Chapter23[1] - Chapter 23 Define: Magnetic Flux, = B A cos...

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Chapter 23 Define: Magnetic Flux, Φ = BA cos θ T · m 2 = 1 Weber (Wb) Simply put, flux is a measure of how many B-field vectors pierce an open area times the size of the area available to it, and also how strong those B-field vectors are. Imagine a large thin hoop or ring. Some (very brave) person is holding the hoop up in the air and a group of archers is shooting arrows through the center. When the person holds the hoop so that the opening is directly facing the archers, they will be able to shoot the most arrows through the opening. If, the person holding the hoop starts to rotate the hoop, the archers will be able to shoot fewer and fewer arrows through the opening as it is rotated, until the orientation of the opening is turned completely perpendicular to the direction that the arrows are going. In this case, no arrows would be able to go through the opening. Flux is the product of the magnitude of the B-field times the area of the opening that is available to the archers. We calculate the magnetic flux by the following equation: , where the little symbol between B and A means “perpendicular to”, and A is the area of the loop. 7. A solenoid with 385 turns per meter and a diameter of 17.0 cm has a magnetic flux through its core of magnitude 12 8 10 4 . 2 . T•m (a) Find the current in this solenoid. a) 0 0 42 Tm 72 0 A B Bn I I B A B nA (1.28x10 T m ) I 11.7 A 385 (4 x10 ) ( (0.085m) ) m        
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b) How would your answer to part (a) change if the diameter of the solenoid were doubled? Explain. How does I vary with A? Faraday’s Law of Electromagnetic Induction Do/recall demonstration of induced emf. What did we observe? Deflection was greater with 1) greater speed of magnet and 2) greater number of loops. The rules are these: 1. A changing magnetic field induces an electric field. 2. A changing electric field induces a magnetic field. (We already knew this!) The above rules are known as electromagnetic induction . Remember that both the electric field and the magnetic field are vectors. A vector can change if we change its magnitude, and it can also change if we change its direction. If we consider a single loop of wire, then what we call the “induced voltage” in that loop is given by the following equation: E =  t cos BA N t N  What can change with time? B, A, θ 17. •• The area of a 120-turn coil oriented with its plane perpendicular to a 0.20-T magnetic field is 00 . 50 2 . m Find the average induced emf in this coil if the magnetic field reverses its direction in 0.34 s. E = f0 2 BB B NN A c o s N A c o s tt 0.20T 0.20T (120)(0.050m )(1) 7.1V 0.34s        t Lenz’s Law: Lenz’s Law is the “WHY?” of electromagnetic induction. The easiest way to understand Lenz’s Law is to keep in mind that “Mother Nature” doesn’t like CHANGE.
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This note was uploaded on 10/13/2011 for the course PHY 102 taught by Professor Alexandrakis during the Fall '06 term at FIU.

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Chapter23[1] - Chapter 23 Define: Magnetic Flux, = B A cos...

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