Presentation on theme: "Electromagnetism Topics Covered in Chapter 14: 14-1: Ampere-turns of Magnetomotive Force (mmf) 14-2: Field Intensity (H) 14-3: B-H Magnetization Curve."— Presentation transcript:
Electromagnetism Topics Covered in Chapter 14: 14-1: Ampere-turns of Magnetomotive Force (mmf) 14-2: Field Intensity (H) 14-3: B-H Magnetization Curve 14-4: Magnetic Hysteresis 14-5: Magnetic Field around an Electric Current Chapter 14
14-1: Ampere-turns of Magnetomotive Force (mmf) The strength of a coil’s magnetic field is proportional to the amount of current flowing through the coil and the number of turns per given length of coil. Ampere-turns = I × N = mmf I is the amount of current flowing through N turns of wire. This formula specifies the amount of magnetizing force or magnetic potential (mmf).
14-2: Field Intensity (H) The length of a coil influences the intensity of a magnetic field. Intensity is different from mmf. Equation: H = mmf/length Units: A·t/m ampere-turns per meter H is the mks unit Shorter magnetic circuits produce a greater field intensity
14-2: Field Intensity (H) How well current can go through a wire? V I Conductance ( G )
14-2: Field Intensity (H) H B Permeability ( ): How easily magnetic flux can be set up inside a piece of material?
14-2: Field Intensity (H) Permeability ( ) is a measure of the ability to concentrate magnetic fields. Materials with high permeability can concentrate flux, and produce large values of flux density B for a specified H. The amount of flux produced by H depends on the material in the field. These factors are reflected in the formulas: B = × H μ = B / H The unit is teslas per ampere-turn per meter: A · t/m T
14-2: Field Intensity (H) Ferromagnetic materials have high values of permeability (as high as 10,000). Paramagnetic materials The permeability is slightly more than 1. Diamagnetic materials The permeability is less than 1. Non-magnetic materials The permeability is approximately 1.
14-2: Field Intensity (H) http://hyperphysics.phy-astr.gsu.edu/hbase/solids/hyst.html
14-4: Magnetic Hysteresis Hysteresis refers to a situation where the magnetic flux lags the increases or decreases in magnetizing force. Hysteresis loss is energy wasted in the form of heat when alternating current reverses rapidly and molecular dipoles lag the magnetizing force. For steel and other hard magnetic materials, hysteresis losses are much higher than in soft magnetic materials like iron.
14-4: Magnetic Hysteresis Demagnetization: To demagnetize a magnetic material completely, the retentivity B R must be reduced to zero. To demagnetize a magnetic material completely, the retentivity B R must be reduced to zero. A practical way to do so is to magnetize and demagnetize the material with a decreasing hysteresis loop. This method of demagnetization is called degaussing. Applications of degaussing include: Metal electrodes in a color picture tube Erasing the recorded signal on magnetic tape.
14-4: Magnetic Hysteresis A magnetic field is produced by alternating current. The magnetic field and the magnetic material are moved away from each other, or the current amplitude is reduced. The hysteresis loop then becomes smaller and smaller until it effectively collapses.
14-5: Magnetic Field around an Electric Current The left-hand rule for conductors: Grasp the conductor with your left hand so the thumb points in the direction of electron flow. Your fingers will encircle the conductor in the same direction as the circular magnetic field lines.
14-6: Magnetic Polarity of a Coil Bending a straight conductor into the form of a loop produces two effects: The magnetic field lines are more dense inside the loop. All the lines inside the loop aid in the same direction. This makes the loop field the same as a bar magnet, with opposite poles at opposite faces of the loop.
14-8: Induced Current Generator Action When a moving conductor cuts across flux lines, a voltage is induced. The amount of induced voltage is proportional to: The conductor velocity The amount of flux The number of turns of wire
14-9: Generating an Induced Voltage The motion of flux across a conductor in an open circuit forces free electrons to move. Since the ends are open, electrons accumulate at them, creating a potential difference. The potential difference is an electromotive force (emf), generated by the work of cutting across the flux. Induced emf increases with the number of turns in a coil. The polarity of the induced voltage follows from the direction of induced current.
14-9: Generating an Induced Voltage Faraday’s Law of Induced Voltage The amount of induced voltage can be calculated by Faraday’s law: N = number of turns dΦ/dt = how fast the flux cuts across the conductor. v ind =N dΦ (webers) dt (seconds)
14-9: Generating an Induced Voltage Faraday’s Law of Induced Voltage The amount of induced voltage is calculated by Faraday’s law: The induced voltage is directly proportional to the number of turns times d /dt. To generate more voltage: Increase the number of turns Increase the flux (stronger magnet) Decrease the time (faster movement of the coil) v ind = N d (Wb) dt(seconds)
14-9: Generating an Induced Voltage The polarity of induced voltage is determined by Lenz’s law. The induced voltage has the polarity that opposes the change causing the induction????????? Movement of a wire/coil in the presence of magnetic field induces current/voltage. Induced current/voltage interacts with the magnetic field to produce a force. The direction of the force depends on polarity of the induces current/voltage. The direction of the force is opposite to the direction of the movement of the wire/coil. It will try to slow down the wire/coil.
14-10: Relays A relay is an electromechanical device that operates on the basis of electromagnetic induction. It uses an electromagnet to open or close one or more sets of contacts. Relays, like switches, have poles and throws. Relays can switch or control high power loads with a low amount of input power. In remote-control applications, relays can control high power loads long distances away more efficiently than can mechanical switches.