A metallic bar is placed in front of the solenoid, on the axis of the solenoid but not inside it. When a DC current is passed through the solenoid, the bar will be strongly attracted, weakly attracted or weakly repelled if it is ferromagnetic, paramagnetic or diamagnetic respectively. What happens AC current flows through the solenoid? We have to consider induced currents in the bar as well, because of changing flux.

My question is: suppose the bar is made of iron and it is an AC solenoid. Will the bar be attracted or repelled or will it oscillate? I am confused about the fact that induced current will always oppose the change in flux and thus the bar will be repelled. On the other hand, the magnetization created within the bar should attract it to the solenoid. Further, in the second case, does the hysteresis loop play a role?

Thanks in advance for your detailed replies.

 

My question is: suppose the bar is made of iron and it is an AC solenoid. Will the bar be attracted or repelled or will it oscillate?
Thanks in advance for your detailed replies.

How do you suppose the typical AC solenoid operates?
Always in the attract mode, one typical way they prematurely fail is when someone pushes the armature out of the coil while powered to test the action!
Also one reason I always use DC solenoids where possible.
Max.

 

A solenoid (AC or DC) will create a magnetic field in space that has a known permeability.
The iron core has a relative permeability of about 200, i.e. its permeability is about 200 times greater than free space.
The magnetic field wants to concentrate in the iron core. This creates an attractive force.

 

So if the coil is switched on when AC voltage is zero we follow the broken line up to point ' a ' this is peak voltage in the coil ... the voltage and field reduce to zero at 'b' ... at this point the voltage polarity switches , and the magnetic field in the coil flips ... but the iron core doesn't flip it's field , so it will be repelled between points 'b' and 'c' !!! ...

At point 'c' the induced magnetism in the iron flips and attraction starts ...

So there 's push/pull going on at double the frequency of the supply , but there's more pull than push .

So DC will pull iron stronger than the equivalent AC supply

 

So if the coil is switched on when AC voltage is zero we follow the broken line up to point ' a ' this is peak voltage in the coil ... the voltage and field reduce to zero at 'b' ... at this point the voltage polarity switches , and the magnetic field in the coil flips ... but the iron core doesn't flip it's field , so it will be repelled between points 'b' and 'c' !!! ...

At point 'c' the induced magnetism in the iron flips and attraction starts ...

So there 's push/pull going on at double the frequency of the supply , but there's more pull than push .

So DC will pull iron stronger than the equivalent AC supply

Thanks! I got that.

 

How do you suppose the typical AC solenoid operates?
Always in the attract mode, one typical way they prematurely fail is when someone pushes the armature out of the coil while powered to test the action!
Also one reason I always use DC solenoids where possible.
Max.

Thanks, Max. But how about the induced (eddy) currents? They will cause repulsion, by Lenz's Law. Would like your views on this.

 

Without a core in the solenoid there is very little magnetic field in front of it. Like Max said without a core in and AC solenoid it will soon burn out. A solenoid without a core piece only has a high magnetic factor in it's center.

 

Thanks, Max. But how about the induced (eddy) currents? They will cause repulsion, by Lenz's Law. Would like your views on this.

The magnetic force will be stronger than the force created by eddy currents.