Electromagnet polarity switching....

[Nidum]

Just for experimental purposes use a motor driven commutator or drum selector switch . Can be as elaborate as you like but a cotton reel and drawing pins will do the job .

 

[Bo Cash]

To change polarity rapidly you must meet two requirements.

Firstly; thick iron will not work. You must use a core that is laminated steel, iron powder or ferrite. That is because the magnetic field must quickly penetrate the magnetic core material. Laminations will give you up to 100 changes per second, iron powder will give 10 thousand CPS, ferrite will give you 10 million CPS.

Secondly; you must use fewer turns, with more current and less voltage. The limit to switching time is inductance which increases as the square of the number of turns.

Magnetic field is proportional to current times the number of turns of wire. To change magnetic field you must change current. The inductor voltage sets the rate of change of current; V = L ∙ di/dt ; which is why you need low inductance, few turns. Sudden changes of inductor current will need or produce high voltage spikes.

You might find the magnetic core and wire in second hand transformers, or from old electrical contactors in industrial scrap.

Thank you for your information.

 

[jim hardy]

I know next to nothing about subject. so the frequency is, to me unimportant at this stage.

Page 121 of this book starts the chapter on magnetics. It's a petty decent introduction .
https://books.google.com/books?id=qEUOAAAAYAAJ&pg=PA1&source=gbs_toc_r&cad=3#v=onepage&q&f=false

i have the 1901 edition and find his explanations wordy but detailed and helpful. Keep in mind that back then this was cutting edge..

Firstly; thick iron will not work. You must use a core that is laminated steel, iron powder or ferrite. That is because the magnetic field must quickly penetrate the magnetic core material. Laminations will give you up to 100 changes per second, iron powder will give 10 thousand CPS, ferrite will give you 10 million CPS.

Baluncore is quite well versed in this subject

For a home experiment that's easy to construct from readily available stuff,
i think i'd suggest

soft black iron wire from the lumberyard for a core. The black oxide gives a little insulation and the small diameter approximates laminations.
Make a bundle and wrap it with tape , bend it into a U shape
Wrap a few hundred turns of copper wire around the bundle.
Pillage an old car alternator for a source
these old Mopars are dirt cheap and simple, no internal regulator and both brushes wired to the back.

Hmmm a circle might work better than a "U" shape..

Good Luck

 

[sophiecentaur]

But what is the OP planning to do with these alternating fields? He has not mentioned anything about this and seems to assume that the effect will be somehow obvious by just looking at it? He has stimulated a load of positive replies from PF but has given away very little.
I think he has another agenda which he is not prepared to talk about. Either that or he just wants to tinker about with some electromagnetic equipment. As far as I'm concerned this thread is not going anywhere and nor is the OP, if he is not prepared to discuss his purpose.

 

[davenn]

But what is the OP planning to do with these alternating fields? He has not mentioned anything about this and seems to assume that the effect will be somehow obvious by just looking at it?

he won't tell us
and I find it so infuriating when OP's do that ... they think they have discovered some new thing in physics
that all the greats in the field haven't found before them

they expect help without giving us all the info and then we play the 20 questions guessing game
It just wastes everyone's time

To @Bo Cash please stop wasting everyone's time

you were told waaaaaay back at the beginning of the thread, by @berkeman, how to produce an alternating signal
and for the large part you ignored it and kept asking random questionsDave

 

[jim hardy]

I think he has another agenda which he is not prepared to talk about.

he won't tell us

honestly it sounds to me like one of those perpetual motion "free energy from magnets" schemes

i wouldn't admit to it either.

Suggest close the thread unless he comes clean pronto.

 

[Bo Cash]

Page 121 of this book starts the chapter on magnetics. It's a petty decent introduction .
https://books.google.com/books?id=qEUOAAAAYAAJ&pg=PA1&source=gbs_toc_r&cad=3#v=onepage&q&f=false

View attachment 213028
i have the 1901 edition and find his explanations wordy but detailed and helpful. Keep in mind that back then this was cutting edge..
Baluncore is quite well versed in this subject

For a home experiment that's easy to construct from readily available stuff,
i think i'd suggest

soft black iron wire from the lumberyard for a core. The black oxide gives a little insulation and the small diameter approximates laminations.
Make a bundle and wrap it with tape , bend it into a U shape
Wrap a few hundred turns of copper wire around the bundle.
Pillage an old car alternator for a source
these old Mopars are dirt cheap and simple, no internal regulator and both brushes wired to the back.
View attachment 213035Hmmm a circle might work better than a "U" shape..
View attachment 213037
Good Luck

Thank you. Your explanation is clear. I wll see if I can implement it.

 

[Bo Cash]

But what is the OP planning to do with these alternating fields? He has not mentioned anything about this and seems to assume that the effect will be somehow obvious by just looking at it? He has stimulated a load of positive replies from PF but has given away very little.
I think he has another agenda which he is not prepared to talk about. Either that or he just wants to tinker about with some electromagnetic equipment. As far as I'm concerned this thread is not going anywhere and nor is the OP, if he is not prepared to discuss his purpose.

I thought I had explained. Sorry. I just wish to experiment as described in which my grandchildren could participate.

 

[Bo Cash]

he won't tell us
and I find it so infuriating when OP's do that ... they think they have discovered some new thing in physics
that all the greats in the field haven't found before them

they expect help without giving us all the info and then we play the 20 questions guessing game
It just wastes everyone's time

To @Bo Cash please stop wasting everyone's time

you were told waaaaaay back at the beginning of the thread, by @berkeman, how to produce an alternating signal
and for the large part you ignored it and kept asking random questionsDave

It is simply my lack of knowledge which makes me ask about the subject. I understand and appreciate all the responses and hope I am not wasting time. That was never my intention. A considerable proportion of what has been suggested is being explored. Whilst it may seem straightforward to many, I have to confess I have difficulty. All input is scrutinised and considered seriously.

 

[Bo Cash]

Thank you to all who contributed. Sorry if you feel I have not supplied sufficient information. I can only say I cannot think of a way to make, "I just want to be able to rapidly change the polarity of ANY two electro magnets simultaneously. (Interaction with permanent magnets experiment.)" any clearer.
Please don't bother to respond further. I do not wish to upset anyone. Much of the advice will be followed to the best of my ability and I can assure you, it wasa not my intention to cause confusion.
Again, all help greatly appreciated.

 

[jim hardy]

Well, please let us know what you discover.

Many of us have grandkids too...
in case you use that sketch ---
You could wrap your copper turns around the whole length of your core there's no need to make them like the donut i showed.
And
Core in shape of big circle with a gap would concentrate flux in the gap.
With no idea what you're up to i don't know what geometry you need.
That 1892 book shows a lot of the experimental apparati the old timers used trying to figure things out. You'll find they experimented with bundled wire cores...
...

that's just one basic approach to achieve what i think you asked.
No esoteric parts required. Do your grandkids watch "Junkyard Wars "?

 

[Bo Cash]

Well, please let us know what you discover.

Many of us have grandkids too...
in case you use that sketch ---
You could wrap your copper turns around the whole length of your core there's no need to make them like the donut i showed.
And
Core in shape of big circle with a gap would concentrate flux in the gap.
With no idea what you're up to i don't know what geometry you need.
That 1892 book shows a lot of the experimental apparati the old timers used trying to figure things out. You'll find they experimented with bundled wire cores...
...

that's just one basic approach to achieve what i think you asked.
No esoteric parts required. Do your grandkids watch "Junkyard Wars "?

Thanks. I will outcome post if and when I am fit enough and get the necessary equipment.
Never heard of programme in UK.

 

[CWatters]

I still think two loudspeakers would do the job at least initially.

 

[sophiecentaur]

I thought I had explained. Sorry. I just wish to experiment as described in which my grandchildren could participate.

If you want a demonstration of some Physics and you don't know much about the subject then you should copy some of the many demos that a Google search will give you. You don't want the grandkids to be disappointed so make sure your demo can actually work and deliver an appropriate message.
Here's one. http://practicalphysics.org/simple-electromagnet.html

 

[jim hardy]

I still think two loudspeakers would do the job at least initially.

One could also remove the voice coils and place them around a core that he's built to his own taste..
Voice coil by industry standard has DC resistance ~80% of nominal speaker impedance so watch how much voltage you put across it. A few watts will heat it , so feel of it often and adjust excitation accordingly.

 

[NTL2009]

Thank you to all who contributed. Sorry if you feel I have not supplied sufficient information. I can only say I cannot think of a way to make, "I just want to be able to rapidly change the polarity of ANY two electro magnets simultaneously. (Interaction with permanent magnets experiment.)" any clearer. ...

No, it is not clear, and part of the reason is there is no context.

Provide some context, what exactly are you trying to show your grandchildren? It is not clear what you are trying to accomplish by 'rapidly' changing polarity "once per second to hundreds of times per second". Why those numbers? What are you trying to show?

At one point you said "If outcome is as I anticipate," - OK, what do you anticipate? If you communicated this, people could likely tell you how to accomplish this, or maybe something better/easier to educate/entertain your grandchildren.

You may not need much in the way of equipment - there are online tone generators on the web, here's one, search turns up many more:

http://onlinetonegenerator.com/

Some electromagnets (or loudspeakers, as has been mentioned several times), connected to an audio amplifier might be all you need to demonstrate something, but we don't know what that "something" is.

Despite your feeling that you are being clear, it should be clear from all the questions that you are not.

 

[sophiecentaur]

Thanks. I will outcome post if and when I am fit enough and get the necessary equipment.

What necessary equipment?
I think you do not realize that a random set of scientific equipment, used in a random way will produce very little and probably nothing. It's monkeys and typewriters.

If you are trying to entertain and inform your grandchildren then you owe it to them to do it right and plan a proper demonstration. You really don't want to teach them that Science Never Works, do you?

 

[Tom.G]

At the risk of being ostracised, here goes.

if anyone could advise where I could obtain a device (including magnets) which will change the polarity of two electro magnets from once per second to hundreds of times per second?

Come On Guys! There was a straight forward request for information from the start, and I think we let him down. He admitted to no knowledge in electronics or magnetics and with a ballpark size estimate added later. However some responders wanted to answer a different question, some wanted to use a computer and some didn't, some wanted more context, some asked how the results were going to be measured, some suggested a 'free energy' connection, some wanted to know the anticipated results, some accused the OP of being unclear in the requirements; and I'm tired of typing.

The OP is obviously trying to learn by doing an experiment. Good for him! That's the Scientific Method.

I would say the electro magnets would be rectangular and about two inches long.
Ideally, I see the device to have a dial which could be rotated to increase the number of times the polarity would be switched.

I just need to get ANY two electromagnets polarity simultaneously, rapidly controlled.

basically I just need to be able to control rapid switching of polarity of two electro magnets.

virtually ANY size and power will be sufficient

I don't have problem with electro magnets per se. Just how to rapidly change polarity.

Tom

 

[sophiecentaur]

The OP is obviously trying to learn by doing an experiment. Good for him! That's the Scientific Method.

That may be a way of teaching when the "doing" is well directed by a teacher. It is not 'scientific' to blunder about randomly because, as I have already said, the results are nearly always zero. The scientific method is to build up on an established body of knowledge. The Brownian approach to a journey is not the best.
The advice in this thread has been (typically for PF) a bit confusing for a beginner and that's because the OP asks about a very vague 'effect' of an alternating field from two electromagnets.
10/10 for wanting to give the grandchildren an interesting time but a good lesson depends entirely on good lesson preparation.

 

[Bo Cash]

It is not my intention to upset people or mislead them. This was never my intention. If people are frustrated by the lack of information I can supply, I can only apologise.
As Tom.G says, as far as I believed, it was a straightforward request. Nothing more.
If, I repeat, IF someone knows how I may set up ANY two magnets (I do know how to make or purchase such) and rapidly, simultaneously change their polarity, that is all I require. I don't understand the need to be supply size, current or even which colour might be best. I just require a system which will work for ANY two electro magnets. I reckon I would start with magnets around two inches long. That is the only information I can think of to offer.
My stage of comprehension at this stage is that I need to have a signal generator connected to an audio amp connected to the magnets. I am exploring this.
With signal generators on eBay at $3 and others at £3,000, I am trying to discover what I need. With no knowledge of how to check compatibility or actual connections, I trust the experts understand my dilemmas.
I would like to make it clear I appreciate the time and effort from all contributors.

 

[NTL2009]

It is not my intention to upset people or mislead them. This was never my intention. If people are frustrated by the lack of information I can supply, I can only apologise. ...

No need to apologize, I don't think people are upset, only frustrated, because we are trying to help you and your grandchildren. But you admit you have little knowledge in this area, yet, you question why we ask for the info we ask for. There is a reason - we want to help you and we need more info to do that. We know enough to know that and you (respectfully) do not.

So help us help you.

As Tom.G says, as far as I believed, it was a straightforward request. Nothing more.
If, I repeat, IF someone knows how I may set up ANY two magnets (I do know how to make or purchase such) and rapidly, simultaneously change their polarity, that is all I require. I don't understand the need to be supply size, current or even which colour might be best. I just require a system which will work for ANY two electro magnets. I reckon I would start with magnets around two inches long. That is the only information I can think of to offer.
My stage of comprehension at this stage is that I need to have a signal generator connected to an audio amp connected to the magnets. I am exploring this.
With signal generators on eBay at $3 and others at £3,000, I am trying to discover what I need. With no knowledge of how to check compatibility or actual connections, I trust the experts understand my dilemmas.
I would like to make it clear I appreciate the time and effort from all contributors.

No one system will work for ANY two magnets. Saying "two inches long" tells us very little, because you have not really told us why you want to switch polarity, what effect you hope to demonstrate, or why the several to hundreds of cycles per second is relevant to anything.

Why so secretive - just spit it out! What is it you are actually trying to demonstrate. Details, please! If you can't do that, I'm out.

To many of us, your question is a lot like "How long is a string?", and expecting a reasonable answer.

 

[sophiecentaur]

It is not my intention to upset people or mislead them.

I think most of us realize that but you need to help us by saying what you actually expect out of this. Would I be right in thinking that you want to hold the two magnets in your hand and feel the force between them?
Are you aware that, altering the polarity of each at the same time will not change the direction of the force between?
N-N poles, whilst S-S poles repel and N-S and S-N will attract so going from SS to NN will have the same effect.
One electromagnet only is needed if you want a force that changes direction.
As far as equipment is concerned, the sort of signal generator that would be idea for you would be a School Laboratory Oscillator, which will produce sine waves or square of very low frequency (Fractions of a Hz) up to several tens of kHz. You need a few Volts peak to peak output (say 5V) and the ability to feed a few Amps (more than that would make your coils too hot). Does that rough specification help you in selecting from eBay?
[Edit: I have seen transformer C Cores on google. These are marvellous for producing a horseshoe magnet (winding perhaps 50 turns on the centre of the C) . Two such magnets will stick together with just an amp or two through them and you cannot pull the apart. I wouldn't be surprised if you could lift a grandchild with the force.

 

[Windadct]

It seems a basic "elementary" curriculum in magnetism and motors - there are kits and pre-designed experiments available to cover this, and then there is a bit of guidance, vs miscellaneous playing around - which can still be done ( and needs to be) - but a foundation of specific learning steps is important.

Keep in mind a basic DC motor - has a polarity reversing electromagnet, that is how it works.

I think what threw a lot of people is the Frequency requirement - that seemed pretty specific, a loudspeaker does fit the bill.

 

[jim hardy]

The car alternator i suggested requires no electronics knowledge
and would let you excite the electromagnets simultaneously
or synchronously with 1/3 cycle of phase difference.
But it lacks the precision and easy adjustability of a function generator.

This outfit is in my hometown. They make permanent magnets. Do you need those too? What size?
https://azind.com/
Check their permanent magnet informational leaflets here. ...
https://3bjfj65j2skfse42ir91lknd-wp.../11/Permanent-Magnet-Guidlines-MMPAPMG-88.pdf
https://3bjfj65j2skfse42ir91lknd-wp...11/Permanent-Magnet-Materials-MMPA0100-00.pdf

That's a good site for speakerbuilding supplies surely there's somebody similar in UK.

This cheap little amplifier behind a function generator should drive a small electromagnet over the audio range.
https://www.parts-express.com/lepai...-stereo-amplifier-with-power-supply--310-3000
search on the brand - lots of places sell it and I'm not advertising for any of them.
I don't know how low it will go in frequency - its internal IC is capable of DC so probably changing an internal capacitor or two would get you down to subaudible frequencies. You'd want a stout power supply for that.
555 timer and function generator electronic hobbyist kits abound

Trying to be practical here and consider OP's probable experience level. - they'll learn as they go.

If the electromagnets don't work out grandkids can use that amp with their Iphones.
Teach them about Bass Reflex speakers and Thiele parameters - fine quality sound will bring them much joy in teenage years..
That's a good site for speakerbuilding supplies and surely there's somebody similar in UK.

old jim