In dynamos or motors, electric and magnetic forces induce currents and voltages {electromagnetic induction}|.
outside force
If force moves conducting material through magnetic field or moves magnetic field near conducting material, protons and electrons in conductor move relative to protons and electrons that caused magnetic field. Moving protons and electrons make two electric currents that make two magnetic fields around conductor. Outside force provides energy to make magnetic fields.
However, no net charge moves, and test charges detect no electric current, because protons and electrons move together, so charges cancel.
induction
The original magnetic field interacts with both generated magnetic fields, setting up relativistic electric forces. Forces move electrons in conductor, but protons cannot move. Moving electrons make electric current opposite to movement and create magnetic field around current opposite in polarity to original magnetic field. Magnetic field created by moving electrons tends to resist relative movement between conductor and original magnetic field.
moving wire
For example, wire can moves through magnetic field. Moving wire moves wire protons and electrons, creating proton current and electron current, and currents make magnetic field around motion direction. Original magnetic field interacts with moving magnetic fields. Wire electrons are free and move down wire. Wire protons cannot move, though they feel magnetic force in opposite direction. Net current appears. Relativistic electric force separates electrons from protons, to make voltage that then makes current.
Energy for charge separation comes from outside mechanical energy used to move wire through magnetic field. Induced current makes net magnetic field that resists wire movement. Mechanical energy used to move wire makes electric field, induces current, and creates induced magnetic field.
energy transfers
In electromagnetic induction, potential energy in electric field causes voltage that makes current with kinetic energy, then current makes magnetic field with potential energy, then magnetic field slows current and builds voltage, which is potential energy in electric field. Cycle repeats.
Electric field and magnetic field, and voltage and current, are out of phase, because energy in one transfers to the other and then back again.
When electric-field change is zero and electric field maximizes, voltage maximizes and current is zero, and magnetic-field change maximizes and magnetic field is zero. As electric field decreases to zero, voltage decreases and current increases. As current increases, magnetic field increases and maximizes when current maximizes, electric-field change maximizes, and electric field is zero. As magnetic field decreases to zero, voltage increases and current decreases. As voltage increases, electric field increases and maximizes when voltage maximizes and electric field change is zero. Magnetic-field phase lags electric-field phase by 90 degrees.
examples
Electromagnetic induction happens in dollar bills in magnet, inductance coils, transformers, solenoids with iron bars, motors, and generators.
In conductors with current in magnetic fields, magnetic field pushes charges to conductor sides and makes electric field {Hall resistance, magnetism} opposed to magnetic field. Hall resistance varies with magnetic field and current.
semiconductor
In semiconductors, high magnetic field separates charges across width, not length, and so causes transverse current {Hall effect}.
quantum Hall effect
Quantum Hall resistance {quantum Hall effect} is inverse of small positive integer n times Planck's constant h divided by electron charge e squared: (1/n) * (h/e^2).
spin
In semiconductor ribbons with electric current, magnetic field from spin-orbit coupling causes excess electrons with one spin on one edge and excess electrons with opposite spin on other edge {spin Hall effect}.
In conductors with current in magnetic fields, magnetic field forces charge to conductor sides and makes electric field opposed to magnetic field {Hall resistance, current}, that varies with magnetic field and current.
Wire coil with current creates magnet with north and south poles {magnetic dipole}|.
field
Magnetic-field direction relates to current direction. By right hand rule, if positive current points in right-hand finger direction, magnetic-field direction points in thumb direction for north magnetic pole, and the opposite direction is south magnetic pole.
force
Like magnetic poles repel. Opposite magnetic poles attract. Force between magnetic poles equals space magnetic permeability k' times one magnetic-pole strength P times other magnetic-pole strength p, divided by distance r between poles: F = k' * P * p / r.
pole
Current i times path length L is pole strength p: p = i*L. Pole strength p equals charge q times velocity v: p = q*v.
infinitesimal
Infinitesimal wire loops can have unit current {elementary magnet}, to make idealized unit dipoles.
5-Physics-Electromagnetism-Magnetism
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Date Modified: 2022.0225