What if now both circuits are stationary, but the current la in the circuit a changes? The magnetic field B produced by Ia also changes The changing B(t )also induces the electromotance v in b do OB B. da dt S at Now this v in the circuit b can be written as a path integral of some electric field e =4bE·dl=s(V×E)da rom these two expressions we arrive at OB V×E at