Moving-electron and stationary-molecule interactions oppose electric current flow {resistance}| and turn electric energy into heat. Electrical resistance depends on path length, cross-sectional area, and material resistivity.
voltage
Resistance makes heat from electrical kinetic energy and, as potential energy decreases, drops potential across resistor.
current
For same voltage, more resistance makes less current, because flow slows.
factors
Resistance is more for poor conductors with few electrons that can move, for longer conductor length, or for less cross-sectional area. If cross-sectional area is more, conductor perimeter is more, fewer electron collisions happen, and resistance is less. If conductor length is more, distance is longer, and total resistance is more. If material resistivity is more, conductor has fewer free charges, and resistance is more.
factors: temperature
In conductors at higher temperature, resistance is more, because random motions are more. In insulators or semiconductors at higher temperature, resistance is less, because more electrons are free to move. Alloys have smaller resistance change when temperature changes, because alloys have fewer free electrons than pure metals.
resistor
Electrical devices {resistor} can have resistance. Conductor resistance R equals material resistivity r, which differs at different temperatures, times conductor length l divided by conductor cross-sectional area A: R = r * l / A.
resistivity
Resistivity is 10^-6 to 10^-1 ohm-cm for conductors, 10^-1 to 10^8 ohm-cm for semiconductors, and 10^8 to 10^21 ohm-cm for insulators.
examples
Resistance in incandescent light bulbs creates light. Resistance in electric heaters creates heat. Fuses, circuit breakers, voltmeters, ammeters, tube resistors, rod resistors, and coil resistors demonstrate electrical resistance. Lie detectors detect skin electrical resistance, which varies with sweat amount.
Resistance reciprocal {conductance}| measures current-flow ease.
At same temperature, electrical-conductivity to heat-conductivity ratio is the same for all metals {law of Wiedemann and Franz} {Wiedemann and Franz law}.
Materials {conductor}|, such as metals, can allow electrons to move almost freely.
dipoles
Because electrons are free to move, no dipoles form. Conductor dielectric strength is zero, and dielectric constant is infinite, because charges can move freely. Rubbing metal with cloth cannot rub off charges, because electrons move freely and quickly in conductors.
spark
If charge touches conductors, electrons flow to neutralize charge, typically making sparks.
spread
Potential difference between conductor points is zero, because all electrons already repel each other equally. No electrons flow.
compression
Compressing metal increases conductivity, because crystals have fewer imperfections.
Materials can allow electrons to move almost freely {conduction, electricity}|. Semiconductors allow electrons to move with high resistance. Insulators do not conduct electricity. Circuit loads are either conductors or semiconductors.
Electric force causes average drift velocity per unit force {mobility, charge} {charge mobility, conductor}|.
Metallic bonds are electron deficient and leave electrons free {free electron}. Metal has electrons that can move among atoms around metal surface. Outside electric force can pull electrons completely away from atoms.
In conductors, moving electrons have average time between collisions {mean free time}| and have average distance, mean free path, between collisions. Collisions tend to reduce charge velocities.
In conductors, voltage V equals resistance R times current I {Ohm's law} {Ohm law}: V = R*I.
Most materials {insulator}| {dielectric} allow no free electron movement. Air, vacuum, paper, and glass are insulators.
dipoles
Outside electric field separates electrons and protons, to make induced charge. Inducing charge can be easy or hard. Dielectric strength is ratio between material capacitance and vacuum capacitance. For vacuum, dielectric constant is 1. For insulators, dielectric constant is 1 to 8. For water, dielectric constant is 81, because water has high polarization and free dipole rotation. For conductors, dielectric constant is infinite. Lustrous metals have negative dielectric constant.
Materials have ease by which electric fields can go through {permittivity}|. Metals have free electrons and cannot have electric fields inside. Insulators have charges that move relative to electric field and oppose electric field. Empty space has no charges and allows electric field. Electric-force constant k inversely depends on permittivity.
Insulators have different abilities to make dipoles {polarizability}| {polarization, electricity}. If polarization is more, refraction index is more. Polarization K is refractive index n squared: K = n^2. Metal has free electrons and cannot make dipoles. Empty space has no charges and cannot make dipoles.
Materials {semiconductor}| can have electrons that can move from atom to atom in atomic-orbital conducting bands. Silicon and germanium are semiconductors. Semiconductor compounds include indium gallium arsenide and indium antimonide.
impurities
Semiconductors can be silicon with added gallium {P-type semiconductor} or arsenic {N-type semiconductor}. P-type semiconductors transfer electron vacancies. N-type semiconductors transfer electrons. Holes and electrons must move in opposite directions to complete circuit.
electric charge
If charge touches semiconductor, no change happens, because semiconductor electrons are not free to move.
Impurities {doping}| added to silicon or germanium supply more negative or positive charges, to make more conduction.
donor
Adding material with five electrons in highest orbital {donor impurity} adds extra electron. Antimony, arsenic, and phosphorus are donors {n-type semiconductor}.
acceptor
Adding material that has three electrons in highest orbital {acceptor impurity} results in extra proton {electron deficiency}. Gallium, indium, aluminum, and boron are acceptors {p-type semiconductor}.
junction
If p-type semiconductor touches n-type semiconductor, electrons in n-type semiconductor flow into holes in p-type semiconductor until reaching balance, with voltage across junction. p-type semiconductor has become slightly negative. n-type semiconductor has become slightly positive. No more free charges exist. Junction width is 50 atoms.
diode
If voltage across np junction makes p side positive, current flows greatly, because p side attracts electrons. If voltage across np junction makes p side negative, no current flows, because p side repels electrons. np junctions allow current in only one direction and allow current to be ON or zero OFF, like diodes.
Semiconductors can emit light {electroluminescence}| across pn junctions when current flows. Phosphors can glow if AC current passes through. Machine sprays glass panel with thin transparent metal layer, adds a phosphor layer, and adds thin metal foil. Electroluminescence is efficient and cool but allows only low light levels.
One electron and one hole {exciton} can bind electrostatically for 4 to 40 microseconds, 1150 nanometers apart. Electric forces cause free electrons and holes to drift in opposite directions, at same velocity. Electric force causes average drift velocity per unit force {charge mobility, exciton}. When electron meets hole, they merge. At constant force, ejections and recombinations are in equilibrium.
A thin layer of electrons is between two semiconductors. Near 0 K in high magnetic field [1982], pairs {quasiparticle, pair} of excited superposition of electron states have fractional charges {fractional quantum Hall effect}, with edge effects {edge state}. Fractional quantum Hall effect can extend to four dimensions, as on five-dimensional-sphere surfaces, which have three-dimensional edge states that emerge with relativity. Excitations can carry magnetic-flux units.
Adding electron-deficient materials, with three electrons in highest orbital, results in extra protons, because of electron vacancies {hole}|.
Two semiconductors can have insulator between them {Josephson junction}. Microwaves can supply energy to electron pairs. Voltage V is n = 1/2, 1, 3/3, or 2 times Planck's constant h times frequency f divided by electron charge e: V = (n*h*f) / (2*e). Third semiconductor can supply control current. Control current sets voltage at zero or one, using quantum-mechanical tunneling.
Two semiconductors, or semiconductor and conductor, can meet in region {junction}| 50 molecules thick. Contact point between metal and semiconductor has resistance that does not follow Ohm's law, because current depends on surface properties.
Semiconductors {metal oxide semiconductor} (MOS) can be metal oxides, which can be unipolar, rather than just bipolar.
Solid-state semiconductor circuit elements {transistor, electronic}| amplify current.
types
N-type semiconductor, P-type semiconductor, and N-type semiconductor can join in sandwich {NPN transistor}. P-type semiconductor, N-type semiconductor, and P-type semiconductor can join in sandwich {PNP transistor}. NPN or PNP transistors are bipolar transistors {junction transistor}, with two junctions. Weak signals control current flow. Junction transistors are current operated.
parts
Transistors have cathode emitter, anode collector, and base controller. Collector is between emitter and base. In PNP transistors, electrons flow from emitter to middle collector and from base to middle. In NPN transistors, electrons flow from middle collector to emitter and from middle to base.
process
Holes and electron diffusion across semiconductor np junction continues until electric force equilibrium, preventing further diffusing. Voltage is across emitter and base and across collector and emitter. Applying small positive charge to base attracts electrons and amplifies current 10^5 times. Electron flow from emitter to collector multiplies directly with voltage from base to emitter.
surface barrier
A depletion layer {space charge, transistor} between metal and semiconductor can control conductivity {point contact semiconductor} {surface barrier transistor}.
field effect
Electric field at right angles to silicon surface causes lateral conductance {field-effect transistor}. Insulated field plate can have field that induces conducting surface channel between two surface pn junctions {gate}, as in field-effect transistors, such as metal oxide semiconductors. Field-effect transistors have slow response and high impedance. They are voltage operated, rather than current operated.
electron tubes
Transistors can replace all electron tube types.
In PNP transistors, electrons flow from emitter to collector, and from other sandwich side {base, transistor} to middle.
In PNP transistors, electrons flow from emitter to middle {collector, transistor}, and from base to middle.
In PNP transistors, electrons flow from one sandwich side {emitter, transistor} to collector, and from base to middle.
At low temperatures, substances can be electrical conductors {superconductor} with no electrical resistance {superconductivity}|. Liquid oxygen, liquid nitrogen, and liquid hydrogen are superconductors. Organic crystals, metal oxides, and insulators can have superconductivity.
high temperature
Most high-temperature superconductors are copper oxide {cuprate} layers between other layers. Mercury-barium-calcium copper oxide superconducts at 164 K under 10,000 atm pressure and at 138 K at 1 atm.
Iron and arsenic layers between a lanthanum, cerium, samarium, neodymium, or praseodymium layer and an oxygen or fluoride layer can superconduct up to 52 K. Magnesium boride superconducts at 39 K. Bismuth, strontium, calcium, copper, and oxygen atoms can combine to make BSCCO high-temperature superconductor. Yttrium, barium, copper, and oxygen atoms can combine to make YBCO high-temperature superconductor.
cause
Large-scale quantum effects cause superconductivity, which happens when energies are small, such as at low temperature. Bosons in same quantum state can condense {Bose-Einstein condensation, superconductivity} (BEC) from gas to liquid. Repulsive bosons condense better. Materials can Bose-Einstein-condense at cold temperatures.
Fermions, such as electrons, form Cooper pairs at temperature lower than temperature at which material becomes degenerate Fermi gas. Both electrons have same spin. Making electron pairs makes positive metal ions. Cooper pairs have streamline flow through metal ions and travel with no resistance.
Fermions can pair more easily if attraction increases. Electrons can resonate {Feshbach resonance} in magnetic fields {magnetic resonance} and so pair better.
Magnetic flux has quanta in superconductors. Electric field has no quanta but quantizing it can make calculations easier.
magnetic field
Outside magnetic field can enter only short distance into superconductors with current, because photons acquire mass as electromagnetic gauge symmetry breaks spontaneously.
insulator
Forcing atoms {Mott insulator} in Bose-Einstein condensates to have definite positions changes quantum properties.
In superconducting materials, electrons distort positive-ion lattice to make phonons, which interact with other electrons, causing attraction and electron pairing {BCS theory}. Critical temperature is higher if more electrons can go to superconductive state, if lattice-vibration frequencies are higher, and if electrons and lattice interact stronger. Mercury-barium-calcium copper oxide does not follow BCS theory. Magnesium boride follows BCS theory. Liquid oxygen, liquid nitrogen, and liquid hydrogen follow BCS theory.
Fermions can pair {Cooper pair} at temperatures much lower than temperature at which system is degenerate Fermi gas.
Systems {degenerate Fermi gas} can have one fermion in each low-energy quantum state.
Devices {transition edge sensor} can detect photons at transition temperature to superconductivity.
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Date Modified: 2022.0225