1. Electromagnetic Induction
Or Suduction is all about Induction
Standards:
5h Students know changing magnetic fields produce electric fields, thereby inducing currents in nearby conductors.

2. Faraday’s and Henry’s Discovery
A changing magnetic field induces a voltage difference.
Also induces a current if inside a conductor
Induced voltage difference is called electromotive force to distinguish it from battery voltage
EMF is directly related to speed of change in magnetic field and number of loops intercepting changing magnetic field (Faraday’s Law e = D B L v)
Question: What did Michael Faraday and Joseph Henry discover about magnetism and electricity in 1735? When is a current also induced? What quantities does the induced voltage depend upon? How does the so called electromotive force differ from other voltage differences? Write the mathematical equation for Faraday’s law.
Activities:
Demonstrate that changing magnetic field induces voltage using overhead device. Also demonstrate that any relative movement between magnet and loops results in voltage and that the quicker the change the greater the induced voltage. Show how more loops also mean more voltage.
Show video detailing Faraday’s and Henry’s discovery.
Return to overhead demo and show that increasing speed of change and number of coils increases voltage.
State that Faraday wanted to distinguish this voltage from battery voltage so he called it electromotive force or EMF.
Write equation for Faraday’s law on board

3. Lenz’ Law
If a current is induced by a changing magnetic field it will create a second current and a second magnetic field that oppose the original change.
Magnitude of back current and magnetic field directly proportional to speed of change.
Result of conservation of energy
Examples:
Magnet being plunged into coil
Magnet and conducting tube
Burning out a saws motor by stopping blade
Moving magnet and loop problems
Question: State Lenz law. What conservation law is this a result? What happens to the force required to plunge a magnet into a coil as the number of coils is increased? Why does a saw motor quickly burn out when it jams against a knot in a piece of wood?
Activity
Start by asking what energy is causing the charges in the wire of the coil to move, It is the work done by you moving the magnet or loops of wire. Conservation of energy says that you can’t get something for nothing so what ever energy you move the coil or magnet with is what you get out. But what would happen if you plunged the magnet through a whole bunch of coils. Wouldn’t you get a lot of current for a little amount of work and thus break the law of conservation of energy?
2) Demo: Conducting tube and falling magnet vs. conducting tube and non-magnet. Point out that tube is conducting but not magnetic. Draw magnet on board and label weight. Say that there must be an upward force that slowed the magnet’s fall, draw it on the diagram and call it the Lenz force. Point out that this force always resists motion much like friction.
Refer to Faraday’s and Oerstead’s observations to show that a second current and magnetic field will be generated. These will be in directions that oppose any change in the original field and current. Furthermore the strength of these fields or force will be proportional to the rate of change of the original magnetic field.
Write Lenz law on board.
Explain that Lenz force resist magnet plunging through coils of wire limiting the speed it can travel.
Explain Lenz force resists falling magnet falling through conducting tube limiting speed it can fall. Also show that the direction of the current can be derived in this case.
Explain burning out of motor when suddenly stopped and back current ceases to exist resulting in much larger forward current than wires are designed to handle.
Show slide and then take students through several versions of the magnet and coil problem pictured on the slide.
Demo: magnetic breaking with pendulum

4. The Electric Generator
Device which changes kinetic or mechanical energy into electrical energy
Works by some outside source moving wires inside of magnetic field
Has same parts as motor
Induced voltage is greatest when wires are moving perpendicular to magnetic field
Generates AC or pulsed DC
Fuels vs. Energy transformation
Question: What is an electric generator? How does it work? Compare and contrast an electric generator and motor. When is the induced voltage of a generator greatest? What type of current is generated by a generator?
Activities:
Show model or picture of AC generator and state that it does the opposite of a motor. Namely it takes mechanical kinetic energy and converts it to electric energy.
Show picture of generator and identify permanent magnet, loops, armature and commutators.
Demonstrate that motor and generator are opposites using genecon, power supply and auto lamp
Show picture and discuss how voltage varies as loop rotates. Make an analogy with a swimmer crossing a swift moving river and the loops crossing the magnetic field river. More work is done when moving perpendicular to current and field and this is when the maximum voltage occurs.
State that generator does not create energy but merely transforms energy from one type to another. You need an energy source or fuel like fossil fuels, falling water, or Uranium to provide energy to move the wires in the magnetic field.

5. Other Uses of EM Induction
Magnetic identification strips on ATM cards
Smart Lights
Retail Metal detectors
Airport metal detectors
Theramin
Speakers and microphones
Rail guns and mag lev trains
Questions: List some common uses besides generators for EM induction.
Activities:
Show and demo magnetic strip by passing it though overhead device.
Show Smart light video and then add idea about flashing brights
Discuss retail metal detectors and magnetic inventory tags.
Discuss airport metal detectors and what they can and cannot detect. Thus the need for X ray and CT scans
Demonstrate speaker made from plastic cup, magnet and small coil. State that microphone has same components but start with sound and convert it to current rather than visa versa.
Demonstrate Elihu Thompson device as levitator and as rail gun. Talk about how could use these two versions of EM induction to levitate and move a train. This is exactly what mag lev trains in Europe and Japan do. Trains currently reach 400 mph but theoretically could reach mph (break sound barrier). Could go from LA to Vegas in 45 minutes or so.