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It's about an investigation carried out on the transformer effect . The primary coil of 20 turns and the secondary coil of 50 turns is wound around a laminated iron core.

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For an applied AC voltage, the primary coil has an impedance from self-inductance which limits the current (amperes) flowing through the copper (unless you draw current from the secondary coil).

For an applied DC voltage, the impedance from self-inductance is zero, which causes a large current to flow. This current heats up the wire with a power $P=I^2R$, where $I$ is the current and $R$ the resistance of the wire. The heating is much smaller in the AC case because $I$ is much smaller in that case.

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  • $\begingroup$ @Monia Reza Please keep in mind that copper has a very low resistance whereas the inductance depends on its magnetic properties like core material, number of turns, frequency of the current etc. and is actually very large compared to DC resistance for usual transformers built for usual AC. $\endgroup$
    – hsinghal
    Jun 5, 2016 at 17:42

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