The speed of electric currents

People think that electricity moves at the speed of light.  Well, the influence of electric fields moves at the speed of light.  The surprising truth is that actual electrons carrying current in wires move slowly in DC systems, and scarcely move at all in AC systems.

Consider a 0 gauge aluminum wire carrying 100 amperes DC.  Aluminum has an atomic weight of 26.98 and a density of 2.70, with 3 valence electrons available to conduct current.  This means that 1 cc of aluminum has almost exactly 0.1 moles of atoms and about 1.8056*10²³ conduction-band electrons in it.

A 0 gauge wire has a cross-sectional area of 0.535 cm², so each cm of wire has about 9.66*10²² conduction-band electrons in it.  100 amperes is 100 coulombs/second, and there are 6.2419*10¹⁸ electrons in a coulomb, so a 100-amp current means there are 6.2419*10²⁰ electrons moving across any given cross-section per second.  Dividing this number by 9.66*10²² electrons per centimeter of wire yields an electron speed of just 6.46*10^-3 cm/sec, less than 100 micrometers per second!

A micrometer is small for a human, but big for an atom; the electrons are moving more than fast enough to run into many atoms per second, thus generating heat.  But the contrast between the speed of electric potential waves and the actual speed of the electrons is something that people have difficulty grasping.

Labels: electricity, physics, science

  ¶ 7/17/2021 02:31:00 PM 0 comments

 

When physics is on the line, face-palm edition

Via a correspondent who asked to remain anonymous and unquoted, I received some screenshots of panels from a brand-new comic called Alt*Hero.  Story authorship is "Vox Day", the pen-name of one Theodore Beale.  He claims to have a 150-ish IQ and refers to Aristotle regularly.

I'm passingly familiar with this guy; he comes up with clever expressions.  He used to get held up as an example of scientific pig-ignorance on scienceblogs, back when I read that site.  So what does he write into his comic?

A 90-kilogram object with an acceleration of 3,825 kilometers per hour strikes with the force of 10 tons

In short, physics bullshit.  It's bullshit from the units (acceleration is in units of distance/time²) through to the figures.  Per the story, the "Redshift" character can hit supersonic speeds from a standing start in just a few meters.  Figuring constant acceleration from 0 to 1000 m/s in 10 meters, the force required isn't "10 tons", it's 4.5 meganewtons (the weight of about 460 metric tons).  That's almost 2 orders of magnitude greater.  Impact into an immobile object would be orders of magnitude more.

Suppose you launched 90 kg to 1000 m/sec from a building.  A 460-ton shock would probably cave floors in and might even knock the building down.  That's physics.  More to the point, a skydiver in free-fall in the arrowhead configuration reaches terminal velocity at about 200 MPH.  There's no way a runner, however strong, could reach > 1 km/sec speeds by pushing against the ground (and at Mach 3 he'd broil himself from the air heated by his own shock front).  Another character flies, without using wings or any other aerodynamic method.  It is obvious that Physics As We Know It is not operating here (it's a comic book, Pure Fucking Magic is not just allowed but expected).

This begs the question:  why the pseudo-physics bullshit?  Does he not know any better?

Vox Day has long harped about scientific fraud and error, and per my correspondent even made up several new words purporting to help describe science accurately.  Ironically, this supposedly 150-IQ guy can't even hack first-semester physics.  (Does this extend to things like chemistry and biology?  Almost certainly the latter.)  He has some massive gaps in his abilities and understanding—worse, in subjects that people of much lesser IQ scores have mastered without undue difficulty.  That has to sting.  It looks like his enmity towards science and its practitioners comes down, not to principled differences, but a large dose of envy.  Face-palm time.

(As for the comic itself, it looks fun.  I may buy it, and that will make it one out of perhaps 5 I ever spent money on.  But seriously, if diversity box-checking in fiction is a turn-off, so's cringeworthy stuff like this.  Best just not to go there.)

Linkage where linkage is due:

• Alt*Hero on Amazon.
• Review discussion page at the author's blog.

Labels: physics

  ¶ 5/25/2018 05:16:00 PM 0 comments

 

Book review: "Earth Unaware"

After reading "Ender's Game" and so many of his other works, I thought that it was mighty hard to go wrong with Orson Scott Card.

I can now say that it is a sinking feeling to find out just how wrong I was.  At least, when he affixes his name to something as a co-author.

I read "Earth Unaware", and shouldn't have bothered finishing it.  Is OSC phoning it in, or what? Based on what's in the book, I'm assuming that his contribution was his name and some of the characters which show up in later novels.  His co-author Aaron Johnson has no concept whatsoever of orbital mechanics, radio propagation, or even basic physics like elementary particles.  The least one can do if one can't remember the difference between gamma rays and cosmic rays is look it up, and it's excruciatingly obvious that he didn't bother.

There are scientific howlers throughout the off-earth parts of the book.  An alien ship moving at a large fraction of lightspeed... takes weeks to travel inside the orbit of Jupiter, and human ships of far inferior capability nevertheless match velocities with it!  This ship generates radio noise of sufficient intensity to block non-laser communications... yet humanity appears to have forgotten how to make and use radio telescopes, because it hasn't been detected, pinpointed and intensively studied because of those very emissions!  Dust particles and gas in the Kuiper belt are a hazard to space-suited persons outside ships moving at speed... ignoring the fact that the solar wind sweeps dust into interstellar space, and the solar wind itself is moving as fast or faster than the velocities given in the book!  Oh, best of all:  the simple expedient of orienting the ship so that the hull intercepts anything from the direction of motion never occurs to the author.

If Card cared for his reputation, he'd ask Tor to remove his name from this book.  If Tor had any integrity, they'd stop selling it as SCIENCE fiction.

Labels: diversity, physics, science fiction

  ¶ 4/01/2015 01:00:00 AM 0 comments