Chapter 25 Electromagnetic Induction 19-Apr-17 Physics 1 (Garcia) SJSU

WARNING Strong magnets will be passed around the room. Keep these magnets away from iPods and laptops (especially my laptop)! 19-Apr-17 Physics 1 (Garcia) SJSU

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. Can show this with presentation galvanometer, single wire, and large magnet. Very weak signal. 19-Apr-17 Physics 1 (Garcia) SJSU

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. Connect large coil to demo galvanometer. Move large magnet near the coil; move coil near large magnet. Pass-around box with coil, bar magnet, and galvanometer. 19-Apr-17 Physics 1 (Garcia) SJSU Note: Do this in lab too.

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). Hard disk 19-Apr-17 Physics 1 (Garcia) SJSU

Electric Generator Electric generator moves a conductor in a magnetic field to produce voltage via electromagnetic induction Bring small generator, similar to the one pictured (one in stock room, another in 2B lab) 19-Apr-17 Physics 1 (Garcia) SJSU

Demo: Electric Generator Turn the shaft by hand and as the coils pass the magnets a voltage is induced. DC current is generated. DC Output SHAFT Magnet Set this up in a pass-around box using St. Louis motor (many in Phys 1 lab) and galvanometer. Magnet Coils Note: Do this in lab too. 19-Apr-17 Physics 1 (Garcia) SJSU

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. 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. 19-Apr-17 Physics 1 (Garcia) SJSU

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. Large Voltage Small Voltage Medium Voltage 19-Apr-17 Physics 1 (Garcia) SJSU

Demo: Electromagnetic Oscillations 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 Oscillating Magnetic Field Electro- Magnet Just like it’s shown. Point out that the ring becomes hot. Connect to alternating current (AC)

Demo: Electric Motor Can create an electrical motor by passing a current through a set of electro-magnets mounted on a rotating shaft. Use big St. Louis motor and lawnmower battery Current out Current in Electro-magnets

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. Current N S N S N S Current Use big St. Louis motor and lawnmower battery

Demo: Generator Becomes Motor Pass a current into the generator and it becomes an electric motor. DC Input Battery Magnet St. Louis motor in a pass-around box (with battery and switch). Magnet Rotor Spins 19-Apr-17 Physics 1 (Garcia) SJSU Note: Do this in lab too.

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. Primary Magnetic Field Current Self-Induced Magnetic Field 19-Apr-17 Physics 1 (Garcia) SJSU

Demo: Lenz’s Law Oscillating 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. Metal ring is levitated by self-induced secondary magnetic field Electro- Magnet Just like it’s shown. Point out that the ring becomes hot. Connect to alternating current (AC) 19-Apr-17 Physics 1 (Garcia) SJSU

Check 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? Electro- Magnet 19-Apr-17 Physics 1 (Garcia) SJSU

Check Yourself 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? Oscillating Magnetic Field Electro- Magnet Just like it’s shown. Point out that the ring becomes hot. Connect to alternating current (AC) 19-Apr-17 Physics 1 (Garcia) SJSU

Demo: Magnetic Brakes 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. 19-Apr-17 Physics 1 (Garcia) SJSU

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. Primary Magnetic Field Eddy Currents Use large, strong magnet or rare earth magnets passed over a piece of copper. Can drop rare earth down aluminum pipe. Self-Induced Magnetic Field 19-Apr-17 Physics 1 (Garcia) SJSU

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. 19-Apr-17 Physics 1 (Garcia) SJSU

Demo: Back Electromotive Force Changing magnetic field in an electromagnet induces a voltage in the electromagnet itself. 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. Connect lawnmower battery to large electromagnet coil using a switch. Large spark when switch opened. 19-Apr-17 Physics 1 (Garcia) SJSU