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When I was a child, I brought a nail (metal bar) and I made some turns around the nail. It was something like a coil and the nail was the core of that coil. When I applied DC on the coil using 1.5 V battery, the nail had become a magnet. It was attracting other nails and metals.

Now, I've learned that the magnetic field is generated due to change of current in a wire, and that's why we apply AC to transformers and coils.

I have two questions:

Why the nail is magnetized although there is no changes in current (DC) ?

Why we don't apply DC to transformers and coils ?

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Constant magnetic field is generated when there is a constant current. An alternating electromagnetic field is generated then the current is alternating. An alternating magnetic field will induce a current in a nearby conductor such that the magnetic field generated by that current will "resist" to the changes in the overall magnetic field, as the magnetic field "doesn't like" to be changed. This is why transformers won't work for DC.

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