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Wee-Kiat Kong wrote:
> 
> I'm doing as assignment for Physics about induced EMF and I have to
> describe
> how a transformer works.
> 
> My problem is that I don't quite understand what the iron core in
> transformers actually do. Does an alternating current get induced in
> the core
> due to the magnetic field caused by the primary coil's alternating
> current?
> Does it (the iron core) therefore create another stronger magnetic
> flux which
> induces an EMF in the secondary coils?
> 
> Also how is the direction of the current in the iron core and the
> secondary coil determined?
> 
> Please reply by e-mail and the newsgroup.
> 
> Thank you in advance.
> 
> Wee-Kiat

Cue lecture....without diagrams this could be tricky...
A good way to understand what the core of a transformer (or any
electromagnet) does is to remember that lines of magnetic force would
much rather go through a ferrous metal than through air, and that
they're not allowed to cross each other. (Look at some textbook diagrams
and hopefully you'll see what I mean.)

Thus one reason for having a core in a transformer is to make sure that
ALL the lines of force from the field of the primary coil end up passing
through the secondary coil, rather than heading off into the classic
"bar magnet and iron filings" pattern.

Yes, currents *are* induced in the core by the primary's field, but
these are called "eddy currents" and are actually sources of loss.
That's why transformer cores are laminated (made of layers of iron
rather than one big lump), to get in the way of eddy currents.

So the primary coil produces a *changing* magnetic field, and it's the
*changes* in that field which induce the EMF in the secondary coil.

As for the direction of the currents/EMFs, look up the "Right-hand Grip
Rule", "Lenz's Law", also "Fleming's Left Hand Rule" and his "Right hand
Rule" may help you indirectly.
Basically Lenz's law says that any induced current will produce a field
that *opposes* the field which created it - which therfore determines
which way that current must be flowing (right hand grip rule).

Hmmm... it only makes sense to me because I can see the diagrams in my
head, it's not a subject that lends itself to text communication! Have
you tried any CD-ROMs? (Encarta 97 isn't much help!)

Hope this helps

Andy
'A' level physics teacher
Weston-super-Mare

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