Workshop: Using Visualization in Teaching Introductory E&M AAPT National Summer Meeting, Edmonton, Alberta, Canada. Organizers: John Belcher, Peter Dourmashkin, Carolann Koleci, Sahana Murthy

Faraday’s Law Presentation Materials

MIT Class: Faraday’s Law Week 9, Day 2 MIT Class: Faraday’s Law Class 21

Fourth (Final) Maxwell’s Equation Underpinning of Much Technology Faraday’s Law Fourth (Final) Maxwell’s Equation Underpinning of Much Technology

Demonstration: Falling Magnet

Magnet Falling Through a Ring Falling magnet slows as it approaches a copper ring which has been immersed in liquid nitrogen.

Demonstration: Jumping Rings

Jumping Ring An aluminum ring jumps into the air when the solenoid beneath it is energized

What is Going On? It looks as though the conducting loops have current in them (they behave like magnetic dipoles) even though they aren’t hooked up

Faraday’s Law Applets Discovery

Faraday’s Law Applets Discovery Activity

Demonstration: Induction

Electromagnetic Induction

Faraday’s Law of Induction A changing magnetic flux induces an EMF

Looks like potential. It’s a “driving force” for current What is EMF? Looks like potential. It’s a “driving force” for current

Faraday’s Law of Induction A changing magnetic flux induces an EMF, a curling E field

Magnetic Flux Thru Wire Loop Analogous to Electric Flux (Gauss’ Law) (1) Uniform B (2) Non-Uniform B

Minus Sign? Lenz’s Law Induced EMF is in direction that opposes the change in flux that caused it

Faraday’s Law of Induction Week 9, Day 2 Faraday’s Law of Induction Changing magnetic flux induces an EMF Lenz: Induction opposes change Class 21

Ways to Induce EMF Quantities which can vary with time: Magnitude of B Area A enclosed by the loop Angle q between B and loop normal

Group Discussion: Magnet Falling Through a Ring Falling magnet slows as it approaches a copper ring which has been immersed in liquid nitrogen.

Magnet Falling Through a Ring Week 9, Day 2 Magnet Falling Through a Ring Falling magnet slows as it approaches a copper ring which has been immersed in liquid nitrogen. Class 21

Example: Magnitude of B Magnet Falling Through a Ring Week 9, Day 2 Example: Magnitude of B Magnet Falling Through a Ring Falling magnet approaches a copper ring or Copper Ring approaches Magnet Class 21

As ring approaches, what happens to flux? Week 9, Day 2 Moving Towards Dipole Move ring down As ring approaches, what happens to flux? Flux up increases Class 21

PRS Question: Faraday’s Law Week 9, Day 2 PRS Question: Faraday’s Law Class 21

PRS: Faraday’s Law: Loop Week 9, Day 2 PRS: Faraday’s Law: Loop :00 A coil moves up from underneath a magnet with its north pole pointing upward. The current in the coil and the force on the coil: Current clockwise; force up Current counterclockwise; force up Current clockwise; force down Current counterclockwise; force down Class 22

PRS Answer: Faraday’s Law: Loop Week 9, Day 2 PRS Answer: Faraday’s Law: Loop Answer: 3. Current is clockwise; force is down The clockwise current creates a self-field downward, trying to offset the increase of magnetic flux through the coil as it moves upward into stronger fields (Lenz’s Law). The I dl x B force on the coil is a force which is trying to keep the flux through the coil from increasing by slowing it down (Lenz’s Law again). Class 22

PRS Question: Loop in Uniform Field Week 9, Day 2 PRS Question: Loop in Uniform Field Class 21

PRS: Loop in Uniform Field Week 09, Day 1 v Bout A rectangular wire loop is pulled thru a uniform B field penetrating its top half, as shown. The induced current and the force and torque on the loop are: Current CW, Force Left, No Torque Current CW, No Force, Torque Rotates CCW Current CCW, Force Left, No Torque Current CCW, No Force, Torque Rotates CCW No current, force or torque Class 20

PRS Answer: Loop in Uniform Field Week 09, Day 1 PRS Answer: Loop in Uniform Field v Bout Answer: 5. No current, force or torque The motion does not change the magnetic flux, so Faraday’s Law says there is no induced EMF, or current, or force, or torque. Of course, if we were pulling at all up or down there would be a force to oppose that motion. Class 20

Group Problem: Changing Area Conducting rod pulled along two conducting rails in a uniform magnetic field B at constant velocity v Direction of induced current? Direction of resultant force? Magnitude of EMF? Magnitude of current? Power externally supplied to move at constant v?

Changing Angle