and the original amount of heat added. Using these values, we can solve for the work done by the system.

30J=50J−W

−20J=−W

20J=W.







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