Let sleeping birds fly, Or sleep on one foot, While electrons limp

Q: What bird flies while sleeping?  (Wen, Selangor, Malaysia)

A: Our best candidate for birds that sleep while flying is the Laysan Albatross, which dwells in skies above northern oceans (such as the Bering Sea).

Albatross off Stewart Island of New Zealand; albatrosses regularly circle the globe. Courtesy of Thomas Mattern, University of Otago, New Zealand and Wikipedia

Once hatched, an albatross spends her life at sea, except to breed.  "They are sometimes seen asleep on the water," says the Arctic Studies Center, a part of the National Museum of Natural History.  "But this makes them easy targets for killer whales and stealthy hunters in kayaks; most albatrosses apparently sleep while gliding in the air."

The Laysan Albatross, one of the largest of all flying birds, has a wingspan greater than six feet (2 m), and, like all albatrosses, flies almost effortlessly.  An albatross heart rate while flying is almost the same as when resting.

Swifts are another possibility.  Swifts eat, mate, bathe and, perhaps, sleep on the wing.  Several thousands of feet up, radar observers in airplanes have monitored flocks of swifts spending the night aloft, write editors Maurice and Robert Burton in The Wildlife Encyclopedia.  "Presumably they take short 'catnaps' as they glide between bouts of flapping."

Further Reading:

The Laysan Albatross, Arctic Studies Center

The albatross, Wikipedia

Chimney Swift, Cornell Laboratory of Ornithology, Cornell University

Q:  Why does electrical resistance exist?  Why does metal have low resistance? (Mayank, Gwalior, India)

A: The electrical resistance of a material is the degree to which the material opposes a flow of charge (current) through it.  So, to understand how a material opposes charge flow, let's first investigate how charge moves through metals. 

How charge flows through metals.  When we apply a voltage (an electrical potential difference) across a metal, the potential difference creates an electric field that causes electrons to drift from one end of the metal to the other. 

A single crystal of metallic silver.  Copyright 1995-2006 by Michael W. Davidson and the Florida State University, used with permission.

Understanding why electrons drift, gets us into quantum theory.  Metals consist of a crystal-like lattice of atoms.   

Each metal atom in the lattice has associated electrons that inhabit only discrete energy levels.  The most tightly bound electrons (yellow ring in the figure) are closest to the atom's nucleus, and have the lowest energy levels.  We call these energy levels the valence band. 

Energy levels of a hydrogen atom (yellow is lowest energy). The nucleus is shown as the center blue blob, and an electron (green) is shown at its lowest energy state, closest to the nucleus. Figure by the author.

Electrons in the next band of energy levels, the conduction band, are free to move about within the lattice of atoms (leaving their "parent" atoms, and forming ions).  Moreover, it doesn't take much energy to kick an electron from the valence band to the conduction band.  Indeed, applying a small electrical voltage does the trick.  The electrons drift in the direction of the potential difference, towards the positive charge.  That's how charge moves through a metal. 

What causes electrical resistance?  Heat causes the most resistance to charge flow.  As the temperature rises, so does the kinetic (motion) energy of the atoms in the metal conductor.  As the atoms move around more, they jostle each other more and this increases the resistance that the conductor offers to the flow of charge.  That's the simple explanation. 

To understand resistance more fully, we've got to switch from thinking of electrons as discrete particles to thinking of electrons as waves oscillating through the lattice.  (Electrons are both particles and waves.) 

Looking at the phenomenon this way, we can visualize the effects of heat on the electron wave that zips along through the lattice of metal atoms.  The greater the heat, the more the metal ions (atoms that have lost electrons) within the lattice vibrate.  Moreover, this vibration destructively interferes with the wave motion of the electron.  The ion vibration is a different wave motion that messes up the electron wave just as two dropped pebbles mess up each other's waves.  Disturbing the electron wave motion increases the resistance of the conductor to the flow of charge. 

Also, impurities in a metal cause irregularities in the lattice structure, which disrupts the electron wave motion and, therefore, resists the charge flow. 

Electron energy levels in an insulator. Courtesy of Wikipedia.

Why metal has low resistance.  Metal atoms require little energy (a small potential difference suffices) to move electrons from the valence to the conduction bands.  Once an electron is in the conduction band, it is free to move and, thereby, conduct electricity.  Thus, metals have low resistance. 

Such is not the case for insulators and semiconductors.  Instead, a forbidden band of energy levels exist between the valence and conduction bands.  See figure.  Electrons cannot exist at these levels.  Therefore, we must apply great voltages across an insulator, for example, to make an electron leap across the forbidden band into the conduction band.  Thus, the resistance to charge flow is great.

Further Reading:

Microscopic view of Ohm's Law by Rod Nave, HyperPhysics

Electrical resistance by Keith Welch, Jefferson Lab

Electrical resistance, Wikipedia

Speed of electricity by Lois A. Bloomfield, How things work

Molecular expressions:  images from the microscope, Florida State University

Q:  What speed does electricity travel along a wire?  (Larry, London, England)

A: This question is slightly perplexing because we mean many things by "electricity." 

When we apply voltage across a wire, this creates an electromagnetic wave that zooms down the wire at the speed of light in the medium. 

An applied electric field superimposes a tiny drift velocity on individual electrons that moves them toward the positive charge.  Courtesy of Rod Nave, HyperPhysics, copyright, used with permission.

An individual electron travels almost as fast as the signal — about a million meters per second (m/s), but this motion is almost entirely a random, bouncing-around motion.  The electron travels this fast without contributing to charge flow.  It undergoes many collisions, and doesn't get far from one collision to the next.

Free electrons in the metal act almost like a gas, having billions of collisions per second, says Rod Nave, physics professor at Georgia State University.  The electric force gives them a net tendency to drift down the wire, but the collisions "thoroughly scramble" their progress.   See figure. 

The drift velocity is a slow limp in the order of a millimeter per second (about ten times slower than a garden snail). 

(Answered June 6, 2006)