Some Weird Stuff November 5, 2010 Induction
calendar Today – QUIZ Discussion about Lenz and Induction Experiments on Inductors Monday – Finish the Unit Additional Discussion Exam Date will be Announced Labs will be collected again shortly … make sure you are ready. calendar Induction
Important Definition – Magnetic Flux Magnetic Field AREA Important Definition – Magnetic Flux Induction
Induction
The Essence of this Topic Consider a conductor that is shaped in a loop but is continuous. The conductor has a magnetic field through the loop that is not necessarily uniform. There is a MAGNETIC FLUX through this loop. If the FLUX CHANGES, an “emf” will be induced around the loop. This emf can cause a current to flow around the loop. Induction
How Can You Change the Magnetic Flux Going Through The Loop? Huh? Divide the area of the loop into a very large number of small areas DA. Find the Magnetic Field through each area as well as the angle that it makes with the normal to the area. Compute the total flux through the loop. Induction
The Magnetic Flux Going Through The Loop: Add up all of these pieces that are INSIDE the loop. Induction
Changing F Change any or all of the And the Flux will change! Bi DAi fi Change the SHAPE of the loop Change the ANGLE that the loop makes with the magnetic field (subset of above) And the Flux will change! Induction
WAIT A SECOND ……. You said that there is a conducting loop. You said that there is therefore a VOLTAGE or emf around the loop if the flux through the loop changes. But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop? ‘tis a puzzlement! Induction
Induction
REMEMBER when I said E Fields start and end on CHARGES??? DID I LIE?? Induction
BUT WAIT, YOU STILL LIE! The truth Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously. Electric Fields created by changing magnetic fields can actually be shaped in loops. BUT WAIT, YOU STILL LIE! Induction
Why do you STILL think I am a liar? Because you said that an emf is a voltage so if I put a voltmeter from one point on the loop around to the same point, I will get ZERO volts, won’t I Induction
Yes … but this doesn’t make me a liar! Let Me Explain. Induction
The POTENTIAL between two points Is the WORK that an external agent has to do to move a unit charge from one point to another. But we also have (neglecting the sign): Ds Induction
So, consider the following: x x x x x x x E So, consider the following: Conductor Induction
THEREFORE WHAT WILL A VOLTMETER READ FROM A to A? x x x x x x x E A The emf B Zero C Can’t tell A Conductor THEREFORE WHAT WILL A VOLTMETER READ FROM A to A? Induction
How Big is the emf? Faraday’s Law Michael Faraday (1791-1867) MINUS???? Induction
Q: Which way does E point? A: The way that you don’t want it to point! (Lenz’s Law). Lenz’s Law Explains the (-) sign! Q: Which way does E point? Induction
Faraday’s Experiments ? Faraday’s Experiments Induction
Insert Magnet into Coil Induction
Remove Coil from Field Region Induction
Push a magnet into a coil of two wires and a current is produced via an emf. In this case, 2 coils, each have the SAME emf so we add.. Ohm’s Law still works, so Induction
The Strange World of Dr. Lentz Induction
Murphy’s Law for Magnets Lenz’s Law Induced Magnetic Fields always FIGHT to stop what you are trying to do!... Murphy’s Law for Magnets Induction
Example of Nasty Lenz The induced magnetic field opposes the field that does the inducing! Example of Nasty Lenz Induction
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Let’s do the Lentz Warp again ! Induction
An inductor resists CHANGES in the current going through it. 2/27/2019 Induction
An inductor resists CHANGES in the current going through it. 2/27/2019 Induction
An inductor resists CHANGES in the current going through it. 2/27/2019 Induction
Faraday says this is the emf! If the FLUX changes a bit during a short time Dt, then the current will change by a small amount Di. Faraday says this is the emf! This is actually a calculus equation Inductance Defined 2/27/2019 Induction
So … E= The UNIT of “Inductance – L” of a coil is the henry. SYMBOL: There should be a (-) sign but we use Lenz’s Law instead! The UNIT of “Inductance – L” of a coil is the henry. SYMBOL: So … 2/27/2019 Induction
Move on to the experiment in UNIT 14 Induction