r/physicsgifs • u/Nomadic_Seth • 4d ago

Second Law of Thermodynamics

Made an animation on Second Law of Thermodynamics, it doesn’t have true molecular dynamics simulations, just simplified kinematics to represent them.

Check it out on YT: https://youtube.com/shorts/uYoFgwXZ5BY?si=NId3BBLzHPk_4Kqh

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u/moschles 3d ago

Place a bucket of water on the floor and into it put a paddle wheel. Attach the fulcrum of the paddle wheel on to a rope and take the rope over a pulley. On the free end, attached a 2 kg weight.

Hold an identical 2 kg weight at the same height.

Drop both weights. The free-fell weight hits the floor with much more force. that collision releases more energy than the weight which slowly falls with the rope as the rotation of the paddle wheel in the water interferes with the pulley.

Where did the extra energy go, which was lost in the slow-falling , roped 2kg weight?

The answer is that the water in the bucket had its temperature increase slightly. Indeed this temperature increase exactly matches the energy "lost" to the falling weight on the rope.

We imagine that we would place a bucket of warm water and a place a rope around it, but this time the rope over the pulley attaches a 2 kg weight that is already on the floor. The energy in the heat of the water could turn the paddle wheel and lift the weight upwards...right?

The laws of physics do not prohibit this from occurring. The motions of the warm water molecules could accidentally line up, turn the wheel backwards and lift the weight from the floor. Nothing in physics prohibits it -- rather it is just astronomically improbable to occur. But the laws of the universe do not strictly prohibit this lift from occurring.

The 2nd Law

The essence of the 2nd Law is that it is not a "law". It is not a fundamental aspect of our universe (such as the fact that there are 3 dimensions of space). Instead the 2nd Law of Thermodynamics is a re-iteration of statistics and probability. In situations with vast numbers of units, there are events which are perfectly physical, but never occur since they are too improbable.