Month: October 1996

Why did Fahrenheit choose 32° for the freezing point of water and 212° f…

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Why did Fahrenheit choose 32° for the freezing point of water and 212° for the boiling point of water? These seem like such awkward numbers to use.

Daniel Gabriel Fahrenheit chose as the zero of his temperature scale the temperature at which ice melts when it’s mixed 50/50 with salt. He then set the temperature at which pure ice melts to be 30° above zero and normal body temperature to be 90° above zero. These values were adjusted several times over the years as temperature measurements became more accurate and are now 32° and 98.6° respectively. Having established the temperature scale based on these various situations, he had no choice about water’s boiling temperature. Water’s boiling temperature at normal atmospheric pressure simply turns out to be roughly 212° on his temperature scale.

Are divining rods and their abilities to locate ground water fact or myth?

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Are divining rods and their abilities to locate ground water fact or myth?

I’m afraid that I think they’re myth. Despite extensive searches, physicists have found only four forces in nature: gravity, the electromagnetic force, the strong force, and the weak force. Of these, only gravity and the electromagnetic force are noticeable outside of atoms. Since ground water has no electric charge, it can’t affect a divining rod through the electromagnetic force. That leaves only gravity as a possibility and the gravity between modest sized objects such as a stick and a pool of water is so incredibly weak that I can’t imagine anyone detecting it with their hands. Having eliminated all the possible external forces that would bend a stick downward when it’s near water, it’s clear that this bending is done by the hands of the person holding it. Perhaps a good dowser can see features in the environment that prompts the dowser, consciously or unconsciously, to believe that water is nearby. In short, I think that there are people who are good at identifying signs that indicate ground water is present and who can find that water. The divining rod itself is unimportant.

Why is it that when you have water on your skin and an air current travels over …

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Why is it that when you have water on your skin and an air current travels over it, your skin gets cold?

Whenever water is exposed to air, the water and air begin to exchange water molecules. By that, I mean that water molecules leave the surface of the liquid water to become water vapor in the air and water molecules that are already vapor in the air leave the air to become liquid water. If the relative humidity of the air is less than 100% (meaning that the air can still hold more water vapor), more water molecules will leave the liquid water than will return to it and the liquid water will gradually evaporate into water vapor. If the relative humidity of the air is greater than 100% (meaning that the air is holding more water vapor than it can tolerate), more water molecules will return to the liquid water than leave it and the water vapor will gradually condense into liquid water.

For a water molecule to leave the surface of liquid water, it needs a substantial amount of energy because it must break several hydrogen bonds which are holding it to its neighbors. It obtains this extra energy from nearby molecules and they become colder. Whenever a water molecule returns to the surface of liquid water, it returns this energy to the nearby molecules and they become hotter. Thus whenever liquid water is evaporating, the water molecules that leave the liquid water are taking away its energy so that it becomes colder. And whenever water vapor is condensing, the water molecules that return to the liquid water are giving it energy so that it becomes hotter.

When your skin is wet and water is evaporating from it, your skin also becomes colder. Blowing additional air across your skin prevents any build-up of humid air near its surface so that far more water molecules leave your skin than return to it. The evaporation then proceeds rather quickly and your skin feels quite cold.

Both hydrogen and oxygen fuel flame, but together they make water and that can p…

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Both hydrogen and oxygen fuel flame, but together they make water and that can put out a flame. Why?

In a sense, water is the “ash” that forms when hydrogen burns in oxygen. Like all fully burned materials, water can’t burn any further. When you put cold water on a fire, it extracts heat from the fire because the water is much colder than the fire and heat naturally flows from hotter objects to colder objects. Since heating the water doesn’t cause the water to burn (it can’t burn), the heat that’s lost by the fire doesn’t create new fire (as would be the case if you threw gasoline on the fire instead of water). So the water gradually cools down the fire until the fire no longer has enough thermal energy to sustain its own chemical reactions. The fire then goes out.

What are the two substances in a Lava Lamp, and why do they react the way they d…

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What are the two substances in a Lava Lamp, and why do they react the way they do?

I’m afraid that I’m unable to determine exactly what substances the lamp contains. However, I believe that one of them is water and the other is a high-density wax. When the lamp is cold, the wax is a crystal solid with a density slightly higher than that of water. Because the buoyant force this wax experiences from the water is less than its weight, the wax sinks to the bottom of the lamp. But when the lamp is on, the bottom of its container heats up and the wax begins to melt. Like most materials, wax’s liquid phase is substantially less dense than its solid phase. As it melts, the wax expands so much that its density drops below the density of water and it floats upward to the cool top of the container. Once it reaches the top, the wax begins to solidify. As it solidifies, the wax contracts so much that its density rises above the density of water and it sinks downward to the bottom of the container. Thus when the lamp is in full operation, the rising bubbles of wax are liquid and the descending bubbles of wax are solid. Dyes are added to the two materials to make them more visible—the water is colored by a water-soluble dye (perhaps food coloring) while the wax is colored by an oil-soluble dye (like those used in permanent markers).

How does a mass spectrometer work and why must it be evacuated before being used…

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How does a mass spectrometer work and why must it be evacuated before being used?

A mass spectrometer is a device that measures the masses of the atoms or molecules in a sample. There are many different types of mass spectrometers but they all work on roughly the same principle: they give each atom or molecule a single electric charge and look at how easy or hard it is to accelerate that atom or molecule by pushing on it with electric or magnetic fields. The more mass the atom or molecule has, the more slowly it will accelerate in response to a particular force. Some mass spectrometers use an electric field to push the atoms or molecules forward until they all have the same amount of kinetic energy and the more massive particles end up traveling more slowly than the less massive particles. Their masses can then be determined by timing how long it takes them to travel a certain distance or by sending them through a magnetic field that bends their flight paths. Because the force that a magnetic field exerts on a moving particle increases with that particle’s speed, the paths of slow moving massive particles bend less than those of fast moving less massive particles. Since all of this mass analysis occurs while the particles are traveling through space, it’s important that they not collide with any gas particles inside the mass spectrometer. That’s why the mass spectrometer must be evacuated before use.