Resistance
Electrical resistance and conductance
It should be noted that it is being presumed that the current is uniform across the cross-section of the wire, which is true only for Direct Current. For Alternating Current there is the phenomenon of " skin effect " in which the current density is maximum at the maximum radius of the wire and drops for smaller radii within the wire. At radio frequencies, this becomes a major factor in design because the outer part of a wire or cable carries most of the current. The inverse of resistivity is called conductivity. There are contexts where the use of conductivity is more convenient.
The combination rules for any number of resistors in series or parallel can be derived with the use of Ohm's Law , the voltage law , and the current law.
The electrical resistance of a wire would be expected to be greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made resistivity. Enter data and then click on the quantity you wish to calculate in the active formula above. The materials shown in the last four rows of the above table have such high resistivity that they would not even be considered to be conductors.
Resistance is a numerical quantity that can be measured and expressed mathematically. The standard metric unit for resistance is the ohm, represented by the Greek letter omega -. The equation representing the dependency of the resistance R of a cylindrically shaped conductor e.
Consistent with the discussion above, this equation shows that the resistance of a wire is directly proportional to the length of the wire and inversely proportional to the cross-sectional area of the wire. As shown by the equation, knowing the length, cross-sectional area and the material that a wire is made of and thus, its resistivity allows one to determine the resistance of the wire. Resistors are one of the more common components in electrical circuits. Most resistors have stripes or bands of colors painted on them. The colors reveal information about the resistance value.
Perhaps you're doing a lab and need to know the resistance of a resistor used in the lab. Use the widget below to determine the resistance value from the colored stripes. Household circuits are often wired with two different widths of wires: Thus, gauge wire has a wider cross section than gauge wire. A Amp circuit used for wall receptacles should be wired using gauge wire and a Amp circuit used for lighting and fan circuits should be wired using gauge wire. Explain the physics behind such an electrical code. A gauge wire is wider than gauge wire and thus has less resistance.
The lesser resistance of gauge wire means that it can allow charge to flow through it at a greater rate - that is, allow a larger current. Thus, gauge wire is used in circuits which are protected by Amp fuses and circuit breakers.
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On the other hand, the thinner gauge wire can support less current owing to its larger resistance; it is used in circuits which are protected by Amp fuses and circuit breakers. Based on the information stated in the above question, explain the risk involved in using gauge wire in a circuit that will be used to power an ampere power saw. Thus, gauge wire can safely support a circuit that uses an appliance drawing up to 20 Amps of current. In fact, a Amp circuit is protected by a fuse or circuit breaker that will flip off when the current reaches 20 Amps. If a gauge wire was used on the same circuit, then the breaker would allow up to 20 Amps to flow through it.
All objects show some resistance, except for superconductors , which have a resistance of zero. The resistance R of an object is defined as the ratio of voltage across it V to current through it I , while the conductance G is the inverse:. For a wide variety of materials and conditions, V and I are directly proportional to each other, and therefore R and G are constants although they will depend on the size and shape of the object, the material it is made of, and other factors like temperature or strain.
This proportionality is called Ohm's law , and materials that satisfy it are called ohmic materials. In other cases, such as a transformer , diode or battery , V and I are not directly proportional. In the hydraulic analogy , current flowing through a wire or resistor is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop that pushes water through the pipe.
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Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. Conductance and resistance are reciprocals. The voltage drop i. In hydraulics, it is similar: The pressure difference between two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe.
If these pressures are equal, no water flows. In the image at right, the water pressure below the pipe is zero. The resistance and conductance of a wire, resistor, or other element is mostly determined by two properties:. Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In the same way, a long, thin copper wire has higher resistance lower conductance than a short, thick copper wire. Materials are important as well. 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, and they essentially cannot flow at all through an insulator like rubber , regardless of its shape. The difference between copper, steel, and rubber is related to their microscopic structure and electron configuration , and is quantified by a property called resistivity.
In addition to geometry and material, there are various other factors that influence resistance and conductance, such as temperature; see below. Substances in which electricity can flow are called conductors.
resistance | Definition of resistance in English by Oxford Dictionaries
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. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs.
For many materials, the current I through the material is proportional to the voltage V applied across it:. Therefore, the resistance and conductance of objects or electronic components made of these materials is constant. This relationship is called Ohm's law , and materials which obey it are called ohmic materials. Examples of ohmic components are wires and resistors. The current-voltage IV graph of an ohmic device consists of a straight line through the origin with positive slope.
Other components and materials used in electronics do not obey Ohm's law; the current is not proportional to the voltage, so the resistance varies with the voltage and current through them. These are called nonlinear or nonohmic. Examples include diodes and fluorescent lamps. The IV curve of a nonohmic device is a curved line. The resistance of a given object depends primarily on two factors: What material it is made of, and its shape.
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For a given material, the resistance is inversely proportional to the cross-sectional area; for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistance R and conductance G of a conductor of uniform cross section, therefore, can be computed as.
The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity are reciprocals: Resistivity is a measure of the material's ability to oppose electric current. This formula is not exact, as it assumes the current density is totally uniform in the conductor, which is not always true in practical situations.
However, this formula still provides a good approximation for long thin conductors such as wires. Another situation for which this formula is not exact is with alternating current AC , because the skin effect inhibits current flow near the center of the conductor.