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23-1 Induced Electromotive Force Faraday’s experiment: closing the switch in the primary circuit induces a current in the secondary circuit, but only while the current in the primary circuit is changing. 23-1 Induced Electromotive Force • The current in the secondary circuit is zero as long as the current in the primary circuit, and therefore the magnetic field in the iron bar, is not changing. • Current flows in the secondary circuit while the current in the primary is changing. It flows in opposite directions depending on whether the magnetic field is increasing or decreasing. • The magnitude of the induced current is proportional to the rate at which the magnetic field is changing. 23-1 Induced Electromotive Force Note the motion of the magnet in each image: 23-2 Magnetic Flux Magnetic flux is used in the calculation of the induced emf. 23-3 Faraday’s Law of Induction Faraday’s law: An emf is induced only when the magnetic flux through a loop changes with time. 23-3 Faraday’s Law of Induction There are many devices that operate on the basis of Faraday’s law. An electric guitar pickup: 23-3 Faraday’s Law of Induction Tape recorder: 23-4 Lenz’s Law Lenz’s Law An induced current always flows in a direction that opposes the change that caused it. Therefore, if the magnetic field is increasing, the magnetic field created by the induced current will be in the opposite direction; if decreasing, it will be in the same direction. 23-4 Lenz’s Law This conducting rod completes the circuit. As it falls, the magnetic flux decreases, and a current is induced. 23-4 Lenz’s Law The force due to the induced current is upward, slowing the fall. 23-4 Lenz’s Law Currents can also flow in bulk conductors. These induced currents, called eddy currents, can be powerful brakes. 23-5 Mechanical Work and Electrical Energy This diagram shows the variables we need to calculate the induced emf. 23-5 Mechanical Work and Electrical Energy Change in flux: Induced emf: