Hole

[alphamale]

If you have a steel washer and you heat it to cherry red does the hole in the middle get smaller because the metal has expanded, or does the hole get bigger because the metal has expanded?

[Skids]

The hole gets bigger.

[gen2]

You can demonstrate it to yourself by drawing a washer on the surface of a deflated balloon and then inflating it.

[MrPahoehoe]

I tried that gen, but the ballons keep getting holes....I think the washer must be too hot ;-)

[alphamale]

but dosn't the metal expand in all directions? Wouldn't the hole in the middle get smaller?

[Hippy]

For the hole to get smaller the metal would have to contract. Try stretching an elastic band to see the answer. OK that is stretching, I know, but the hole gets bigger as the band expands (stretches), and yes the band gets thinner which your washer doesn't, but it is the expansion of the inner boundary of the hole that it demonstrates.

The molecules of metal on the inside edge, next to the hole get further apart due to heating. That is your expansion. So ..... the hole gets bigger!

[alphamale]

Thanks EDDIE51, now I know the answer (after 31 years since the question was first posed to me). Thank you very much.

[JudgeJ]

Perhaps an easy way to grasp this would be to imagine the washer expanding to 100 times its original size instead of the relatively small increase provided by heating.

If it were to expand to 100 times its original size would you really expect the hole in the middle to shrink?

[d0gsb0dy]

A good example is the fitting of metal tyres to wooden spoked wheels. The tyre is heated and then dropped over the wheel, after which it is doused with cold water to fit it tightly to the rim.

[alphamale]

Hello Dogsbody and Eddi51,

Maybe the hot ring test is not the explanation. The heated ring will expand in length and flex. I am thinking, as JuddgeJ has alluded to, a rigid body. Maybe it would just expand like a big thick ring - I'm not sure.

AlphaMale

[Bert]

This question was asked quite a long time ago. Some of the answers above offer proofs of what alphamale obviously knows but does not understand - the ring and ball demonstration and fitting metal hoops onto wooden wheels. One of the answers points to the correct explanation - imagine the atoms on the inner surface of the washer, they have to get further apart as the meat heats up, i.e. the circumference of the hole must get larger, so its diameter must get bigger too.