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Magnetism

Magnetism. AP Physics B Chapter 21 Notes. What is E/M Induction?. Electromagnetic Induction is the process of using magnetic fields to produce voltage, and in a complete circuit, a current .

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Magnetism

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  1. Magnetism AP Physics B Chapter 21 Notes

  2. What is E/M Induction? Electromagnetic Induction is the process of using magnetic fields to produce voltage, and in a complete circuit, a current. Michael Faraday first discovered this by wondering if a moving current created a magnetic field, could a magnetic field create a current?

  3. Induced EMF He tried unsuccessfully to get emf from a number of set-ups—all involving creating a static B, such as this one. He did note the galvonometer spiked when he opened or close the switch at X!

  4. Induced EMF He concluded that only a changing magnetic field produced an emf (induced emf) Only relative motion is required—either coil or B move

  5. Magnetic Flux What really drives emf is changing magnetic flux—the dot product of B and area Flux is proportional to the number of field lines going through a fixed area

  6. Magnetic Flux Magnetic flux for a uniform magnetic field through a loop of area A is the dot product: Units: Tm2=Wb (Weber) Flux changes as a coil rotate in a uniform B—this will induce emf

  7. Changing Magnetic Flux So how can magnetic flux be changed? • Change magnetic field strength • Change area of loop • Change the angle of the loop of wire to the magnetic field by rotating it If we do this all quickly, we can get induced emf and current.

  8. Faraday’s Law Faraday completed many investigations on all of these and derived one of the basic laws of electromagnetism: What’s with the negative sign? One Loop N Loops

  9. Lenz’s Law Lenz's law gives the direction of the induced emf and current resulting from electromagnetic induction--the induced emf and the change in flux have opposite signs. Lenz’s Law

  10. Lenz’s Law—Practice Problems Determine the direction of induced current in each case.

  11. Faraday’s and Lenz’s Laws—Example A coil of wire consists of 20 turns each of which has an area of 0.0015 m2. A magnetic field is perpendicular to the surface. Initially, the magnitude of the magnetic field is 0.050 T and 0.10 s later, it has increased to 0.060 T. Find the average emf induced in the coil during this time.

  12. Lenz’s Law Summary Lenz’s Law Reasoning Strategy 1. Determine whether the magnetic flux that penetrates the coil is increasing or decreasing. 2. Find what the direction of the induced magnetic field must be so that it can oppose the change in flux by adding or subtracting from the original field. 3. Use RHR-2 to determine the direction of the induced current.

  13. EMF Induced in a Moving Conductor What if a conductor is moved in a magnetic field as shown below? The area of the loop increases by ΔA = LΔx = LvΔt

  14. EMF Induced in a Moving Conductor Substituting into Faraday’s Law gives: Known as Motional EMF

  15. EMF Induced in a Moving Conductor Another way to consider what happens is to look at the force acting on an e in the rod:

  16. Motional EMF--Example Suppose the rod is moving with a speed of 5.0 m/s perpendicular to a 0.80-T magnetic field. The rod has a length of 1.6 m and a negligible electrical resistance. The rails also have a negligible electrical resistance. The light bulb has a resistance of 96 ohms. Find (a) the emfproduced by the rod, (b) the current induced in the circuit, (c ) the electric power delivered to the bulb, and (d) the energy used by the bulb in 60.0 s.

  17. Changing B Flux and Electric Field Changing magnetic flux produces an induced current which implies an E field in the wire since electrons move. It is found by: E= F/q = qvB/q = vB

  18. Applications--Generator An electric generator is the opposite of an electric motor—it converts mechanical energy into electrical energy. AC here. The axle is rotated by an external force such as falling water or steam. The brushes are in constant electrical contact with the slip rings.

  19. Applications--Generator DC generators are the same except they have a split ring commutator

  20. Applications--Microphone Microphones use the same principle to convert sound pressure into electrical signal Sound pressure moves a diaphragm which is connected to a coil of wire in a magnetic field – the moving coil generates an electrical signal with the same frequency response

  21. Applications--Transformers A transformer consists of two coils, linked by an iron core. A changing emf in one induces an emf in the other. Transformers allow emf to be increased or decreased, but require changing current to work—only work for AC

  22. Applications--Transformers The ratio of emf’s is equal to the ratio of number of coils Energy is conserved, so the ratio of currents must be the inverse of the ratio of turns

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