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chapter20_updated

# chapter20_updated - First exam Saturday 8am(posted on...

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2/20/2009 1 First exam: 02/21/09, Saturday; 8am (posted on UBLEARN) Room: Check for announcement Makeup exam: O’Brian 112, 02/24/09, Tuesday 5pm See rules posted online Students with disabilities: Please contact your office for exam scheduling. Exam format: 12 MC and 4 Problems (12x 3 + 4x16 = 100 points). Sample exam available on UBLEARN (Note materials covered/uncovered may be different.) Covers Ch15-Ch18 Sections without HW problems will not be covered in exam Review: Wed. Feb 18.

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2/20/2009 2 Room assignment: Last names starting with: A - F : NSC 225 G - I : NSC 215 J - K : NSC 220 L - N : Knox 110 O - Q: NSC 210 R - U: Knox 109 V - Z: Knox 104 Good Luck! Induced Voltages and Inductance How does electric generator work?
2/20/2009 3 A carton box A strong bar magnet A small flashlight light bulb A long thin conducting wire (with insulating layer) One of the greatest scientists in history Invented electric motor, generator and transformers Discovered electromagnetic induction Discovered laws of electrolysis 1791 1867

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2/20/2009 4 A current can be produced by a changing magnetic field (more precisely, changing magnetic flux ) First shown in an experiment by Michael Faraday A primary coil is connected to a battery A secondary coil is connected to an ammeter An electrical current is produced by a changing magnetic field The emf is actually induced by a change in the quantity called the magnetic flux rather than simply by a change in the magnetic field How can you tell it is the changing field or flux that matters? What is magnetic flux?
2/20/2009 5 A current loop in a uniform magnetic field The loop has an area A The magnetic flux is defined as where θ is the angle between B and the normal to the plane B ) cos( BA A B B When the field is perpendicular to the plane of the loop (a) θ = 0, Φ B = Φ B, max = BA When the field is parallel to the plane of the loop (b) θ = 90° , Φ B = 0 SI units of flux are T . m² = Wb (Weber) ) cos( BA A B B

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2/20/2009 6 ) cos( BA A B B Magnetic field is also known as magnetic flux density The flux can be visualized with respect to magnetic field lines The value of the magnetic flux is proportional to the total number of lines passing through the loop For same area of lands Near the magnetic equator: the flux passing through the land is small Near the magnetic poles, the flux is approaching a maximum value
2/20/2009 7 Magnetic flux If the angle between the magnetic field and the normal direction of the plane is 60 o , T 0 . 2 m 0 . 1 B r x x x x x x x x x x x x x x x x x x x x x x x x x x x x Wb 28 . 6 ) 1 14 . 3 ( 0 . 2 ) ( ) cos( 2 2 r B BA A B B Wb 14 . 3 5 . 0 ) 1 14 . 3 ( 0 . 2 ) 60 cos( ) ( ) cos( 2 2 o B r B BA A B The induced emf = rate of change of magnetic flux where is the change in magnetic flux during the time interval D t .

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