Final answer:
The most energy-efficient transformer cores are made from amorphous metal or laminated steel, as these materials minimize hysteresis and eddy current losses, respectively.
Step-by-step explanation:
The most energy-efficient transformer cores are constructed with amorphous metal or laminated steel. Both materials are well-suited for use in transformers because of their magnetic properties. Amorphous metal has a disordered atomic structure that leads to low hysteresis loss, which is energy lost during the cyclic magnetization process. Laminated steel, often silicon steel, minimizes eddy currents. Eddy currents are closed loops of electric current induced within conductors by a changing magnetic field, which lead to energy losses. By laminating the steel into thin insulated sheets, these undesired currents are disrupted, thereby reducing the energy loss.
Transformer cores constructed from these materials follow Faraday's law of induction, where a time-varying magnetic field induces an electromotive force (emf) in a circuit. For instance, Figure 20.31 shows a laminated-coil transformer that utilizes this principle, with the iron core trapping and enhancing the magnetic field, analogously to how a dielectric increases electric field strength in a capacitor.