Author: Lou Bloomfield

What is the difference between internal and external combustion engines?

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What is the difference between internal and external combustion engines?

External combustion engines burn a fuel outside of the engine and produce a hot working fluid that then powers the engine. The classic example of an external combustion engine is a steam engine. Internal combustion engines burn fuel directly in the engine and use the fuel and the gases resulting from its combustion as the working fluid that powers the engine. An automobile engine is a fine example of an internal combustion engine.

When you walk on snow when it is cold (-20° C), the snow squeaks; but when i…

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When you walk on snow when it is cold (-20° C), the snow squeaks; but when it is relatively warm (-5° C) the snow doesn’t squeak. Why? — PW, Alberta, CA

Near ice’s melting temperature, the surfaces within warm snow become more and more liquid-like. These liquid-like surfaces not only allow the warm snow to stick together as firm snowballs, but they act as lubricants so that the snow is particularly slippery. At much lower temperatures, the snow’s surfaces are much more solid and they slide uneasily and noisily across one another. The cold snow squeaks because it hasn’t “been oiled.”

When an object is free falling, I understand that the earth’s gravity causes its…

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When an object is free falling, I understand that the earth’s gravity causes its velocity to increase at 10 meters/second2 in the downward direction. Is there a point at which this object would reach a “terminal velocity” in the earth’s atmosphere and cease to accelerate? — CS, Sykesville, MD

Yes, most objects will reach a terminal velocity and stop accelerating downward. The faster an object drops, the more air resistance it experiences. This air resistance pushes the object upward and at least partially cancels the downward force of gravity—the object’s weight. When the object’s downward speed becomes high enough, the upward air resistance force exactly cancels the object’s downward weight. At that point, the object experiences zero net force and it no longer accelerates. Instead, it descends at a constant downward velocity—its terminal velocity. This terminal velocity is determined partly by the object’s density and size and partly by its aerodynamics. Large, dense, and aerodynamic objects tend to have very large terminal velocities while small, low-density, non-aerodynamic objects tend to have very small terminal velocities.

I’ve heard that, technically speaking, our atmosphere is a fluid. Can you discus…

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I’ve heard that, technically speaking, our atmosphere is a fluid. Can you discuss this?

Since both gases and liquids are fluids, the earth’s atmosphere is certainly a fluid. Any material that flows in response to sheer stress (tearing) is considered a fluid. The earth’s atmosphere flows in responses to sheer stress—for example when you drive your car past another car, the air in between experiences this tearing and it flows in a complicated fashion. Winds are another important example of fluid flow in the earth’s atmosphere.

When raisins are added to a solution containing water, baking soda, and vinegar,…

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When raisins are added to a solution containing water, baking soda, and vinegar, why do the raisins dance? — RE, Troy, IL

Baking soda and vinegar react in water to release carbon dioxide molecules. If the chemicals are sufficiently dilute in the water, the carbon dioxide molecules may remain dissolved in the water almost indefinitely. But when the water has impurities in it, the carbon dioxide molecules tend to come out of solution as gas bubbles at those impurities. The impurities allow the molecules to form tiny gas bubbles—a process called nucleation. In the present case, the raisins serve as the impurities that nucleate gas bubbles. As the gas bubbles grow on the surfaces of the raisins, the raisins experience upward buoyant forces from the surrounding water. The bubbles float upward, carrying the raisins with them and causing the raisins “to dance.”

In his Lectures on the Elements of Chemistry, Joseph Black discussed his …

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In his Lectures on the Elements of Chemistry, Joseph Black discussed his difficulty in understanding latent heat. He performed an experiment where water in a tube was brought below freezing without a phase change. The water remained in this equilibrium as long as the tube of water was not disturbed. When it was disturbed, the water instantly turned to ice, releasing enough heat to raise the temperature of the ice to 0° C. Please explain why the system remained in equilibrium until it was acted upon by some external motion. — EDH, Annapolis, MD

The water in Black’s tube was in an unstable equilibrium state known as supercooled water. Supercooled water tends to spontaneously convert into ice. When part of this supercooled water does convert to ice, it releases enough latent heat energy to raise its temperature and that of the remaining water to 0° C, thereby terminating the phase transition before all of the water has become ice.

But in the experiment you describe, the supercooled water was having trouble nucleating the initial seed ice crystal on which the remaining water could crystallize. Given enough time, that water would have spontaneously formed a seed crystal and the growth of the ice crystal would have proceeded rapidly after that. However, Black accelerated the formation of the seed crystal by shaking the tube. A defect at the surface of the tube or a piece of dust then acted as the trigger and helped the seed ice crystal form. The water then crystallized rapidly around this seed crystal. After the ice had formed, the water was truly in equilibrium.

Why does light travel slower in some media than in a vacuum? For example, in gla…

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Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible light is not absorbed and yet it slows down. What’s going on? — FH, Waltham, MA

When a light wave enters matter, the light wave’s electric field causes charged particles in the matter to accelerate back and forth. That’s because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don’t retain the light’s energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened—the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That’s because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons’ motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons’ motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren’t able to jiggle back and forth as far as they should and they don’t slow the light wave down as well.

What is infrared light?

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What is infrared light? — AC, Teaneck, NJ

Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second). Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can’t see infrared or ultraviolet lights because the cells of retinas aren’t sensitive to these lights. Nonetheless, we can often tell when those lights are present—we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.

Is it true that water that has been previously boiled will boil faster than wate…

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Is it true that water that has been previously boiled will boil faster than water that hasn’t been boiled? — HE, Haddonfield, NJ

I don’t think so. The only effect that bringing water to a boil has on the water is to drive dissolved gases out of solution. Once the water returns to room temperature, it’s essentially the same as it was before it was heated to boiling, except that it contains very little dissolved air. It may be that this absence of dissolved air will allow the water to boil slightly faster the next time around, but I doubt that you’d be able to detect a difference.