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Welcome to our Physics lesson on The Meaning of Entropy, this is the third lesson of our suite of physics lessons covering the topic of Entropy and the Second Law of Thermodynamics, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
In simple words, Entropy represents the degree of disorder in a thermodynamic system. Entropy therefore is high when the particles in a thermodynamic system move in irregular way. We have explained in the previous tutorials that particles motion is related to temperature, i.e. higher the temperature, more irregular the particles motion. Therefore, entropy is related to the temperature of system.
Also, we know that more heat supplied to a system, higher the kinetic energy of its particles and therefore, higher the degree of disorder in the system. This means entropy is related to the heat supplied to the system.
Combining these two factors together, we obtain the equation of entropy S. It is
where Q is the heat of system and T its temperature.
However, just like in Kinematics, where we were not that interested in the object's actual position but in the change in position, as it gives a clearer idea about the object's motion, here we are more interested in the change in entropy ΔS rather than in the actual entropy of a thermodynamic system. Therefore, we can write:
The unit of entropy is joule per kelvin [J/K].
200 g ice at 00 is inserted inside a closed room containing water steam at 100°C. Given that the latent heat of fusion for ice is 334 000 J/kg, calculate:
a) Since the room is closed, the system is considered as isolated.
Given that 200 g = 0.2 kg and Tice = 0°C = 273 K, we obtain for the change in entropy for ice:
b) The steam is at 100°C = 373 K. Given that the heat absorbed by ice is equal to the heat released by steam (with negative sign), we obtain for the change in entropy of steam:
c) The change in entropy in the universe is
You have reached the end of Physics lesson 13.10.3 The Meaning of Entropy. There are 6 lessons in this physics tutorial covering Entropy and the Second Law of Thermodynamics, you can access all the lessons from this tutorial below.
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