Please help! Science project about light/sound in a vacuum.

[mtrome]

We have set up an enclosed, sealed glass dome. A light and bell are in the center. Our goal is to create a vacuum inside, still seeing the light but no longer hearing the bell. We are having trouble making this work. I have tried different vacuums (maybe not strong enough) and no luck. The glass is sealed tightly too. Any ideas????

[deleted-71395]

That's a fun project. I've only seen it, not set it up, but I have two suggestions:

-- A pump that could create a vacuum with bell jar is a pretty standard piece of equipment for upper-grade science teachers, so you might be able to borrow it from school.

-- I assume you're using a normal vacuum cleaner that you're just connecting to the side of your home-made bell jar. I'm guessing, though, that you don't have any sort of one-way valve there; you're just directly connecting it. I don't know, but I'd guess that the friendly folks at Home Depot or the ilk might be able to set you up with some sort of one-way valve that would assist.

I tried a Google search for other suggestions, but didn't find anything.

[Jim Lewandowski]

If your bell is in contact with the base of the jar you can have sound conduction through that material too. The speed of sound through some solid materials, such as metals, can be much faster than through air too.

You may need to put your bell on some standoff's to lessen the surface area in contact with the base of your bell jar.

Another cool vacuum experiment is to make water boil at room temperature.
If the sound thing doesn't work out, give that a try. Put a glass of water in the bell jar and pump it down until the water begins to boil.

Jim

[mtrome]

I have tried a couple of different vacuums. I tried a inflator/vacuum that you get with one of those air mattresses. I also tried a home vacuum. No luck. I have the glass dome sitting on top of plexiglass. I sealed the bottom perimeter with silicon sealer. A copper tube (standard plumbing) comes in through the bottom and stretches out about a foot to be able to attach a vacuum. Could the narrow copper tube be restricting the ability for the vacuum to adequetly suck the air out of the glass dome????? Should I try a wider tube??

Additionally, I have the bell mounted on a dowel. Yes, it is touching the base and may be causing some vibration, but I don't believe that you could hear the metallic bell sound from that. Could you?

[deleted-71447]

From your description, you may not be getting a good vacuum in your dome. A home vacuum and an air mattress inflator are designed to draw a very high rate of flow, but they do not produce a very high pressure gradient. Assuming there are no leaks, something like a bicycle pump would be able to produce a stronger vacuum inside your dome.

You could possibly hear some metallic sound from transmission of sound waves from the bell, to the dowel, to the dome. You can check by pressing your ear against the solid surface of the dome to hear how much sound is being produced by the dome itself. To minimize such problems, you can use foam or rubber to dampen the transmission of vibrations between rigid parts.