Will transformers work on alternating current or direct current?

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Transformers rely on electromagnetic induction, which requires a changing magnetic field. That changing field comes from alternating current because it continually reverses direction, keeping the magnetic flux in the core varying at the operating frequency. This changing flux induces a voltage in the secondary winding, allowing energy to transfer and the voltage to be stepped up or down.

With direct current, the magnetic field would quickly rise to a steady value and then stop changing. Once the flux stops changing, no additional voltage is induced in the secondary, so no continuous power transfer occurs. The core can also saturate and overheat if DC is applied. So transformers work with alternating current, not direct current.