What is the need for laminating the core of a transformer?

The correct answer is "To reduce the eddy currents".

When an alternating current (AC) flows through the primary winding of a transformer, it creates a continuously varying magnetic flux in the iron core.

According to Faraday's Law of Electromagnetic Induction, this changing magnetic flux induces an electromotive force (emf) not only in the secondary winding but also within the conductive iron core itself. These induced voltages cause circulating loops of electric current to flow in the core material. These circulating currents are known as eddy currents.

Eddy currents flow through the bulk resistance of the core, producing heat due to Joule heating ($P_e=I^{2}R$). This results in significant energy loss (known as eddy current loss) and causes the transformer to heat up, reducing its overall efficiency.

To minimize these losses, the transformer core is not made of a single solid block of iron. Instead, it is constructed by stacking thin sheets of steel, called laminations, which are coated with a very thin insulating layer of varnish or oxide.

Laminating the core in this manner offers two main benefits:
1. It breaks the large continuous path available for currents to circulate, thereby restricting the eddy currents to much smaller loops within each individual thin sheet.
2. By drastically reducing the cross-sectional area of each path, it increases the effective electrical resistance to the flow of these currents.

Since the resistance of the path is high, the magnitude of the induced eddy currents is greatly reduced, thereby significantly minimizing the energy lost as heat.