The induced emf in a loop changes so that the induced current opposes the change in magnetic flux. This describes which principle?

• A. Faraday's law
• B. Gauss's law
• C. Lenz's law
• D. Ampere's law

Answer: (C)

Direction of the induced current to oppose changes in magnetic flux is what this item tests. When the magnetic flux through a loop changes, an emf is produced, but the important point is that the induced current flows in a direction that creates a magnetic field opposite to the change. This is Lenz's law. If the external flux through the loop is increasing in a certain direction, the induced current circulates so that the loop’s magnetic field points opposite to that increase; if the flux is decreasing, the induced current adjusts to try to maintain it by producing a field in the same direction as the original. Faraday's law gives the magnitude of the emf, while Lenz's law gives its direction. The other laws describe different aspects (Gauss's law for electric flux and charge, Ampere's law for magnetic fields due to currents) and don't specify this opposition behavior.