Moving charges can generate a magnetic field Can a magnetic field make charges move current Chapter 20 Induced Voltages and Inductance 20 1 Induced emf voltage and magnetic flux A moving magnet can induce electric currents which can be used to control things Cruise controls and anti lock breaks all involve magnets in motion Monetary Flux Watch how much is coming down and how you position your net Magnetic flux Count number of field lines through loop B A Magnetic flux B A B al rm no Magnetic flux B A cos A is angle between normal and B For HW The angle between the normal and the field Examples B 1 T A 0 5 m2 20 2 Faraday s Law Emf Change in magnetic Flux Time N i N f t f ti t Forget about the sign and use N Since B A cos 3 things can change 1 Area of loop 2 Magnetic field B 3 Angle between A and B Change of Area t Change of B at the position of the loop For HW a loop or field changes from situation A to situation B in some length of time Find voltage In this case v 1 m s Example More examples of changing flux ABcos 20 3 Motional EMF B F qvB v Velocity B Moving charge still feels force downwards Voltage v L E L F qE q L If q L q v B vBL it reaches an equilibrium final stable situation 10 Example What are the emf s for the three moving rods with length L each Quick way to remember only the velocity component that is cutting the field lines contribute to motional emf Example Mechanical energy from the hand goes into the light bulb If v 1 m s R 2 L 1 m and B 1T what is the power burned by the light bulb 20 5 Generators The last part of a power plant fuel could be gas coal nuclear power water bike The fuel or input in general is used to spin the coil and the rest is taken care of by Faraday s Law Voltage across generator 1 N BAsin N BAsin t Vmaxsin t v 2x v r Angular Velocity N number of turns A area B magnetic field Vmax N BA Similar design If the input is voltage it generates motion electric energy to mechanical or thermal energy If the input is motion it generates a voltage mechanical energy to electrical energy A motor is also a generator The voltage generated the spike always opposes the applied voltage back emf A generator is also a motor always resisting the applied torque v 0 v 0 Can you generate electricity without moving the magnet Yes if you can change the strength of the magnet 20 6 Self Inductance AC Generator Changing current Changing B field Changing proportional to I N LI Induced EMF voltage Recall Faraday s law Direction N Opposes change in current A source of power surge Inductance N t t I t Unit of L Henry L A For a solenoid of N turns length l and cross sectional area A L 0 N l 2V V Al volume 20 7 RL Circuit Solenoid resists flux change either higher or lower flux is proportional to current Solenoid resists current change After switch is closed to B
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