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Electrical resistance and conductance | HearLore
Electrical resistance and conductance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is conductance, measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with mechanical friction. The SI unit of electrical resistance is the ohm (Ω), while electrical conductance is measured in siemens (S). Siemens were formerly called the mho and then represented by the symbol S. Objects made of electrical insulators like rubber tend to have very high resistance and low conductance. Objects made of electrical conductors like metals tend to have very low resistance and high conductance.
Material Properties And Geometry
A wire's resistance is higher if it is long and thin, and lower if it is short and thick. All objects resist electrical current, except for superconductors, which have a resistance of zero. The resistance R of an object is defined as the ratio of voltage V across it to current I through it. For a wide variety of materials and conditions, V and I are directly proportional to each other. This proportionality is called Ohm's law, and materials that satisfy it are called ohmic materials. A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. Similarly, electrons can flow freely and easily through a copper wire, but cannot flow as easily through a steel wire of the same shape and size. The difference between copper, steel, and rubber is related to their microscopic structure and electron configuration. Resistivity and conductivity are reciprocals: σ = 1/ρ. The conductivity of teflon is about 10^30 times lower than the conductivity of copper.
The SI unit of electrical resistance is the ohm (Ω). Electrical conductance is measured in siemens (S), which were formerly called the mho.
How does wire length and thickness affect resistance?
A wire has higher resistance if it is long and thin, and lower resistance if it is short and thick. All objects resist electrical current except for superconductors, which have a resistance of zero.
Who discovered Joule heating and what is its formula?
Joule heating is named after James Prescott Joule and follows the formula P = I^2R. This phenomenon converts electrical energy into thermal energy to heat resistors or filaments.
Why do metals increase in resistivity as temperature rises?
Near room temperature, the resistivity of metals typically increases as temperature is increased while semiconductor resistivity decreases. Components that use this property are called resistance thermometers or thermistors.
What conditions allow superconductors to achieve zero resistance?
Superconductors require cooling to temperatures near 10 K with liquid helium for most metallic types like niobium. Ceramic high temperature superconductors need cooling to temperatures near 77 K with liquid nitrogen.
Other components and materials used in electronics do not obey Ohm's law; the current is not proportional to the voltage. These are called nonlinear or non-ohmic. Examples include diodes and fluorescent lamps. In other cases, such as a transformer, diode, incandescent light bulb or battery, V and I are not directly proportional. The ratio R = V/I is sometimes still useful, and is referred to as a chordal resistance or static resistance. In other situations, the derivative dV/dI may be most useful; this is called the differential resistance. The current, voltage graph of an ohmic device consists of a straight line through the origin with positive slope. A piece of conducting material of a particular resistance meant for use in a circuit is called a resistor. Conductors are made of high-conductivity materials such as metals, in particular copper and aluminium.
Alternating Current And Impedance
When an alternating current flows through a circuit, the relation between current and voltage across a circuit element is characterized not only by the ratio of their magnitudes, but also the difference in their phases. For example, in an ideal resistor, the moment when the voltage reaches its maximum, the current also reaches its maximum. But for a capacitor or inductor, the maximum current flow occurs as the voltage passes through zero and vice versa. Complex numbers are used to keep track of both the phase and magnitude of current and voltage. The complex angle φ is the phase difference between the voltage and current passing through a component with impedance Z. For capacitors and inductors, this angle is exactly -90° or +90°, respectively. Ideal resistors have an angle of 0°. In general, AC systems are designed to keep the phase angle close to 0° as much as possible. At commercial power frequency, these effects are significant for large conductors carrying large currents, such as busbars in an electrical substation.
Energy Dissipation And Joule Heating
Running current through a material with resistance creates heat, in a phenomenon called Joule heating. This is called Joule heating after James Prescott Joule, also called ohmic heating or resistive heating. Resistors (and other elements with resistance) oppose the flow of electric current; therefore, electrical energy is required to push current through the resistance. This electrical energy is dissipated, heating the resistor in the process. High voltage transmission helps reduce the losses by reducing the current for a given power. On the other hand, Joule heating is sometimes useful, for example in electric stoves and other electric heaters. Incandescent lamps rely on Joule heating: the filament is heated to such a high temperature that it glows white hot with thermal radiation. The formula for Joule heating is P = I^2R, where P is the power converted from electrical energy to thermal energy.
Environmental Dependencies Of Resistance
Near room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased. As a consequence, the resistance of wires, resistors, and other components often change with temperature. When temperature-dependent resistance of a component is used purposefully, the component is called a resistance thermometer or thermistor. A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer. Just as the resistance of a conductor depends upon temperature, the resistance of a conductor depends upon strain. By placing a conductor under tension, the length of the section of conductor under tension increases and its cross-sectional area decreases. Some resistors, particularly those made from semiconductors, exhibit photoconductivity, meaning that their resistance changes when light is shining on them. These are called photoresistors or light dependent resistors.
Zero Resistance In Superconductors
Superconductors are materials that have exactly zero resistance and infinite conductance, because they can have R = 0 and G = infinity. This also means there is no joule heating, or in other words no dissipation of electrical energy. Therefore, if superconductive wire is made into a closed loop, current flows around the loop forever. Superconductors require cooling to temperatures near 10 K with liquid helium for most metallic superconductors like niobium, tin alloys. They also require cooling to temperatures near 77 K with liquid nitrogen for the expensive, brittle and delicate ceramic high temperature superconductors. Nevertheless, there are many technological applications of superconductivity, including superconducting magnets.