Electromagnet

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

Permeability  The permeability  defines the strength of the B field in a material.  The vacuum permeability is  0. Defined as  0 = 4  x T m / ADefined as  0 = 4  x 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

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

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 Tm/A)(1.0 A)2 / 2  (1.0 m) F/l = 2 x 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 newton per meter of length.

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.

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

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

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