Introduction to Active Learning: Faraday’s LawToday’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 and get to know each other.IntroductionsWhat we are trying to get a feel for: You Tube Link: http://youtu.be/YywaJkGKOaY5 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 5 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.6 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 tubeSeeing the Unseen: Faraday’s Law Applet Applet -- Faraday’s law applet (with a magnet and a coil): http://public.mitx.mit.edu/gwt-teal/FaradaysLaw2.html Play with the application until you are familiar with all the features. In the Actions Menu: try checking Motion on and unchecking Motion on(manual 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?9 Examples of Flow Electric Current: Flow Of Charge Electric Current I: Charge ΔQ flowing across area A in time Δt I =ΔQΔt10 Current and Magnetic Field Current produces a magnetic field as shown in figure11 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 SSeeing the Unseen: Magnetic Field Run the Applet on motion on and stop the magnet when it is near the ring Scroll down on the panel on the right and click on Magnetic Field: Iron FilingsSeeing 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?14 Magnetic Flux Thru Wire Loop ΦB= B⊥A Flux is the Generalization of Flow Product of perpendicular component of magnetic field and areaDiscussion 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. 2. 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?20 Faraday’s Law of Induction I ∝dΦBdt=d(BA)dt=dBdtAChanging magnetic flux induces a current21 Electromotive Force, Current and Resistance Vary the resistance in the applet and observe the current. Electromotive force looks like a voltage difference. It’s a “driving force” for induced current ε= IRε22 Faraday’s Law of Induction ε∝dΦBdt=d(BA)dt=dBdtAChanging magnetic flux is proportional to electromotive force23 Demo: Electromagnetic Induction24 Demo: Electromagnetic Induction25 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 25 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.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 26 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 Lenz’s Law Direction of Induced CurrentDiscussion 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? Try flipping the coil and see what result you get for the current.Direction of Induced Current The slope of the total magnetic flux (blue) is proportional to the negative of the induced current30 Minus Sign? Lenz’s Law Induced EMF is in direction that opposes the change in flux that caused it ε= −dΦBdt31 Current Loop Acts Like a Magnet Force between magnet and current loop is like force between magnetsFalling Conducting Wire http://web.mit.edu/viz/EM/visualizations/faraday/fallingCoil/FallingRingResistive/FallingRingResistive.htm33 Concept Question: Faraday’s Law: Loop A coil is falling downward directly beneath a magnet with its north pole pointing upward. The current in the coil and the force on the coil are: 1. Current clockwise; force up 2. Current counterclockwise; force up 3. Current clockwise; force down 4. Current counterclockwise; force down34 Demonstration: Induction At this point, students again move the aluminum tube in their desktop apparatus to feel the forcce35
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