Pub trick – making a coin rattle
Jearl Walker www.flyingcircusofphysics.com
September 2010   The challenge this month is about a coin placed on the top of a beer bottle. Can you make the coin rattle without touching it or rattling the bottle or the table, and without even being close to the bottle?

Here is a hint to delay the answer --- both the answer and the physics are simple and something you heard about in high school chemistry and physics glasses when you learned about an ideal gas and the ideal gas law.

Open a cold bottle of beer and pour its contents into a drinking class. Then dip the coin into the beer. As you bring it out of the beer, spread the clinging beer over the full surface of the coin. Then place the coin over the open top of the bottle and step away. Within a minute, the coin begins to occasionally rattle up and down. You can see all this in the following video:

http://www.youtube.com/watch?v=yWbLTqs278U&feature=related

The air that flows into the bottle as you empty the beer is at room temperature but after the bottle is set down, the air begins to be chilled by the cold wall of the bottle. By the time you place the coin on the bottle, the air is at about its lowest temperature. For the next several minutes, the walls and trapped air begin to warm toward room temperature. This temperature increase causes an increase in the kinetic energy of the air molecules and thus also an increase in the air pressure.

If you do not wet the coin, then the coin forms an imperfect seal on the top and the increase in air pressure merely pushes some of the trapped air out through the tiny openings between the coin and the top of the bottle. However, with the coin wet, beer fills those openings and completes the seal.

Every now and then, the air pressure increases enough to lift the coin, momentarily breaking the seal and allowing some of trapped air to escape. As the pressure drops slightly, it is then not high enough to maintain the lifted edge of the coin, and so the coin drops back down on the top of the bottle, striking the glass with a noticeable sound --- part of the rattle. This process can be repeated many times (and in short bursts) until the air pressure is no longer able to lift the coin and the internal temperature is approximately room temperature.

I used an empty beer bottle, chilled it for 30 minutes in a freezer, and then placed a water-coated United States quarter (a coin of value 0.25 US$) over the top. The rattle of the coin began almost immediately and reoccurred frequently for about 10 minutes.
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About 15 months ago, I hung a beer bottle on my basement wall and explained why it stuck to the wall. Some of you have asked it if is still there. I am happy to announce that it is still as firmly stuck to the wall as initially. If you want to jump to the explanation, use this link.
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Do you want the pub-trick stories here at the FCP site? Use the following links and then scroll down a page or search for "pub trick". Keep in mind my point --- anyone can do a pub trick but the real trick is to explain it without bluffing or just waving your hands in the air. Physics = the power to explain.
Chapter 1 archives
Water and the disappearing cigarette  click this
Balancing a coin on a folded paper edge  click this
Lifting a bottle with thumb and one finger, click this
Hanging spoons from the nose, click this
Hanging bottle caps on your face click this
Standing eggs on end  click this
Removing a lighter from under a bottle  click this
Removing a bill from between balanced bottles  click this
Removing the cork from a wine bottle  click this 
Balancing a hammer and a lorry  click this
Hanging a bottle on the wall  click this
Champagne cork as a morter round   click this 
Removing a coin from under a mug   click this 
Transferring a stell ball between beer mats    click this 
Matchstick rocket  click this
Tying a ring hitch click this
Chapter 2 archives
Reversing an egg in a tequila glass  click this
Blowing out a candle   click this
Escape from a cellophane pocket  click this
1000 drops from an empty bottle  click this
Exchanging water and whiskey   click this
Yard of ale and beer boot  click this
Tia Maria wormy action  click this
Vortex in a bottle and the vortex beer bottle   click this
Using glug-glug to clear beer foam   click this
Inverted can of Red Bull   click this
Chapter 3 archives
Moving match sticks on a glass rim
Chapter 4 archives
Slam-freezing a beer or soda
Chapter 5 archives
Rotating a matchstick balanced within a glass container