This lecture covers the derivation of Maxwell's equations, focusing on the need for an equation to describe time-varying fields and the relationship between magnetic field B and current distribution. It then delves into Ampère's circuital law, illustrating its equivalence to Bio-Savart's law and its application in systems with high symmetry. The lecture emphasizes the importance of identifying symmetry, choosing appropriate integration paths, and determining enclosed current. Practical examples and calculations are provided to demonstrate the application of Ampère's law.