試題詳解

88. A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another. A varying current in any one coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force across any other coils wound around the same core. Transformers are commonly used in our daily life, and provide the voltage we need, for example, from220 volt to 110 volt. Such emerging work is achieved by varying                 .
(A)the size of transformer's magnetic core
(B)the number of turns in a winding
(C)the magnetic flux in the core
(D)the material of the core