# How to test toroidal permanent magnet with inside field

• I
As the permanent magnet is magnetized by donut shape coil, thus field B = 0 outside.
How can I know if seller screwed me or not?

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berkeman
Mentor
As the permanent magnet is magnetized by donut shape coil, thus field B = 0 outside.
How can I know if seller screwed me or not?
You test it the same way you test Write-Only Memory.

There no N & S polar for the donut magnet.
If I cut into 2 semi circles, then there are N & S polars. But I will keep it integral.

berkeman
Mentor
There no N & S polar for the donut magnet.
If I cut into 2 semi circles, then there are N & S polars. But I will keep it integral.
Is it a pretty paperweight?

Is it a pretty paperweight?

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Through the center, if I vertically insert a straight wire carrying 100A current, and make sure the induced magnetic field is opposite to the inside of the said donut magnet, will the donut spin (assuming friction zero) ? I think there should be torque in theory if the 2 fields are fighting in counter directions.

You test it the same way you test Write-Only Memory.
You are kidding me. No such thing of write-only memory.

berkeman
Mentor
You are kidding me. No such thing of write-only memory.
Yes, but the object you are asking about seems to be of about as much use as write-only memory...

Yes, but the object you are asking about seems to be of about as much use as write-only memory...
I back-order it by customization in the purpose of experiment of the simplest motor - no windings, only a straight conducting axle as described in previous text. Is there torque anyway?
I know the ball bearing electric motor is named the simplest motor: Video - A strange electric motor If my experiment success, this may be another simplest motor, and more useful than the ball bearing motor.

Homework Helper
Gold Member
I think all you can succeed in doing with a very strong current passing through the hole in the donut is to reverse the direction of magnetization. If this does occur, a loop of wire that wraps the toroidal part would show an EMF when this occurs. In practice, achieving this reversal of the magnetization of the permanently magnetized toroid is likely to be extremely difficult. Wrapping the toroid in a solenoidal manner and sending a large current through the coils would be more effective at reversing the direction of magnetization, but even that method would require many loops and very high currents. ## \\ ## As for making a motor out of it, I think you are likely to get zero motion of this thing. There simply isn't any torque of any significance.

I think all you can succeed...As for making a motor out of it, I think you are likely to get zero motion of this thing. There simply isn't any torque of any significance.
No torque even 2 fighting fields in opposite directions? But, moving a bar magnet with N polar toward the N of another bar magnet, the other bar will be pushed backward.
The manufacturer must use donut coil to magnetize it, but I can not use donut coil in counter direction as a motor, because even there is torque, because of closed wire blockage, no way to output the torque and drive a load. It seems: only by straight wire through the center, the magnet spin can drive a load.
Anyway, I am waiting for its shipping and delivery, then I can try.

One day, the room temperature superconductor age will come, even 2000A current just small deal, then, this simplest motor can be put in applications.

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Homework Helper
Gold Member
In the manufacturing process, (I'm no expert in this part, but I think I have it correct), the material is typically magnetized starting in the molten state above the Curie temperature. It only requires a weak applied magnetic field to get the alignment of the magnetization in one direction. Once that magnetization is established , requiring cooling below the Curie temperature, , it can be very difficult to reverse it, unless you heat the magnet above its Curie temperature. ## \\ ## Meanwhile, toroidal magnets would not have the same type of repulsive or attractive forces properties that you find from cylindrical "pole" magnets. The force in the case of two pole magnets is caused by the gradient in the magnetic field from one, interacting with the magnetization in the other. In the case of a pole magnet in a completely uniform magnetic field, it experiences zero force. In the case of the toroidal magnets, not only is the external magnetic field nearly uniform, it is also nearly zero.

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berkeman
Mentor
One day, the room temperature superconductor age will come, even 2000A current just small deal, then, this simplest motor can be put in applications.
That is speculation, which is not allowed at the PF. This thread is done.