10.2.2—Kirchhoff’s second law and understand that it is a consequence of

Around any closed circuit loop, the algebraic sum of e.m.f.s and potential changes is zero: ΣV=0.

Choose a loop direction and keep rises and drops signed consistently. Include internal resistance where the loop contains a real source.

A 12 V source feeding 4 V and 8 V drops satisfies 12−4−8=0.

A loop equation is not a claim that every component has the same voltage; it balances signed energy transfers per charge.