Why don't the transformers work with a dc supply?

A transformer is based on a very simple fact about electricity: when a fluctuating electric current flows through a wire, it generates a magnetic field (an invisible pattern of magnetism) or "magnetic flux" all around it. The strength of the magnetism (which has the rather technical name of magnetic flux density) is directly related to the size of the electric current. So the bigger the current, the stronger the magnetic field.
Now there's another interesting fact about electricity too. When a magnetic field fluctuates around a piece of wire, it generates an electric current in the wire. So if we put a second coil of wire next to the first one, and send a fluctuating electric current into the first coil, we will create an electric current in the second wire. The current in the first coil is usually called the primary current and the current in the second wire is (surprise, surprise) the secondary current. What we've done here is pass an electric current through empty space from one coil of wire to another. This is called electromagnetic induction because the current in the first coil causes (or "induces") a current in the second coil. We can make electrical energy pass more efficiently from one coil to the other by wrapping them around a soft iron bar (sometimes called a core).

As we came to know that the transformer works with changing in flux with out any change in physical parts (i.e, we can say the transformer is a static device). So, to provide this fluctuating current it only possible with ALTERNATING CURRENT (A.C) because for A.C we have frequency. But in D.C we don't have frequency so the steady current constantly flows in the same direction so we can't get fluctuating electric field. That is why TRANSFORMERS DON'T WORK ON DC.


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