Brass Plate Heating: Newsletter Challenge (January 2014)

I remember many years ago when we still did general science at school we had this neat little experiment to illustrate the expansion of metals. it was a brass ring attached to a small stand and in it sat a brass ball which would not pass through the ring until the ring was heated. simple but effective way to demonstrate the principle. I agree with your comments.
Regards
Andy

"when we still did general science at school" Unbelievable. Where are you?

The thread could just as well be closed at this point, but it may be amusing to see if anyone disagrees with given answer.

Surprisingly, some do.

I think that at least one is thinking of the differentials caused by the heat spreading outward from the hole, and stopping before the temperature of the whole plate reaches the new temperature.

Just as well the metal was specified because some metals can shrink when heated.

With the old school experiment, the hole became larger because as the metal expanded evenly, the center was under compression and could not move in. As the outer rim moved out it pulled the center out in tension. A plate of this shape would also expand the hole but it would distort the opening from a true circle because of differing amounts of material in the sides and corners.

Hole gets bigger. Old freshman physics lab question about "the coefficient of thermal expansion"

Good Luck, Old Salt

Gets bigger. All dimensions of the plate get larger on heating - including circumference of the hole and hence its diameter.

Ash55 has hit the nail on the head, very logical answer explaining the relativeness of the hole's circumference to its diameter.

GA to Ash55

Talk to any machinist or mechanic, it gets bigger. It's how we often fit bearings. Heat the bearing, cool the shaft,, press together, when the reach the same temperature good luck getting them apart.

Martin

My best thoughts would be to maybe expect the metal to go say from possibly being flat after the hole was bored at say temperature "A" (stress relieving), to faintly "Fuji Volcano" shaped, the hole getting slightly bigger of course at the same time, at a higher temperature of "B".

Therefore if the temperature of the metal was to have been significantly different to room temperature when the hole was bored, all bets are off as to what happens......higher temperatures possibly resulting in a bigger hole maybe.....

Any further ideas?

A hole reacts as if it were the parent material. Thus it expands.

I remember this one from my school days.

(KrisDel corporation has a special range of press fit high stability holes for critical applictions)

Del

Do you have half holes, or just whole holes...?

For sale:

What is the room temperature

That's one cold room. I ain't sleeping with you!

http://www.youtube.com/watch?feature=player_embedded&v=V0ETKRz2UCA

Bigger

This doesn't sound the same as a ring with square edges. I'm not going to argue, but my first thoughts were that it depends on the size of the hole; a large hole would get bigger, but a small hole would get smaller. Has anybody actually tried this, or do you blindly accept your physics text book?

It's logic - linear dimensions behave similarly.

Logic doesn't always work in physics, especially quantum physics. The anchients thought it was logical that a heavy thing would fall faster than a light thing. They were wrong. A hole containing air doesn't have the same coefficient of expansion as brass.

However, air is a fluid, & takes the shape of its container.

Consider the plate with a grid marked on it, Say 5"x5" with 1" grid spacing. Say the plate is uniformly heated and expands 10%. The plate becomes 5.5" square and the grids all become 1.1" square. Consider 1" diameter circles drawn in each grid square, touching the grid lines before heating the plate. Would they still be touching the grid lines after the plate has expanded?

If you imagine you could instantaneously remove the disc of metal from the hole while it was hot or cold you can hopefully visualize that it would just lift smoothly out as the disc and parent metal will be at the same state of expansion. The hole left as the disc is removed isn't going to suddenly get bigger or smaller else the disc (still carefully maintained at the same temperature wouldn't fit back in)
The size is irrelevant... the metal doesn't know if a hole is 'big' or 'small' it has no concept of measurement units or number... or can a brass plate tell that 20 mm is in fact smaller that 1 inch?
Del

That makes sense.

If you take a bunch of balloons, and place them in the shape of this holey plate, and inflate each balloon more, what happens...?

Well, in this case the picture pixels coarsen.

I thought I saw something like this one night on the Playboy channel. But it was late and there was alcohol involved . . .

All three. If it's a large hole it will get bigger. If it's a small hole it will get smaller. Somewhere in the mid size it will stay the same.

it depends on the material of the plate!

most metals (not all) get bigger with increasing temperature.

if the plates outer diameter increases, the inner diameter will increase too.

the inner diameter will do the same like the outer diameter.

The material was specified as brass. Oddball metals are irrelevant.

i don't know all the material constants and i answered before i read any comment.