Re: [femm] high schoool science project

["ernestfreeman" <ernestfreeman@xxxxxxxxxxxxx>]

Greetings jtheobald.

1. It would be helpful to know your academic level, what exams have you
passed in science? I can then pitch my replies accordingly.

2. It is extremely difficult to calculate magnetic fields when you have
iron present. You really need a computer aided design (CAD) package. Try
this website...

http://members.aol.com/gmagnetics/download.htm


This will not be easy for you, but will show you what is possible. Also
look at the listed companies, their websites will give you a good idea of
the state of the art.

3. If the coil is in air, with no iron present, then you can calculate
the field on the axis. This is very easily done for a single circular coil,
see A-level physics texts. Off-axis is very tricky using closed form
algebraic expressions. For a solenoid, divide it up into a large number of
thin circular coils, then add them separate fields together. I use a spread
sheet for that sort of tedious calculation.

4. To calculate forces, you really need fields. Then you would apply
Maxwell stress formulae. Again, you need a CAD package.

There is another technique based on rate of change of inductance. Measure
the inductance with the iron piece at several distances along the axis,
keeping the dc current constant. Plot inductance against distance. Then
the force is given by.....


Force = 0.5*(dc current)^2*(rate of change of inductance with distance) in
Newtons


5. To a first approximation, the power required will be independent of
the number of turns, for a given force. Also, do note that the field is
proportional to (turns*current).


6. The Institution of Electrical Engineers, (IEE), London, has links to
schools. That means that your school could have a direct link to the sort of
expertise you are tapping with this enquiry.

7. I work at Imperial College. It runs the Pimlico Project, to take
science into schools. If your school is in or near london, that might be
worth noting.

8. Please show this to your teacher.

9. As an aside, I enjoyed your joke spelling, but I am sure that for
serious correspondence you would run everything you type through a spelling
checker.

Good luck, E.M.Freeman


Professor E.M.Freeman, FREng, FIEE, FRSA, PhD,
Professor of Applied Electromagnetics,
Electrical and Electronic Engineering Department,
Imperial College of Science Technology and Medicine,
Exhibition Road, London, SW7 2BT, UK.
phone:0171-594-6166, fax 0171-594-6167, ernestfreeman@xxxxxxx