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Why does putting on a coat in cold weather make us warmer?

Why does putting on a coat in cold weather make us warmer? asks a reader.

Go out lightly dressed on a frigid day, and thermal energy will quickly drain away from you into the cold December air. But unlike a run-down mechanical bunny, you don’t need new batteries. A warm coat pulled from the back of the closet will do just fine.

A warm parka

Although you might assume the culprit is heat radiating from exposed skin (such as your red, cold nose), the body actually loses thermal energy in five different ways.

First, we are indeed champion radiators: Exposed or under- insulated parts of our bodies give off infrared radiation. (Think of those infrared warming lights fast food restaurants; they keep the fries hot.) We also absorb infrared radiation from the environment. But when the temperature drops — when the air is cooler than the body’s temperature — we lose more heat than we gain. On a cold winter day, the loss is speedy, and mounts up: Infrared radiation accounts for more than half of all the heat we lose.

How do we lose the rest? First, there’s conduction, direct contact between a warmer object (us) and a cooler object (say, a cold plastic subway seat). Heat slowly transfers from you to the seat. If you’ve ever sat down on a toasty, just-vacated seat in winter, you’re being warmed by the previous sitter’s lost body heat.

(Water is even better at conducting heat than plastic or wood. Which is why falling into a pond in winter can cause a disastrous loss of body heat.)

So we are radiators and conductors. (No wonder cats like to snooze on our chests.) But we also lose heat through convection. Air isn’t a great conductor of heat, which is why a body-warmed layer of air hangs around our skin. But when the wind blows, it carries away this toasty layer. On a windy winter day, we become chilled very quickly. The higher the wind speed, the faster heat is carried away, so the wind-chill index is actually a handy clothing guide.

We also lose heat through evaporation. In the summer, evaporation of our skin’s watery sweat helps keep us from overheating. In the winter, sweating just makes us colder. Finally, we lose thermal energy just by breathing, exhaling warm water vapor into the wintry air.

Winter coats help contain some of this five-pronged heat loss. Coats, hoodies, scarves, gloves and other clothes don’t have built-in heaters like electric blankets. But they do help keep your body’s thermal energy from simply warming the air, seat, and metal pole in a chilly subway car.

How? Coats made of real or synthetic leather create a physical barrier to wind. Fluffy down-type jackets keep a layer of warmed air virtually trapped near your body, minimizing loss by convection. Wool, furry, or sherpa-style coats hold air in pockets created by their fibers. (Such coats are warmest if the fuzzy bits are on the inside.) Sweaters and shirts underneath a coat enhance the insulating effect, adding layer upon snug layer of trapped toasty air, courtesy of the body’s inner furnace.

January 26th, 2012 | Category: human body, nature, weather

How do the body’s own electric currents work?

How do the body’s own electric currents work? asks a reader.

In “I Sing the Body Electric,” a story by Ray Bradbury, an “electric grandmother” arrives to take care of a family of motherless children. This “grandmother” was a robot, but human grandmothers–and children–are electric, too. In fact, every body is electric. Just as the current running through a lamp cord powers a light bulb, the body’s own tiny currents power each and every cell, enabling them to pump blood, secrete hormones, move limbs, sense the environment, and think.

In the copper wires of your home, electrons jump from atom to atom, creating a current. But our body’s currents don’t run along tiny wires, and the currents aren’t made up of wandering electrons.

Lightning

How it works: An ordinary atom is electrically neutral, because its negatively charged electrons are perfectly balanced by its positively charged protons. If atoms weren’t neutral, everything around you, from your desk to your dog to the dandelions in your yard, would be electrically charged.

Ions are atoms (or molecules) that have become electrically charged. These charged-up atoms have gained or lost electrons, upsetting their carefully neutral balance. An atom with too many electrons has a negative charge; with too few electrons, the charge is positive.

Our bodies’ cell currents are made up of ions. In a complicated process, cells separate ions by pumping them through holes in the cell membrane, called channels. Like a wooden toy that allows only triangles to fit through one opening, squares through another, the channels allow certain ions to enter or leave. That keeps the charges — eager to unite, since opposites attract — separated on either side of the thin cell membrane.

Take nerve cells, a.k.a. neurons. A resting nerve cell has a negative charge on the inside, since it’s composed mainly of negatively charged protein molecules, which can’t pass through the membrane. The nerve cell has a kind of “pump” that moves sodium and potassium ions–both positively charged–into and out of the neuron. For every two potassium ions allowed in, three sodium ions are ejected. So when a neuron is resting, there are fewer potassium ions on the inside than sodium ions on the outside. The result is an electrical voltage difference.

Nerve cells use electricity to transmit messages over the miles and miles of pathways running through the body, up the spine, and into the brain. That’s how you sense that your bare feet have just stepped on sharp gravel, or that a cat’s fur is silky smooth under your hand. As a resting nerve cell swings into action to send out a signal, sodium channels open. Since the neuron’s interior has a net negative charge, positively charged sodium ions naturally flood in. As the inside of the neuron becomes more positive, potassium channels open, and repulsed potassium ions stream out of the cell. The result is a kind of electrical current, triggering the channels on neighboring neurons to open and close, too — sending a signal across the universe of your body.

For more on how the body’s cells generate electricity, see http://faculty.washington.edu/chudler/ap.html .

January 26th, 2012 | Category: human body, nature, physics
 
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