# Electromagnets and Permanent Magnets

#### Klepht

Hi everyone

Im an engineering student currently working on a group project. Part of the project is to use a electric magnet and a permanent magnet to balance a weight on the end of a cantilever (digital scale effectively). We would then measure the current through the electromagnet to calculate the weight.

My question is (since I only did physics in first year) how can I quantify the strength of my permanent magnet (cylindrical, 1cm high, 2 cm diameter roughly)

and secondly could someone point me in the right direction for constructing electromagnets?

Thanks very much :)

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#### rsr_life

Hello Klepht

Look at this site for basic ideas on magnetism, starting with the solenoid
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/solenoid.html

The formulas for computing the field strength of permanent magnets (essentially the magnetic induction B) should be available online - just google it out (am a bit lazy right now). But you would quantify the strength of electromagnets using it's magnetic induction (B) value. Usually this tends to be of the order of a few teslas for strong electromagnets. 1T=10,000 Gauss.

Your typical electromagnets on the other hand, aren't going to give you as strong a field - at least not for very long. These are very simple to make - just keep winding the core with several hundred turns of wire. And thats the key - the core. Using an air-core (i.e. just a magnet wire wound in a tight helical fashion), you're not going to get much strength. For your application, I presume you'd need a much stronger field - maybe a 0.5-several teslas. This is going to take quite some work unless you use strong cores.

If residual fields are fine (go for an iron core, or variations of the iron core). These have the advantage of producing very strong fields, but tend to stay magnetized for quite a long time after the current is turned off (unless you switch current directions and demagnetize them). If residual fields aren't fine, go for more specialized cores (like powdered ferrite core, MPP, High-Flux, etc). These tend to have significantly smaller residual fields, aren't too expensive, but tend to produce weaker fields.

So, if you take an iron core, you need lesser turns since the permeability of the core is pretty darn high. Since time isn't a critical issue for your project (if millisecond to second times dont matter much), go for something like this.

Also, note that if you pass too much current, you're going to heat your magnet a lot. So, unless you're modelling the whole thing and making an advanced control setup that takes heating into account, I would recommend just getting better cores, and reducing the current you need.

Good luck!

#### Klepht

Thanks for the reply rsr_life

I couldn't find anything on how to compute/measure the magnetic field of an electromagent but Im going to go find a physics lecturerer today and see what he/she has to say on hte matter.

I haven't decided on the core yet but since the electromagnet only has to balance a wieght of 1 kg maximum I suspect we'll go for a core that suffers from very little residual field. Would be fun setting up an advanced control loop to set the field back to 0 everytime and actually may not be that difficult. Will invetigate.

#### Klepht

This is a question relating to the construction of the electromeagnet spcifically the core:

Ive read that using a 'soft' iron core has low remnance, high permeativity and is relatively inexpensive. Can anyone recommend a different core material or possibly comment on the use of the soft iron core? How much of a problem will eddy currents be? I have no clue what amount of current Ill be passing through the windings. Will have to be able to life 1kg so approx 10 newtons max but there is a permanent magnet to aid with lifting.

Also would the soft iron core be a lump of metal or would it have to be laminated?

I'm using a DC supply so what effect does that have on hysteresis, eddy current etc...

Thanks everyone

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