Science Web Corner I.S.S.N.: 1579-1149

El rincón de la Ciencia         nº 29 (enero-2005)

The jumping coin (pr-42)

M. A. Gómez

English Translation: J. Cantos

This is a English version of the La moneda saltarina

We are going to introduce a brief experience that will probably be very funny for the youngest. But as all of them, it will also have its scientific foundations for the eldest. We are going to take profit of the pression variations that cause the changes of air temperature to make a coin skip.

What do we need?

  • A glass bottle

  • A coin

What are we going to do?

We are going to place the glass bottle in the freezer for a certain time until it is cold enough.

After a time ( for instance, half an hour) we are going to take it and leave it standing on a table.

Afterwards, we are going to cover the mouth of the bottle with a coin and we are going to watch what is happening. If necessary, wait for a while.

What is happening? Why do you think that is happening?

What has happened?

If you have done the experiment well, you will have seen as the coin skips over the mouth of the bottle for a few minutes. This effect is due to the very low temperature of the air in the bottle when we take it out from the freezer, (approximately -15 º C). By placing the coin on the mouth of the bottle, we are covering it and preventing air from entering or leaving.

After a few minutes, as the room temperature is higher (let´s say +20º C), the bottle warms up and so does the air inside. The increase of temperature of the air contained in the bottle implies a rise of its pression as well, until it is high enough to blow the money up and let some air escape. And back to restart.

The coin will keep on skipping at increasingly longer intervals, while increased indoor air temperature causes a pressure increase  high enough to make it skip.

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