Re: Electrical Puzzles - proper answers

[Dave Biggs <D.Biggs@domain.elided>]

JOHN CLAYTON wrote:
> 
> Glenn,
> 
> That all sounds logical to me, except the bit about electrons
> travelling faster when cold. Are you sure? I would've thought
> higher temperatures meant less resistance and therefore
> faster speed. But then I'm not a nuclear physicist.

Glenn is right on this one. The current through the resistance of the
bulb filament causes it to heat up, as it heats up it's resistance
increases which causes the current to decrease. It will reach an
equilibrium when the bulb is dissipating its rated wattage eg 55W =
4.58A @ 12V (2.62 Ohms). So on initial turn on when the filament is cold
and it's resistance is say 0.5 Ohms the dissipation is 12V squared/ 0.5
= 288W (24A @ 12V) ! (This is not quite true as the filament has
inductance as well as resistance which will help to absorb the current
transient on switch on.) This is the reason that bulbs generally fail
during switch on.

Hope this hasn't mudded the waters even more !

Dave.  
-- 
	Dave Biggs                       
	Senior Design Engineer 	
	Fujitsu Telecommunications Europe Ltd, 
	Birmingham Business Park,Solihull Parkway, Birmingham, B37 7YU.
	mailto:D.Biggs@domain.elided  tel: 0121 717 6094  efax: 0121 717 6014