1Physics1Ch 21: Electromagnetic Induction If electric currents produce magnetic fields,can magnetic fields produce electric currents? Yes! Discovered independently by Joseph Henry (American) Michael Faraday (English)Physics2Joseph Henry Lived from 1797 to 1878 Premier American scientist after Ben Franklin Albany native Attended & then taught at Albany Academy conducted experiments in his spare time! Invited to Princeton and stayed there now endowed chair of physics department Joseph Henry Laboratories First Secretary of the Smithsonian Institute One of first members of National Academy ofSciencePhysics3Induced EMF Faraday observed deflection when switchedwas opened and closed changing B field induces currentPhysics4Induced EMF2Physics5Faraday’s Law of Induction The induced emf depends on the area of loop how fast B field is changing = change in magnetic flux, ΦΒwhereFaraday’s Lawof InductionPhysics6Faraday’s Law of Induction If circuit contains N closely wrapped loops: Can induce emf by changing B field area of loop loop’s orientation with respect to B fieldPhysics7Ch 21: Problem 5 A 12.0-cm-diameter loop of wire is initiallyoriented perpendicular to a 1.5-T magneticfield. The loop is rotated so that its plane isparallel to the field direction is 0.20 s. What isthe average induced emf in the loop?Physics8Lenz’s Law A current produced by an induced emf movesin a direction so that its magnetic fieldopposes the original direction of change Test yourself:3Physics9More practice What is the direction of the induced current inthe circular loop due to the current in thewire?Physics10Ch 21: Problem 2 The rectangular loop shown is pushed intothe magnetic field which points inward. Inwhat direction is the induced current?Physics11EMF Induced in a Moving Conductor Area inside loop is increasingas rod moves to right A changing B field inducesan electric field (E = F/q)Physics12Electric Generators4Physics13Ch 21: Problem 20 A simple generator is used to generate apeak output voltage of 24.0 V. The squarearmature consists of windings that are 6.0 cmon a side and rotates in a field of 0.420 T at arates of 60.0 rev/s. How many loops of wireshould be wound on the square armature?Physics14Transformers ΔB/Δt in primary inducesVS in secondary coil Works for AC only DC voltages don’t havechanging B fieldPhysics15Transformers Step-Up Transformer Vs > VP Step-Down Transformer Vs < VP Conservation of energy requires PP = Ps IP VP = IS VSPhysics16Ch 21: Problem 31 A transformer has 320 turns in the primarycoil and 120 in the secondary coil. What kindof transformer is this, and by what factor doesit change the voltage? By what factor does itchange the current?5Physics17Inductance where M = mutual inductance Units of inductance = Henry 1 H = 1 Ω sPhysics18Self-Inductance changing current passes through a coil produces changing B flux which induces an emf Induced emf opposes change in fluxinductance (H)Physics19Ch 21: Problem 39 If the current in a 180-mH coil changessteadily from 25.0 A to 10.0 A in 350 ms,what is the magnitude of the induced