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Electromagnet

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Wire Field A moving charge generates a magnetic field. Symmetry with experiencing force Perpendicular to direction of motion Circles around path Outside a straight wire the magnetic field is cylindrical. Decreases inversely with distance I B

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Permeability The permeability defines the strength of the B field in a material. The vacuum permeability is 0. Defined as 0 = 4 x 10 -7 T m / ADefined as 0 = 4 x 10 -7 T m / A The permeability is constant for most materials. Usually close to 0Usually close to 0 Not constant for ferromagnetic materials like iron > 0Not constant for ferromagnetic materials like iron > 0

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Mutual Attraction Two parallel wires with current will exert a force on each other. Wire length l Separation d Currents in the same direction attract. Currents in the opposite direction repel. I1I1 d I2I2 l

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Wire Force Find the force per unit length experienced by each of two long parallel wires in avacuum separated by 1.0 m carrying currents of 1.0 A. This is the official definition of the SI unit of the ampere (A). 1 A = 1 C/s1 A = 1 C/s Convert the force on the wire into a force per unit length. F/l = (4 x 10 -7 Tm/A)(1.0 A)2 / 2 (1.0 m) F/l = 2 x 10 -7 N/m The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross- section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10 -7 newton per meter of length.

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Loop Field Each segment of a loop of current contributes a circular field. All the same direction inside loop Similar to a dipole pattern Field strength depends on radius of loop R.

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Solenoid Multiple parallel loops form a solenoid. Each of N loops contributes a share of the fieldEach of N loops contributes a share of the field Solenoids create very uniform magnetic fieldsSolenoids create very uniform magnetic fields Tight, short coil

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Close Loops Field in a solenoid varies at the edges from each loop of wire. Closer loops in a solenoid make a stronger field. Turns per length n = N/lTurns per length n = N/l Formula changes for a long coilFormula changes for a long coil A split coil would have half the field. Equivalent to saying half the field leaks out between loops.Equivalent to saying half the field leaks out between loops. B NI l

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Electromagnet next An electromagnet is usually a solenoidal coil of wire. Increasing the permeability will increase the field lines. Iron core solenoid

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