Heating Metal Ring
If you heat a metal ring equally on all sides, will the
hole in the middle increase in size, decrease, or stay the same?
When the temperature increases, the metal expands IN ALL DIRECTIONS. It gets thicker, its inner
as well as its outer diameter increases; every part of it increases by the same proportions.
---Nathan A. Unterman
Since the metal expands all around, the hole in the center will get bigger. Your science teacher
may even have little set up to demonstrate this. (I do at any rate.) It is a metal ring on a
stick and a metal ball on a stick. When cool, the ball is too big to pass through the ring. When
the ring is heated the ball will go through. Now, you tell me, will this work if you heat both
the ball and the ring?
Good question, good thinking.
If you heat a metal ring uniformly, the hole in the middle will increase in size 'almost' all
cases. This is better understood by realizing that the ring in reality is a rod bent into a circle.
That rod will expand upon heating. The same bar in bent into a ring thus also expands. This is an
expansion of the ring perimeter and thus its diameter (the hole).
As you note, I said the hole in the metal ring 'almost' always expands upon heating. This means that
there are some exceptions. They are not very common but do occur due to unusual properties of some
materials, the shape of the ring, and whether or not the ring is constrained (held tight in some
Dr. Ali Khounsary
Argonne National Laboratory
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Update: June 2012