Electromagnetic Induction The Discoveries of Michael Faraday and Joseph Henry Showed That a Current Can Be Induced by a Changing Magnetic Field.

Faraday’s Law AnAn emf is set up in a closed electric circuit located in a magnetic field whenever the total magnetic flux linking the circuit is changing. ThisThis is called electromagnetic induction. Induced emf Induced current

A Galvanometer and a B-Field Induced current is related to the motion of the conductor in the magnetic fieldInduced current is related to the motion of the conductor in the magnetic field The direction of the induced current is directly related to the direction of motion of the conductor in the magnetic fieldThe direction of the induced current is directly related to the direction of motion of the conductor in the magnetic field Direction is dependent on the motion of the B- field or the conductorDirection is dependent on the motion of the B- field or the conductor The motion of the conductor is perpendicular to the B-fieldThe motion of the conductor is perpendicular to the B-field

More Thoughts… IfIf the conductor is moved parallel to B-field, the galvanometer will show no deflection, thus no induced emf or current. the conductor is looped several times, the deflection on the galvanometer will increase. Thus,Thus, the rate of motion, the number of loops and the magnitude of the B-field vary directly with the magnitude of the induced current and emf.

Factors Affecting Emf Emf induced in each coil of the conductor is proportional to rate of change of magnetic fluxEmf induced in each coil of the conductor is proportional to rate of change of magnetic flux Emf = -  tEmf = -  t Emf = -N(  t)Emf = -N(  t) Note that negative sign indicates that induced emf is of such polarity as to oppose the change that induced itNote that negative sign indicates that induced emf is of such polarity as to oppose the change that induced it

Induced EMF Induced current exists only if wire in a field is part of a circuit.Induced current exists only if wire in a field is part of a circuit. Thus, if  = BA then  = BxlThus, if  = BA then  = Bxl So therefore, Emf =  Bxl/  tSo therefore, Emf =  Bxl/  t But x/t = v (velocity)But x/t = v (velocity) Therefore, Emf = BlvTherefore, Emf = Blv

Faraday’s Law The induced emf in a coil of N loops with a changing B – field within in equal to the rate of change of magnetic flux times the number of loops.

Lenz’s Law

The net B-field thru a loop from a changing flux comes from two sources.

TheThe original B – field. B – field caused by the original current…the induced B – field.

Induced B – Field This occurs because of the induced current. This can be used to determine the polarity of the induced emf.

Lenz’s Law The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced B – field opposes the original flux change.