Electromagnetic induction causes a voltage to be induced in a conductor that is moving through a magnetic field or when a magnetic field is moving relative to a stationary conductor. This principle allows for technologies like electric generators, electric motors, magnetic recording/data storage, and transformers. The induced voltage is proportional to the rate of change of the magnetic flux through the conductor based on Faraday's Law of Induction.

1. Electromagnetic Induction Jun 7, 2009

2. Electromagnetic Induction Voltage is induced whether the magnetic field of a magnet moves near a stationary conductor or the conductor moves in a stationary magnetic field. Jun 7, 2009 Physics 1 (Garcia) SJSU

3. Demo : Magnet Induces Current Voltage is induced when a magnet moves towards or away from a coil, inducing a current in the coil. Faster the magnet’s motion, the greater the induced current. Jun 7, 2009 Physics 1 (Garcia) SJSU Note: Do this in lab too.

4. Magnetic Recording Write data by magnetizing recording media (e.g., video tape, hard disk) using electromagnets. Data is read back using the induced current produced when magnetized media moves past receiver coils (reverse of writing data). Jun 7, 2009 Physics 1 (Garcia) SJSU Hard disk

5. Electric Generator Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction Jun 7, 2009 Physics 1 (Garcia) SJSU

6. Demo : Electric Generator Turn the shaft by hand and as the coils pass the magnets a voltage is induced. DC current is generated. Jun 7, 2009 Physics 1 (Garcia) SJSU Magnet Magnet Coils DC Output SHAFT Note: Do this in lab too.

7. Induction: No Free Lunch Takes work to turn the generator crank to produce electric current. The faster we turn the crank to produce more current, the more difficult it is to turn. Jun 7, 2009 Physics 1 (Garcia) SJSU More difficult to push the magnet into a coil with more loops because the magnetic field of each current loop resists the motion of the magnet.

8. Faraday’s Law The induced voltage in a coil is proportional to the product of the number of loops and rate at which the magnetic field changes within the loops. Jun 7, 2009 Physics 1 (Garcia) SJSU Small Voltage Medium Voltage Large Voltage

9. Demo : Electromagnetic Oscillations Connect to alternating current (AC) Oscillating Magnetic Field Electro- Magnet Put alternating current into an electromagnet and you create an oscillating magnetic field. This oscillating magnetic field induces electrical currents by inducing electrical field oscillations. This is, effectively, a very low frequency electromagnetic antenna. Coil with bulb

10. Demo : Electric Motor Can create an electrical motor by passing a current through a set of electro-magnets mounted on a rotating shaft. Current in Current out Electro-magnets

11. Electric Motor, Analyzed Electromagnet mounted on a shaft with opposing magnets on each side. Current direction always such that electromagnet is repelled, causing shaft to turn. N S N S Current Current N S N S

12. Demo : Generator Becomes Motor Pass a current into the generator and it becomes an electric motor. Jun 7, 2009 Physics 1 (Garcia) SJSU Rotor Spins Magnet Magnet DC Input Battery Note: Do this in lab too.

13. Self-Induction When a current is induced by a changing magnetic field, that current itself produces its own magnetic field. This effect is called self-induction . Jun 7, 2009 Physics 1 (Garcia) SJSU Current Primary Magnetic Field Self-Induced Magnetic Field

14. Demo : Lenz’s Law Jun 7, 2009 Physics 1 (Garcia) SJSU Connect to alternating current (AC) Oscillating Magnetic Field Electro- Magnet Metal ring is levitated by self-induced secondary magnetic field Induced current produces a secondary magnetic field that is always opposed to the primary magnetic field that induced it, an effect called Lenz’s law .

15. C heck Yourself Does the ring levitate if we connect the electromagnet to direct current (e.g., to a battery)? Does the ring levitate if it has a gap? Jun 7, 2009 Physics 1 (Garcia) SJSU Electro- Magnet

16. C heck Yourself Jun 7, 2009 Physics 1 (Garcia) SJSU Connect to alternating current (AC) Oscillating Magnetic Field Electro- Magnet Hold the ring down on the shaft and it starts to get hot. Why? What if we cool the ring in liquid nitrogen and repeat the demo?

17. Demo : Magnetic Brakes Jun 7, 2009 Physics 1 (Garcia) SJSU Strong magnet dropped into a copper pipe falls slowly due to secondary magnetic field induced by its motion. Great America’s Drop Zone has a 22 story freefall, lasting four seconds, decelerated by magnetic braking.

18. Eddy Currents Changing magnetic field induces eddy currents within any conductor. These internal currents produce self-induced magnetic fields, which by Lenz’s law are in opposition of the primary magnetic field. Jun 7, 2009 Physics 1 (Garcia) SJSU Eddy Currents Primary Magnetic Field Self-Induced Magnetic Field

19. Metal Detectors Can detect the presence of metals by using a transmitter coil to create an oscillating primary magnetic field. This creates a secondary magnetic field due to eddy currents in the metal. Can detect this secondary magnetic field by using a receiver coil. Jun 7, 2009 Physics 1 (Garcia) SJSU

20. Demo : Back Electromotive Force Changing magnetic field in an electromagnet induces a voltage in the electromagnet itself. Jun 7, 2009 Physics 1 (Garcia) SJSU When we disconnect a large electromagnet the magnetic field rapidly collapses. This changing magnetic field self-induces a large voltage in the coil of the electromagnet. Large spark occurs at the switch.