Tuesday, December 6, 2016

Second Obscure Physics Question

You can wrap an electrically charged coil around a piece of iron to make an electromagnet.  Does that work if you put the coil inside an iron cylinder?  Perhaps I should go out to the shop and try.


Akatsukami said...

I'd guess that an external cylinder would act as a magnetic shield.

StormCchaser said...

No, it doesn't. But you might still get some magnetization because there may be a single turn consisting of the power supply, the wire to the coil, the coil, and the wire back to the power supply.

w rorke said...

Magnetic field surrounds a current-carrying conductor. If you put two conductors next to each other the fields will sum and surround both. If you make many turns the fields sum and surround all the conductors - going down the middle of the coil and returning up the outside. Easiest to think of a section view.

When you impose a magnetic material, the magnetic flux flows preferentially in that material instead of thru the air.

If the coil has small inner diameter and is tight to the metal, the flux is concentrated in the metal - high flux per cross-sectional area. This increases the magnetic strength.

If you put the magnetic material around the outside, the flux will flow thru it too. And it will also act like a magnet, but the cross-sectional area is likely to be much larger due to larger diameter, the flux per area lower, and the magnet strength lower.

Clayton Cramer said...

It isn't clear from these responses if coil on the inside will produce an electromagnet. I will try in shop tomorrow. Presumably, electromagnet strength increases with wattage?

StormCchaser said...

The magnetic field is proportional to the current only, not the power.

In this experiment, there are two issues: the magnetic field in the ferrous metal itself, and the magnetic field in free space. The field in the metal will increase until the metal has reached saturation.

In your experiment, vary the number of turns significantly and see what happens. If the turns are causing the magnetism in the cylinder, then it should be proportional to the number of turns. If it is just the single turn described in my comment above, it should not be.

My bet is that it will not change significantly with the number of turns, if the coil is kept inside the cylinder.