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Name: Andrew
Status: student
Age: 13
Location: N/A
Country: N/A
Date: 1/20/2004

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

Dear Andrew,

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.

Martha Croll


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 manner).

Dr. Ali Khounsary
Argonne National Laboratory

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