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Key Terms

 back emf emf generated by a running motor, because it consists of a coil turning in a magnetic field; it opposes the voltage powering the motor eddy current current loop in a conductor caused by motional emf electric generator device for converting mechanical work into electric energy; it induces an emf by rotating a coil in a magnetic field Faraday’s law induced emf is created in a closed loop due to a change in magnetic flux through the loop induced electric field created based on the changing magnetic flux with time induced emf short-lived voltage generated by a conductor or coil moving in a magnetic field Lenz’s law direction of an induced emf opposes the change in magnetic flux that produced it; this is the negative sign in Faraday’s law magnetic damping drag produced by eddy currents magnetic flux measurement of the amount of magnetic field lines through a given area motionally induced emf voltage produced by the movement of a conducting wire in a magnetic field peak emf maximum emf produced by a generator

Key Equations

 Magnetic flux $$\displaystyle Φ_m=∫_S\vec{B}⋅\hat{n}dA$$ Faraday’s law $$\displaystyle ε=−N\frac{dΦ_m}{dt}$$ Motionally induced emf $$\displaystyle ε=Blv$$ Motional emf around a circuit $$\displaystyle ε=∮\vec{E}⋅d\vec{l}=−\frac{dΦ_m}{dt}$$ Emf produced by an electric generator $$\displaystyle ε=NBAωsin(ωt)$$

Summary

• The magnetic flux through an enclosed area is defined as the amount of field lines cutting through a surface area A defined by the unit area vector.
• The units for magnetic flux are webers, where $$\displaystyle 1Wb=1T⋅m^2$$.
• The induced emf in a closed loop due to a change in magnetic flux through the loop is known as Faraday’s law. If there is no change in magnetic flux, no induced emf is created.

13.2 Lenz's Law

• We can use Lenz’s law to determine the directions of induced magnetic fields, currents, and emfs.
• The direction of an induced emf always opposes the change in magnetic flux that causes the emf, a result known as Lenz’s law.

13.3 Motional Emf

• The relationship between an induced emf εε in a wire moving at a constant speed v through a magnetic field B is given by $$\displaystyle ε=Blv$$.
• An induced emf from Faraday’s law is created from a motional emf that opposes the change in flux.

13.4 Induced Electric Fields

• A changing magnetic flux induces an electric field.
• Both the changing magnetic flux and the induced electric field are related to the induced emf from Faraday’s law.

13.5 Eddy Currents

• Current loops induced in moving conductors are called eddy currents. They can create significant drag, called magnetic damping.
• Manipulation of eddy currents has resulted in applications such as metal detectors, braking in trains or roller coasters, and induction cooktops.

13.6 Electric Generators and Back Emf

• An electric generator rotates a coil in a magnetic field, inducing an emf given as a function of time by $$\displaystyle ε=NBAωsin(ωt)$$ where A is the area of an N-turn coil rotated at a constant angular velocity $$\displaystyle ω$$ in a uniform magnetic field $$\displaystyle \vec{B}$$.
• The peak emf of a generator is $$\displaystyle ε_0=NBAω$$.
• Any rotating coil produces an induced emf. In motors, this is called back emf because it opposes the emf input to the motor.

13.7 Applications of Electromagnetic Induction

• Hard drives utilize magnetic induction to read/write information.
• Other applications of magnetic induction can be found in graphics tablets, electric and hybrid vehicles, and in transcranial magnetic stimulation.

Contributors

Paul Peter Urone (Professor Emeritus at California State University, Sacramento) and Roger Hinrichs (State University of New York, College at Oswego) with Contributing Authors: Kim Dirks (University of Auckland) and Manjula Sharma (University of Sydney). This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).