The Hot Chocolate Effect
Have you ever noticed that the sound that you get from a cup of coffee changes as you stir in sugar? See and Listen to the clip below.
Take a CLEAN cup and fill it with cold water from the tap (it MUST be cold) and put it in the microwave for a minute or two. Don't boil it - just bring it up to 70C or so. A good drinking temperature.
Now add a teaspoon of sugar, stirring as you do it. Make sure you use a metal teaspoon. Listen to the tinkling noise it makes. Notice that the pitch drops. Now stop stirring, but keep tapping the outside of the cup. Notice that the pitch rises again?
Explanation:
Air is more soluble in cold water than in hot water. Cold water out of the tap has lots of dissolved air. When you heat it up in the microwave the air-in-water solution becomes supersaturated. Adding sugar (or any granular material) very quickly nucleates lots of tiny air bubbles out of solution.
The speed of sound in a homogeneous liquid, according to the Newton-Laplace formula:
Water is approximately 800 times denser than air, and air is approximately 15,000 times more compressible than water. When water is filled with air bubbles, however, the fluid's density is very close to the density of water, but the compressibility will be the compressibility of air. This greatly reduces the speed of sound in the liquid. Wavelength is constant for a given volume of fluid, therefore the frequency (pitch) of the sound will decrease as long as gas bubbles are present.
Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.
Source:http://www2.eng.cam.ac.uk/~hemh/coffeecup/coffeecup.htm
https://en.m.wikipedia.org/wiki/Hot_chocolate_effect
Take a CLEAN cup and fill it with cold water from the tap (it MUST be cold) and put it in the microwave for a minute or two. Don't boil it - just bring it up to 70C or so. A good drinking temperature.
Now add a teaspoon of sugar, stirring as you do it. Make sure you use a metal teaspoon. Listen to the tinkling noise it makes. Notice that the pitch drops. Now stop stirring, but keep tapping the outside of the cup. Notice that the pitch rises again?
Explanation:
Air is more soluble in cold water than in hot water. Cold water out of the tap has lots of dissolved air. When you heat it up in the microwave the air-in-water solution becomes supersaturated. Adding sugar (or any granular material) very quickly nucleates lots of tiny air bubbles out of solution.
The note frequency f is equal to the speed v of the wave divided by four times the height of the water column h: 
The speed of sound in a homogeneous liquid, according to the Newton-Laplace formula:
Water is approximately 800 times denser than air, and air is approximately 15,000 times more compressible than water. When water is filled with air bubbles, however, the fluid's density is very close to the density of water, but the compressibility will be the compressibility of air. This greatly reduces the speed of sound in the liquid. Wavelength is constant for a given volume of fluid, therefore the frequency (pitch) of the sound will decrease as long as gas bubbles are present.
Upon initial stirring, entrained gas bubbles reduce the speed of sound in the liquid, lowering the frequency. As the bubbles clear, sound travels faster in the liquid and the frequency increases.
Source:http://www2.eng.cam.ac.uk/~hemh/coffeecup/coffeecup.htm
https://en.m.wikipedia.org/wiki/Hot_chocolate_effect
No comments:
Post a Comment