8.1 Induced Emf p. 300 Magnetism, EMF, and Electric Current An Englishman, Michael Faraday ( ) and an American, Joseph Henry ( ), working independently discovered that magnetism could produce or induce a current in a circuit. Inducing an EMF in a Straight Piece of Wire A current in circuit can be induced if a length of wire, l, is made to move through a perpendicular magnetic field. If the wire (assume it is part of circuit that is not shown) is moved downwards, then according to the right hand rule, free charges, Q, will be forced to move along the wire due the magnetic force acting on them.

8.1 Induced Emf p. 301 Inducing an EMF in a Straight Piece of Wire F B = BQvUsing the right hand motor rule the direction of the force on the charges in the wire shown in the diagram would be out of the page. The amount of work done when a charge, Q, is forced to move the length l, of the segment of wire: W = Fd = BQvl Since there is work done on moving the charges, the charges gain potential energy: V = EpEp Q = EMF = BQvl Q And induced EMF: Ɛ = Bvl (Units: Volt)

8.1 Induced Emf p. 301 The Generator (part 1) Michael Faraday invented the generator. A motor uses a magnetic field, an electric current, and coils of wire to produce motion (kinetic energy). A generator uses magnetic fields and coils of wire, and motion (kinetic energy) to produce (induce) a current in a circuit. This diagram shows the main components of a hand- operated generator. The force acting on the charges in the wire would be a maximum when the coil is horizontal and a minimum when the coil is vertical to magnetic field. The voltage produced by a generator will not be constant. The range will be from zero to a maximum value called the peak voltage, Ɛ max.

8.1 Induced Emf p. 303 The Generator (part 2) These series of diagram shows 2 major concepts: 1)The voltage or EMFis not constant and varies from max +EMF to zero to –EMF. 2)The current direction swicthes direction every ½ turn of the coil generating an alternating EMF. A graph of the EMF versus time as the coil rotates would look like this: Note the locations of max. +EMF, 0 EMF and max. – EMF on the graph.

8.1 Induced Emf p. 303 The Generator (part 3) Ɛ = Ɛ max = Bvl When the coil is perpendicular to the lines of magnetic force the EMF will be a maximum value: At any other position of the coil: Ɛ = Bv l T v T Where is the speed of the coil at any other position. Since there are two parts of the coil in the magnetic field, and the wires are not always perpendicular to the magnetic field and EMF varies from +1 at sin 90 o to -1 at sin 270 o, the equation becomes: Ɛ max = +2NBvlsin Ɵ Ɛ max = Ɛ max sin Ɵ

8.1 Induced Emf Key Questions In this section, you should understand how to solve the following key questions. Page 302 – Practice Problems #2 Page 305 – Quick Check #1, & 3 Page 309 – 310 – Review 7.1 #2,4,6, & 8