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A.S. 12.1.1 – 12.1.6 due Friday, 12/19/14 Things to think about… What happens to electrons as they move through a magnetic field? What would happen if there were a LOT of electrons…like in a wire… What happens… Electrons experience a force when moving through a magnetic field If the electrons are in a wire, they all experience a force and will move in the direction of the force Result: A net shift in charge so that one side of the wire is more negative than the other End result: There is an electric field created in the wire due to the separation of charges: ∆𝑉 𝑉 𝐸= = ∆𝑥 𝐿 Induced (motional) emf The electrons (charge = e) will continue to move to one end of the wire UNTIL… The force of electrostatic repulsion balances the force from the magnetic field: 𝑒𝐸 = 𝑒𝑣𝐵 𝐸 = 𝑣𝐵 𝑉 = 𝑣𝐵 𝐿 𝑽 = 𝑳𝒗𝑩 Electromagnetic Induction Known: An electric current produces a magnetic field Known: A wire moving through (perpendicular to) a magnetic field will develop a potential difference across its ends (Induced emf) More difficult? Producing a current… Observations What happens to the current when: North end moved into the loop? North end moved out of the loop? South end moved into the loop? South end moved out of the loop? North end held above the plane of the loop? Magnet held inside the loop? Magnet moved in/out of the loop at a different speed than before? Variables that affect current: As these increase, so will the current: The relative speed of the magnet with respect to the loop/coil Strength of the magnetic field Number of turns in the coil Area of the loop ALSO: Angle of the magnetic field relative to the plane of the loop. At an angle 90° (field perpendicular to the plane of the loop), the current will be the maximum possible for the conditions Michael Faraday British Physicist and Chemist—born 1791, died 1867 Self-taught…discovered many concepts, had difficulties with some of the math Discovered Electromagnetic Induction in 1831 (also devised the laws relating to electrolysis and the deposition of ions onto metals through the use of electricity) Found the connecting link between each of the observations we just made… The relevant law that bears his name (Faraday’s Law) relates to electromagnetism Magnetic Flux The strength of the magnetic field crossing the plane of a loop of area, A Φ = 𝐵 ∙ 𝐴𝑐𝑜𝑠𝜃 B = magnetic field strength A = area of the loop q = angle between the magnetic field direction and the normal to the plane of the loop Magnetic Flux Units of flux = Weber (Wb) Conceptual visualization: flux is the number of magnetic field lines that are passing through the plane of the loop Increase the flux by: Increasing the area of the loop Increasing the strength of the field (more field lines…) Making the loop and field lines more perpendicular Magnetic Flux Linkage Same as magnetic flux…but specifically when there is more than 1 loop in a coil. Φ = 𝑁 ∙ 𝐵 ∙ 𝐴𝑐𝑜𝑠𝜃 N = number of loops in the coil Sample Problem A loop of area 2.00 cm2 is in a constant magnetic field of 0.100 T. What is the magnetic flux through the loop in each of the following situations: When the loop is perpendicular to the field When the loop is parallel to the field When the normal to the loop and the field have an angle of 60.0° between them? Faraday’s Law The induced emf is equal to the (negative value for) rate of change of magnetic flux: ∆Φ 𝜀 = −𝑁 ∆𝑡 The induced emf, therefore, can cause a current in a conducting wire…but this will ONLY happen when there is a changing magnetic flux! Sample Problem The magnetic field through a single loop of area 0.250 m2 is changing at a rate of 4.25 T·s-1. What is the induced emf in the loop?