How Come?

Why does metal feel colder than clothing if they are both at room temperature? asks a reader.

Have you ever climbed into a cast-iron tub before it’s filled, and touched your bare back to the metal? Even in a warm room, the tub will make you flinch. Or stepped barefoot on marble tiles after walking across a carpet? Common sense tells you that the carpet and the marble are probably at the same temperature. But your bare feet, acting as a handy thermometer, tell a different story: The marble feels much cooler.

While your skin may not be the most accurate thermometer, it is sensing a real difference between materials. It all comes down to the movement of heat between one object and another.

(more…)

How come when I open a can of mixed nuts, the Brazil nuts (which I don’t even like) are always at the top? asks a reader.

It’s like a Brazil nut conspiracy. The big, heavy nuts sit like bullying boulders at the top of the can, shoving all the tasty almonds, pecans, cashews, and (not-so-thrilling) peanuts to the bottom. Doesn’t gravity make heavier things sink? Or is it somehow rendered powerless in the confines of a Planters can?

Scientists first officially identified the mystery in the 1930s, around the same time the first Planters peanut stores opened across the U.S. When a container of particles is shaken up and down, a big particle buried inside will tend to rise to the surface.

More than 70 years later, the Brazil Nut Effect still isn’t completely understood. Why all the attention to a problem with party nuts? Scientists say that the Brazil Nut Effect goes way beyond oppressed almonds. The separation of a material’s particles by size affects everything from geological processes to food and drug manufacturing. On the positive side, the effect allows manufacturers to use vibration to separate particles (like grains of rice) by size. On the negative side, it can result in unevenly-mixed medicine.

(more…)

What causes tea to run down the underside of the spout rather than out of the teapot? asks a reader.

Whether it’s tea ruining a tablecloth or rain rotting a windowsill, scientists call it The Teapot Effect, and still write and publish papers about the annoying phenomenon. Tea seemingly changes its mind about being poured into your waiting cup, turning back to run down the spout. But gravity has the last laugh, as the liquid breaks off the teapot and drips unceremoniously onto the table.

And it’s not just tea. If you’ve ever tried to pour milk into your cereal from a drinking glass, you know that it sometimes takes a side trip down the glass. From rainwater pooling underneath outdoor windowsills to soup running down the pan and into the burner, the teapot effect is a pesky problem in fluid dynamics.

So what’s the story behind tea’s messy retreat? Some theories say the teapot effect is due mainly to surface tension and adhesion. The molecules at the surface of a liquid are attracted much more strongly to each other than they are to molecules in the air above. The result is surface tension, creating a kind of elastic “skin,” allowing some insects to walk across ponds. Surface tension also causes water to bead up on wax paper or other surfaces. Meanwhile, water is also attracted to other materials, causing it to cling a bit to glass and ceramic vessels.

(more…)

How does dry ice go from gas to solid without becoming a liquid? asks Deidre Hocevar, a student in Brookville, NY.

Dry ice is frozen carbon dioxide, the gas that make soda pop bubbly. Like other substances, carbon dioxide (CO2) can also be a liquid or a solid. At normal air pressure, water is a liquid at room temperature, turning to a gas (steam) when heated to 212 degrees F and into a solid (ice) when its temperature drops to 32 F. But to at 32 F, carbon dioxide remains a floaty gas. A block of dry ice has a temperature of about -109 F.

Dry ice gets its name from its peculiar behavior: Instead of melting into a puddle on the floor, it disappears into a cloud of gas. The scientific term for this process is “sublimation.”

But carbon dioxide can and does exist in liquid form. To turn carbon dioxide into a liquid requires a big squeeze–at room temperature, a pressure of about 30 atmospheres. Many fire extinguishers contain liquid carbon dioxide, held under just such high pressure. And in 2006, scientists discovered a shallow lake of carbon dioxide hidden under the floor of the East China Sea.

(more…)

Why does the moon change color in the fall? asks Alex Dickinson, a student in Holtsville, NY.

It just wouldn’t be fall without a huge Halloween moon glowing orange at the horizon, rising above a spooky landscape of black tree limbs and piled-up leaves.

We call the moons of autumn harvest moons, but the official Harvest Moon rose on September 26th. The Harvest Moon is the full moon closest to the autumnal equinox, which fell on September 23rd this year. The Hravest Moon was probably named by farmers. Long before there were huge, gas-powered harvesters with blinding headlights, there was the bright fall moon, lighting the fields as the work of crop-gathering stretched into the night.

The romantic image of the autumn moon even inspired a famous song: “Shine On Harvest Moon,” written more than 100 years ago, remained popular for much of the 20th century. (For a clip of comedic actors Laurel and Hardy performing the song, visit
www.hamienet.com/midi12542_Shine-on-Harvest-Moon.html. )

The moon is a rocky gray-and-brown ball, lit up by brilliant sunlight. But the color we on Earth see depends on where the moon is located in the sky. No matter what the time of year, as the moon first peeks over the horizon, it may appear yellow, orange, or nearly red. Gradually, as the Earth turns eastward and the moon rises higher in the sky, the color pales to white.

(more…)

Why does a penny at the bottom of a glass of water sometimes look like it’s floating on the surface? asks a reader.

Tricks of light and water are all around us. Gaze at a goldfish through the curved wall of its bowl: The fish looks both bigger and at a different depth than it appears when we look down through the open top. And if the fish decides to hang out on the bottom of the bowl, you may see a fishy mirage, floating on the surface.

And then there’s the drinking-straw-in-the-glass illusion. Put a straight straw in a glass of water, and voila: Instant bendy straw. The fact that a straw appears bent or broken is caused by something that really is bent: light.

Light moving from thin air into solid glass slows (and the light wave bends) as it passes into this denser material. It bends again when it travels out of glass and into water. Reflecting off objects underwater, the light travels back through the water and glass, re-emerging in the open air. At each junction, it has bent a bit more. By the time it reaches your eyes, the light is seriously off course. So the part of an object that is submerged in water, like a drinking straw, appears to have moved seriously out of place.

(more…)