[singh awtaar (OP)]

what is the ON - OFF system in the scrap lifting machine,      (based on permanent magnet.) how it work.

[Madidus_Scientia]

It's not a permanent magnet, it's an electromagnet. When a current is passed through wire, it creates a magnetic field. The more current, the stronger the magnetic field. So electromagnets are usually made by wrapping a coil of wire tightly around a peice of iron, or other ferromagnetic material. (Ferromagnetic means something which is attracted by a magnetic field, but is not itself a magnet. Steel or iron, for example.)

The ON and OFF system is achieved simply by turning the power to the electromagnet on or off.

[lyner]

Have you heard of the permanent magnet version which is more 'fail safe'? You use a permanent magnet, which will not 'let go' on its own. You release the load of iron by a burst of electric current in a coil, around the magnet, which nullifies the permanent field for a brief time - then the load falls off.

[Chemistry4me]

Nope, I have never heard of it. Btw...how do they change the poles around? 

[lyner]

With the  current in the coil. It gives a brief canceling field when switched on.

[Chemistry4me]

I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?

[lyner]

That's unlikely because you would need to change the polarity of a modern permanent magnet - that would require hundreds of times more current and, probably, some heating would be involved.

The system I described uses a supplementary coil, merely to interrupt the field produced from the permanent magnet. (Anyway, what would changing the polarity achieve?)

[Chemistry4me]

I was merely wondering []
So can you or can't you (with an electromagnet)? []

[lyner]

Can u or can't you what?
To make a powerfull enough permanent magnet, you couldn't support a suitable electromagnet on a simple crane.

[teragram]

I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?

When the current supplied to the coil of an electromgnet is direct (D.C.), the poles can be reversed by reversing the direction of current flow in the windings. If the current is alternating (A.C.)the poles are continously reversing, ie. at 50 times per second if (U.K.) mains frequency is applied. 

[Madidus_Scientia]

No I hadn't heard of that sophiecentaur, good idea though. But you wouldn't be able to have as powerful a magnet would you? or it would be a hell of a lot heavier than an electromagnet equivalent?

[lyner]

Not with modern magnets. You're not after the sort of high flux density of an LHC magnet. You're just after  two concentric poles - like a loudspeaker(total flux).
I don't  think you could make a micromotor with the power / torque and size of a modern permanent magnet motor using a field winding.

[Chemistry4me]

I thought that you could switch the poles around on an electromagnet, i.e. the North becomes South and vice versa, how do they do that?

When the current supplied to the coil of an electromgnet is direct (D.C.), the poles can be reversed by reversing the direction of current flow in the windings. If the current is alternating (A.C.)the poles are continously reversing, ie. at 50 times per second if (U.K.) mains frequency is applied. 

Okay, thanks teragram []

[lyner]

You can make a workable electromagnet using AC but it makes a fearsome buzzing noise wken supporting a steel load; it still supports things despite the polarity changing.   It is not 'on' all the time and so it isn't as 'strong'.

[singh awtaar (OP)]

i do not know these answer giving friends are scientists or some commen men like me. but it is true that their knowledge about permanent magnet is very short. ( except one friend, massage ID no.231997) they know  electromagnet only. here are many instruments using permanent magnet like scrap lifter, work holding chucks. needle block for lath machine and so many......avtaar singh

[lyner]

"Scrap lifter"?
How would it let go?

[Chemistry4me]

I'm Spartacus a common man!
Yes, you are correct, my knowledge of electromagnets can be counted on one finger []

[singh awtaar (OP)]

thanks for giving answer for my previous question.
my another question is : when the glaciers are melting day by day very speedily due to global warming.
then river should be over flooded. but in India ( how far i know ) the rivers are dry. the ground level is going down day by day. the question is that where the water is going.

[Chemistry4me]

The water is on my front lawn!

[Chemistry4me]

Perhaps you should start another thread in the Environment section? []