A transformer is used in the electricity distribution system to change the voltage of an alternating current supply. Explain why a transformer cannot work with a direct current supply.

A transformer relies on a changing magnetic field to induce a voltage in the secondary coil. A direct current produces a magnetic field that is constant and does not vary with time. Because the field is steady, no electromotive force is induced in the secondary winding. In contrast, an alternating current continually reverses direction, producing a magnetic field that changes magnitude and polarity, which in turn induces a voltage in the secondary coil. Therefore a transformer can only operate with alternating current.