Heating a ring?

Question

I've been told that when you heat a metal ring, the ring expands - but instead of the center hole getting smaller (like I would have thought because of the metal expanding in all directions), the hole actually gets larger with the rest of the ring.

What causes that to happen? I would have thought that since the metal of the ring would expand in all directions, the hole would shrink.

Answer 1

The metal ring is made by bending a metal rod in a circle shape. Whenever a metal rod is heated over a flame, it expands more in its length but not in its width. As such is the nature, the ring made from the metal rod, will increase its circumfernce significantly when heated over the flame. The both outer and inner circumferences will increase proportionately. Thus, there is no chance of shrinkage in the hole of the circle. Another reason, the density of the outer curvature surface of the ring will be lesser than that of the inner curvature. This density variation also contributes much in the non-shrinking of the hole.

Answer 2

What is happening at an atomic level is that the individual atoms are becoming more energetic and are pushing away from each other with more force than normal. Draw the ring with a grid of tiny dots loosely representing the atoms, then imagine what would happen if all the dots suddenly spread out.

The dots on the inside would certainly nudge inwards, but notice that there are more dots (assuming your grid is evenly spaced) along the outside. This will cause more expansion along the outside rim, and this will tend to balance out and indeed, exceed the expansion of the inner circumference. Essentially, the atoms along the outside will "drag" the atoms along the inside outwards as the ring is heated.

Answer 3

It comes from the fact that if you cut a disk from a plate and heat both equally the hole expands exactly as the disk i.e. the expansion of the two pieces are as if thay are still one. The ring is just a particular case of this phenomenon

Answer 4

Because the circumfrence of the ring gets much greater than the thickness of the ring, causing the overall diameter to get bigger, even on the inside.