A metal disk {magnetic brake} rotating between two permanent magnets dissipates energy, because eddy currents make magnetic field opposed to permanent magnetic field and slow disk.
After removing magnetization, magnetic domains return to original orientations {magnetic memory, computer}.
Devices {solenoid}| can have wire coils. If current is in coils, magnetic field is sum of coil magnetic fields. Large magnetic field points down coil middle. Soft iron core in coil middle increases magnetic field by adding atom magnetic fields.
Devices {transformer}| can transfer voltage from circuit with alternating current to voltage from second circuit with alternating current. Transformers induce current in stationary-wire second coil using alternating current in first coil. Power in first coil equals power in second coil. Power is circuit voltage V times wire current I times wire-coil number n: V1 * I1 * n1 = V2 * I2 * n2.
Electronics can use electron charge and spin {spintronics} {magneto-electronics}. Flowing-electron spins {spin current} can align {spin-polarized}.
resistance
Electrical resistance {magnetoresistance} can change in different-polarization magnetic layers. Electrons take curved paths, slow in current direction, and decrease current. Computer hard drives can use magnetoresistant read heads [1998].
spin
Quantum spintronics can control single-electron spin. When nitrogen atoms replace carbon atoms in diamond, adjacent locations can be empty {nitrogen-vacancy center} (N-V center). Doped diamonds can semiconduct. N-V centers make single fluorescing electrons with two energy levels, with no ionization.
Mechanical energy can turn metal coil in magnetic field to generate electric current {generator, electricity} {electric generator}.
current
Electric current is in coil leading and trailing edges. Current changes direction with coil half turns, to make alternating current.
voltage
Voltage V equals magnetic field H times wire movement velocity v times wire-coil length l: V = H*v*l. Voltage V equals magnetic field H times area change dA divided by time change dt: V = H * dA / dt. Voltage V equals flux change dF divided by time change dt. V = dF / dt. Voltage V equals mutual inductance I times current change di divided by time change dt. V = I * di / dt.
example
Water from dams or steam from steam engines can turn wire coils around steel shafts {rotor, generator}, which are inside permanent magnets. Magnets and rotation cause electric current to flow in coils. Electric current changes direction as coil flips.
AC or DC
Rotor shaft {commutator, generator} can have separate conductors {brush, generator} on halves to allow current to leave rotor as direct current. Large-generator shafts {armature} collect alternating current directly.
Alternating current in coil has alternating magnetic field that can interact with outside magnetic field to make magnetic force on coil leading and trailing edges, and so turn coil {electric motor}|.
parts
Direct current or alternating current causes magnetic field in stationary wire coils {stator, motor} and in rotating wire coils {rotor, motor}. As rotor turns, current can go in forward or backward direction, changing magnetic field direction, because rotor shaft has separate conductors {brush, motor} on halves. Rotor magnetic field continually pulls into alignment with stator field, turning rotor by magnetic force. Rotation angular momentum starts cycle again.
torque
Magnetic force causes torque on coil and makes both magnetic fields tend to align. Coil torque T equals coil number n times magnetic field B times current i times coil area A: T = n * B * i * A. When magnetic fields align, force or torque is zero. Just before magnetic fields align, current reverses in coil. Current can reverse every half circle using commutators. Current can reverse using alternating current at needed frequency.
torque: direction
Right-hand palm points in magnetic-force direction, fingers point in magnetic-field direction, and thumb points in positive-current direction {right hand rule, torque}.
types
Series motors have low back emf, high field, and high current when starting and low current, high back emf, and low field when running. Shunt motors have constant field and lower current at high speed. Series and shunt motors can combine. Electric motors use direct current {induction motor}, alternating current {synchronous motor}, or either {universal motor}.
Current can reverse every half circle using devices {commutator, motor}|.
5-Physics-Electromagnetism-Magnetism
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Date Modified: 2022.0225