Index Key:  PHY061
Author:     lois
Subject:    Why do electric heating elements hum?
Text:       I have been wondering why heating elements (in an electric stove, 
in a toaster, etc.) hum when turned on.  I asked our physics teacher here at 
my school and he was unable to come up with a theory.  Do you have any 
thoughts?

Response #:  1 of 1
Author:      sysop
Text:        I use a slight variation of this question for our application for 
employment test for our Engineers!  Our question is more like "Why does a 
Soldering GUN heat up the tip.  While the reason for the heat is different, 
the cause of the sound in a heating element and the source for heat in a U 
shaped soldering tip is the same.  I would assume that the heating elements 
that the questioner is talking about is of the zig-zag type.  The vibration 
that causes the sound (or buzz) comes from the pulsating eddy currents that 
"fight" the adjoining wire.  Those eddy currents collapse at the rate of 60 hz 
in single phase applications.  If the coil was DC, the reaction of this 
element would be different and probably silent.  In a soldering gun, those 
eddy currents actually produce enough heat at the 180 turn that the tip heats 
enough to melt solder.  Remember that the tip of a gun is actually a single 
turn of the secondary of a transformer in the gun.  I am quite surprised at 
how few engineers get this answer on my test.  In fact, most say that it is 
resistive!