Interlude The Hardest Part of this Course Faradays

Interlude: The Hardest Part of this Course Faraday’s Law We will do this twice, today and in about a month (discovered Aug 29, 1831)

Today’s Objectives Introduce key concepts from electricity and magnetism through discovery activities, experiments, concept questions, discussion, and visualizations. Later in the course, we will return to the same concepts. Today we are just going to have some fun.

What we are trying to get a feel for: You Tube Link: http: //youtu. be/Yywa. Jk. GKOa. Y

Concept Question: Loop in Uniform Field While a rectangular wire loop is pulled upward though a uniform magnetic field B field penetrating its bottom half, as shown, there is 1. a current in the loop. 2. no current in the loop. 3. I do not understand the concepts of current and magnetic field. 4. I understand the concepts of current and magnetic field but am not sure of the answer. 4

Demo: aluminum sleeve moving past fixed magnet, students do this at their tables Demo: we show the demo of magnet falling through plastic tube and aluminum tube 5

Seeing the Unseen: Faraday’s Law Applet -- Faraday’s law applet (with a magnet and a coil): http: //web. mit. edu/viz/EM/visualizations/faradays. Law/faradayapp. htm Play with the application until you are familiar with all the features. In the Actions Menu: try both Manual and Generator Mode. You can use the buttons at the bottom to start, pause and reset the simulation. You can move the magnet and the ring back and forth using the mouse. Let each person in the group have a turn.

Seeing the Unseen: First Concept Flow Group Discussion Question What are some examples of flow of “something” through an area?

Examples of Flow Electric Current: Flow Of Charge Electric Current I: Charge ΔQ flowing across area A in time Δt 8

Current and Magnetic Field Current produces a magnetic field as shown in figure 9

Magnetic Field of Bar Magnet (1) A magnet has two poles, North (N) and South (S) (2) Magnetic field lines leave from N, end at S 10

Seeing the Unseen: Magnetic Field Run the Applet on generator mode and stop the magnet when it is near the ring Scroll down on the panel on the right and click on Magnetic Field: Iron Filings

Seeing the Magnetic Field: Iron Filings The iron filings represent the magnetic field present at the instant you stopped the magnet. The direction of the magnetic field is along the direction of the iron filings. Does the magnetic field intercept the area of the circular wire?

Magnetic Flux Thru Wire Loop Flux is the Generalization of Flow Product of magnetic field and area 13

Discussion Question: Magnetic Flux in Ring The first graph on the right in the Applet shows a plots of the external magnetic flux and total magnetic flux in the ring versus time. Briefly describe where the “external flux” (red plot) is coming from: that is, what kind of flux is this, what creates it, over what area is the flux being measured.

More Discussion Questions About Magnetic Flux 1. Describe different ways that you can change the external flux 2. Explain how the total magnetic flux (blue plot) is related to the external magnetic flux (red plot).

Current in Ring The second graph on the right in the Applet shows a plot of the current in the ring versus time.

Proposing a Hypothesis Propose a qualitative relationship between magnetic flux (seen in top graph) and current that flows in the ring (seen in bottom graph).

Testing Hypotheses Groups utilizing the application came up with the following hypotheses. 1. Group A conjectured that the current through the ring is proportional to the total magnetic flux. 1. Group B proposed that the current through the ring is proportional to the change in the total magnetic flux. Use the application to test these two hypotheses. Design and run a virtual experiment that could rule out any of the hypotheses. Which did you rule out and why?

Faraday’s Law of Induction Changing magnetic flux induces a current 19

Electromotive Force Electromotive force looks like a voltage difference. It’s a “driving force” for induced current 20

Faraday’s Law of Induction Changing magnetic flux is proportional to electromotive force 21

Demo: Electromagnetic Induction 22

Demo: Electromagnetic Induction 23

Demonstration: Induction At this point, students again move the coil of wire in their experiment just to observe the current 24

Concept Question: Loop in Uniform Field While a rectangular wire loop is pulled upward though a uniform magnetic field B field penetrating its bottom half, as shown, there is 1. a current in the loop. 2. no current in the loop. 3. I do not understand the concepts of current and magnetic field. 4. I understand the concepts of current and magnetic field but am not sure of the answer. 25

Concept Question Answer: Loop in Uniform Field Answer: 1. The motion changes the magnetic flux through the loop. The magnetic flux is decreasing in time as more of the loop enters a region of zero magnetic field. According to Faraday’s Law there is an induced current through the loop. 26

Concept Question: Loop in Uniform Field While a rectangular wire loop is pulled sideways though a uniform magnetic field B field penetrating its bottom half, as shown, there is 1. a current in the loop. 2. no current in the loop. 3. I do not understand the concepts of current and magnetic field. 4. I understand the concepts of current and magnetic field but am not sure of the answer. 27

Concept Question Answer: Loop in Uniform Field Answer: 2. The motion does not change the magnetic flux through the loop. The magnetic flux is constant in time. According to Faraday’s Law there is no induced current through the loop. 28

Concept Test: Induced Current We define positive current clockwise as viewed from the top. As the coil moves from well below the magnet to well above that magnet, the induced current through the coil will look like: (1) (2) (3) (4) Try to answer this question using your experimental set-up (5) I don’t know

Concept Question: Induced Current Answer Solution (3).

Lenz’s Law Direction of Induced Current 31

Discussion Question: Induced Current Run the Applet and observe the relation between the sign of current and the slope of the plot of magnetic flux. What do you observe?

Minus Sign? Lenz’s Law Induced EMF is in direction that opposes the change in flux that caused it 33

Conclusion: Faraday’s Law of Induction Changing magnetic flux generates electromotive force that opposes that change in flux 34

Jumping Ring An aluminum ring jumps into the air when the solenoid beneath it is energized 35

What is Going On? This is a dramatic example of Lenz’s Law: When the magnetic field created when the solenoid is energized tries to permeate the conducting aluminum ring, currents are induced in the ring to try to keep this from happening! 36