Equation 22-7 tells us that a changing magnetic flux through a loop causes an electric field that circulates around that loop. This is true even if there is no material substance at the location of the loop. The minus sign in Equation 22-7 tells us the direction of the circulating electric field: If a conducting wire is present around the loop, the electric field will cause charges to flow, and the magnetic field due to those moving charges will oppose the change in magnetic field that caused the change in magnetic flux. (This is Lenz’s law, which we introduced in Section 20-3.)