Electromagnetism (Grobs)

• The mmf of a coil is directly proportional to both the current, I, and the number of turns, N.
• The strength of the magnetic field of a coil magnet depends on how much current flows in the turns of the coil.
• The more current, the stronger the magnetic field.
• Magnetomotive force


  the magnetic potential difference that causes magnetic flux to flow through a material or magnetic circuit.
• Magnetomotive force unit


  Ampere-turn
• With thinner wire, more turns can be placed in a given space.
• The cgs unit of mmf is the gilbert
• One ampere-turn equals 1.26 Gb.
• intensity of the magnetic field depends on the length of the coil.
• The field intensity H is at the center of an air core.
• For an iron core, H is the intensity through the entire core.
• The cgs unit for H is the oersted
• A good magnetic material with high relative permeability can concentrate flux and produce a large value of flux density B for a specified H
• Note that the permeability decreases for the highest values of H.
• The effect of little change in flux density when the fi eld intensity increases is called saturation.
• Hysteresis means “lagging behind.”
• The work done by the magnetizing force against this internal friction produces heat. This energy wasted in heat as the molecular dipoles lag the magnetizing force is called the hysteresis loss.
• The faster the magnetizing force changes, however, the greater the hysteresis effect.
• method of demagnetization is also called degaussing
• Hysteresis loss increases with higher frequencies
• Degaussing is done with alternating current.
• The magnetic lines are circular because the field is symmetrical with respect to the wire in the center
• The magnetic field with circular lines of force is in a plane perpendicular to the current in the wire.
• If you grasp the conductor with your left hand so that the thumb points in the direction of electron flow, your fingers will encircle the conductor in the same direction as the circular magnetic fi eld line
• When the magnetic lines of two fields are in the same direction, the lines of force aid each other, making the fi eld stronger
• When magnetic lines are in opposite directions, the fields cancel.
• A coil of wire conductor with more than one turn is generally called a solenoid.
• To determine the magnetic polarity of a solenoid,

  left-hand rule

  : f the coil is grasped with the fingers of the left hand curled around the coil in the direction of electron flow, the thumb points to the north pole of the coil.
• The solenoid acts like a bar magnet, whether or not it has an iron core
• Adding an iron core increases the flux density inside the coil.
• The physical motion from the forces of magnetic fields is called motor action
• We know that like poles repel and unlike poles attract. It can also be considered that fields in the same direction repel and opposite fields attract.
• fundamental reason for motor action

  magnetic field tend to produce motion from a stronger magnetic field toward a weaker field
• When the conductor is at 90°, or perpendicular to the external field, the reaction between the two magnetic fields is maximum.
• When the conductor is at 0°, or parallel to the external field, there is no effect between them
• When the conductor is at an angle between 0 and 90°, only the perpendicular component is effective
• if electron flow or the external field in the reverse direction in the conductor, the motor action will be in the opposite direction
• Reversing both the field and the current, results in the same direction of motion
• This effect of a force in producing rotation is called torque.
• Since torque is proportional to current, the amount of rotation indicates how much current flows through the coil