chapter19-2 - Magnetic Moment Chapter 19 Magnetism The...

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2/18/2010 1 Chapter 19 Magnetism Today s Torque on a current loop, electrical motor s Magnetic field around a current carrying wire. Ampere’s law s Solenoid s Material magnetism Torque on a Current Loop s τ = B I AN sin θ s Applies to any shape loop s N is the number of turns in the coil s Torque has a maximum value of NBIA s When θ = 90° s Torque is zero when the field is parallel to the plane of the loop Magnetic Moment s The vector is called the magnetic moment of the coil s Its magnitude is given by μ = IAN s The vector always points perpendicular to the plane of the loop(s) s The angle is between the moment and the field s The equation for the magnetic torque can be written as τ = μ B sin θ μ r Electric Motor s An electric motor converts electrical energy to mechanical energy s The mechanical energy is in the form of rotational kinetic energy s An electric motor consists of a rigid current-carrying loop that rotates when placed in a magnetic field Electric Motor, 2 s The torque acting on the loop will tend to rotate the loop to smaller values of θ until the torque becomes 0 at θ = 0° s If the loop turns past this point and the current remains in the same direction, the torque reverses and turns the loop in the opposite direction
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2/18/2010 2 Electric Motor, 3 s To provide continuous rotation in one direction, the current in the loop must periodically reverse s In ac motors, this reversal naturally occurs s In dc motors, a split-ring commutator and brushes are used s Actual motors would contain many current loops and commutators Electric Motor, final s Just as the loop becomes perpendicular to the magnetic field and the torque becomes 0, inertia carries the loop forward and the brushes cross the gaps in the ring, causing the current loop to reverse its direction s This provides more torque to continue the rotation s The process repeats itself Force on a Charged Particle in a Magnetic Field s Consider a particle moving in an external magnetic field so that its velocity is perpendicular to the field s The force is always directed toward the center of the circular path s The magnetic force causes a centripetal acceleration, changing the direction of the velocity of the particle Force on a Charged Particle s Equating the magnetic and centripetal forces: s Solving for r: s r is proportional to the momentum of the particle and inversely proportional to the
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chapter19-2 - Magnetic Moment Chapter 19 Magnetism The...

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