You know the principle from your physics class: if something is heated, it expands. This generally applies to solids as well as liquids and gases. There are of course exceptions: For example, water, the volume of which decreases between zero and four degrees when heated and only increases from four degrees.
The thermal expansion can also be observed on coins: the diameter and thickness increase minimally. However, the coin this puzzle is about has a circular hole in the middle - see picture above.
What happens to the diameter of this hole when we heat the coin? Does it gain weight, does it stay the same? Or is it falling because the coin expands in all directions?
Please scroll down to find the solution!
solution
The diameter of the hole in the coin increases as we heat the coin.
A clear explanation goes like this: We can also imagine the coin without a hole. If we were to draw the missing hole on the metal with a pen, the diameter of this drawn circle would definitely increase when heated. This must also apply analogously if the coin has a hole at this point.
Another explanation looks at the distances between the atoms in the metal lattice. These increase as the temperature rises.
Now let's imagine that the coin is only one atomic layer thick. There would be a closed ring of metal atoms around the hole in the middle. Heating increases the distance between the metal atoms. The diameter of the hole must inevitably increase - the ring becomes larger.
The fact that the diameter cannot decrease is ultimately due to the fact that there is not enough space in the area of the hole for the atoms of the metal lattice from the area around the hole. If the coin were to expand towards the inside of the hole, the distance between the atoms in this area would decrease, which would require high pressure and additional energy. None of this is necessary when expanding outward.
I discovered this riddle in the book "The Chickens of Minsk" by Jurij B. Tschernjak and Robert M. Rose.
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