Resistance

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.

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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. 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.

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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. It could overheat and thus lead to the risk of fire.

A Amp circuit should never be wired using gauge wire. Determine the resistance of a 1-mile length of gauge copper wire. Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the diameter of wire B than wire A?

If wire A has four times the resistance, then it must have the smaller cross-sectional area since resistance and cross-sectional area are inversely proportional. In fact, A must have one-fourth the cross-sectional area of B. Put another way, the diameter of wire B is two times greater than the diameter of wire A. Read Watch Interact Physics Tutorial. What Can Teachers Do First find the cross-sectional area: Ohm's Law Power Revisited. Jump To Next Lesson: The resistivity, and thus the resistance, is temperature dependent.

Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance. 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.

Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as. The factor in the resistance which takes into account the nature of the material is the resistivity. Although it is temperature dependent, it can be used at a given temperature to calculate the resistance of a wire of given geometry. 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.

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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.