# Speed of Polarity Switching

1. Jul 23, 2014

### Slakjak1215

I'm not sure if this is in the right section but...Is there a limit to how fast the polarity can be switched on an electromagnet? If there is a limit then how fast can they be switched? Thanks.

2. Jul 23, 2014

### Simon Bridge

You can see that there has to be - if only because there is a limit to how fast you can physically throw a switch - i.e. signal generators do not generate an arbitrarily high frequency.

But I suspect you are more interested in the electromagnet end.

There is a limit there too - and it will depend on how it is made.

Basically an electromagnet does not come fully "on" the very instant it is switched on, and it does not go fully "off" when you cut the power. Toggle the power very fast and you'll get odd effects. What exactly happens depends on how the electromagnet is made.

To give you an idea:
http://ocw.mit.edu/resources/res-6-...elds-and-energy-spring-2008/chapter-10/10.pdf

3. Jul 23, 2014

### Staff: Mentor

Yes. In practice, the more windings you put on a given core, the longer it will take to reverse the current in the winding. More windings lead to it having more inductance.

If you went the superconductivity route, things might be different.

4. Jul 23, 2014

### phinds

5. Jul 23, 2014

### Slakjak1215

Okay well, ill just throw this out there, What about creating a positive negative pulse to levitate a very small object. This is what I am shooting for. The pulses would have to be incredibly fast to not jolt it very far.

6. Jul 23, 2014

### Staff: Mentor

Why do you want to both attract then repel the object? Wouldn't it suffice to repel it for a time, then switch off the field, assuming the magnet is positioned below the object?

7. Jul 23, 2014

### Simon Bridge

... or just use a small field.
... or shape the magnetic field, or accept a high levitation :)
You could experiment by putting a square wave on an electromagnet and vary the period.
You may not need to go to critically small timings.