From: SFPhysics@aol.com
Date: Wed Oct 17 2001 - 16:35:44 PDT
From: SFPhysics@aol.com Message-ID: <a9.1cea77fb.28ff6fd0@aol.com> Date: Wed, 17 Oct 2001 19:35:44 EDT Subject: Re: electromagnets
Hi Sally, et al:
<<
> From: "Sally Seebode" <sseebode@earthlink.net>
> 1) What is the best kind of wire to make an electromagent?
Use "magnet wire" which is copper wire with a high temperature coating of a
special lacquer. It is sold in rolls but it is not inexpensive; however,
small rolls from Radio Shack, etc., are very expensive because of high
packaging markup. Electrical Wire is made is diameters which is called its
gauge. For most electromagnets I liked to have my students use a 22 gauge
wire because it is low in resistance and easy to work with. Some high
current applications require 18 gauge while a sensitive low current compass
galvanometer uses 40 gauge.
> 2) What is the optimal wire thickness to to length/turns of wire?
The gauge and number of turns depends on the project. Everything is
connected by Ohm's Law. Each meter of copper wire of a particular gauge will
have specified resistance. To get a desired current you will then have to
apply a voltage that will drive the current you desire through the
resistance. I = E/R Since all current through a resistance produces heat
the idea is to use low resistance wire of large diameter where possible.
Putting wire in coils keeps the heat from getting out and this too is a
factor.
> 3) If the voltage is constant, what would be the equation to find the
maximum
> magnetic field if the length was to be varied, in comparison to the
thickness
of wire?
You could use a series of equations to relate wire diameter to resistance
then to current for a fixed voltage but your results would be essentially
meaningless. Electromagnet strength in Webers ( F ) is equal to
Newton-Amperes per Meter^2 and ultimately to Ampere Turns on the
electromagnet. i.e. A loop of wire will develop a magnetic field which
concentrates in the axis of the loop. Putting loops together in the same
direction makes a coil where the central axis is termed a core. Each loop
adds its magnetic field to the next loop and they all concentrate the
magnetic force through the core. Coils are rated in Ampere Turns so that one
Ampere of current going through each loop creates force multiplied by the
number of loops. A coil of 3000 loops running a current of 1 Ampere would be
a 3000 Ampere turn coil.
Finding flux density F = 2 x 10^-7(Amperes/radial distance) of a coil
represents a unique problem in finding total force since the radial distance
varies with each successive winding layer of the coil.
The efficiency of the coil core in concentrating the magnetic field has a lot
to do with the force generated. An air-core coil is very inefficient because
air is not magnetically permeable while a soft iron core is extremely
permeable and will concentrate most of the magnetic fields from the
individual loops. What you have for a core is more a determining factor than
how much current you are putting through the coil. An air-core coil may only
be able to generate 10 Gauss at one end can have an iron core placed inside
which will then give a 400 Gauss value.
Look at the definition of the Weber and the Tesla to see if you can use them
for the purposes you were trying to achieve:
1 Weber F = kg * m^2/C * s
where C is Coulombs and s is seconds
1 Tesla B = kg/C * s
> 4) What would be the maximum current in the wire before the wire would fail
(melt, etc.)
Copper melts around 1,083°C but that is not what you need to be concerned
with. It is the failure of the insulation on the wire that will go first.
Plastic insulation on regular wire melts before 200°C for most
thermoplastics. The special insulation on magnet wire will reach 200°C
without degradation but is not recommended to exceed that value or hold that
value for any length of time. The majority of magnetic induction motors try
to keep the operating temperature below 80°C and the "rule of thumb" is "if
you can't keep your thumb on the device for 10 seconds, it is running too
hot."
> Thanks for your help.
> sally
I hope I have been of some help but not knowing what you were trying to
achieve I could only answer in generalizations.
Best of luck,
Al Sefl
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