Ask A Scientist Physics Archive

name         Andrew
status       student
age          13

Question -   If you heat a metal ring equally on all sides, will the
hole in the middle increase in size, decrease, or stay the same?
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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
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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
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Hello,

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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