mind blockage

[Rob Campbell]

Hi Folks,

 

All windows and doors shut?  Even in a draught-free room, air molecules are
batting by at random speeds greater than a rifle bullet.  For any one
component gas (e.g. oxygen) the total KE (linear + rotational) is
proportional to absolute temperature.

 

If you open a window, creating a breeze, the motion is no longer random.
There's a low drift velocity superposed on it.  The particle speeds haven't
changed, just the presence of a non-random component brought about by
pressure differences.

 

The breeze appears to start throughout the room at once, even though it may
be quite 'slow'.  Information about the pressure difference travels far
faster than the rate of air drift.  It travels at the maximum speed of
information in air: the speed of sound.

 

As with air, so with free electrons in a conductor.  The mean electron speed
in a piece of wire may be very high (and within wide limits, is proportional
to absolute temperature) but with no potential difference, the electrons are
going nowhere.  A potential difference imposes a non-random component on
their velocities - current, in effect - which is proportional to the p.d.
producing it.  And while the drift velocity may be low, mm/s or less, news
of the potential difference travels down the wire at the maximum speed of
electromagnetic information: light.

 

So yes, current (drift velocity) 'slows' but the electrons don't.  Their
almost random speed is set by the temperature.

 

Cheers,

 

Rob Campbell

 

  _____  

From: phys-teach-talk-bounces at nzip.org.nz
[mailto:phys-teach-talk-bounces at nzip.org.nz] On Behalf Of loneill
Sent: Friday, 21 November 2008 1:22 p.m.
To: Phys-teach-talk at nzip.org.nz
Subject: mind blockage

 

HI all

 

Mind blockage. If a current decreases, do less charges leave the battery, or
do they leave slower (ie speed changes)?  Does current "slow"?

 

Lee

 

Lee O'Neill

TIC Physics

 

James Hargest College Senior Campus

288 Layard St

Invercargill

 

loneill at jameshargest.school.nz

Ph (03) 2176-129 ext 304

Fax (03) 2170-351

 

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