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A resistor is an electrical component that resists the flow of current. Resistors are used as part of electrical networks and incorporated into microelectronic semiconductor devices. The critical measurement of a resistor is its resistance, which serves as a ratio of voltage to current and is measured by the SI unit Ohms. A component has resistance 1 Ohm if a voltage of 1 Volt across the component results in a current of 1 Ampere, or Amp, which is equivalent to 1.6 · 1019 electrons per second.

Any physical object is a kind of resistor. Most metals are conductors, and have low resistance to the flow of electricity. The human body, a piece of plastic, or even the air we breath has a resistance that can be measured. Materials that have very high resistance are called insulators.

The relationship between voltage, current, and resistance though an object is given by a simple equation, often mistakenly referred to as Ohm's Law:

  V = IR

where V is the voltage across the object in Volts, I is the current through the object in Amperes, and R is the resistance in Ohmss. If V and I have a linear relationship -- that is, R is constant -- along a range of values, the material of the object is said to be ohmic on that range. An ideal resistor has a fixed resistance across all frequencies and amplitudes of voltage or current.

Resistance in theory can range anywhere from 0 to infinity. In practice, acheiving 0 or inifinity is quite difficult or impossible. Superconducting metals at very low temperatures approach 0 resistance. Insulators (such as air, ceramic, or other non-conducting materials) may have extremely high resistance, but it often breaks down under sufficiently high voltage.

The resistance of a component can be calculated from its physical characteristics. Resistance is proportional to cross-sectional area and to the material's resistivity, a physical property of the molecules that comprise the resistor. Resistance is inversely proportional to the length of the resistor.

In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (variation from the marked resistance), on color-coded resistors a leftmost silver band denotes 10% tolerance, a gold band 5% tolerance, and a red band 2% tolerance. Lower tolerance resistors are also available.

A resistor has a maximum working voltage and current above which the resistance may change (drastically, in some cases) or the resistor may be physically damaged (burn up, for instance). Although some resistors have specified voltage and current ratings, most are rated with a wattage rating which is determined by the physical size. Larger resistors are able to dissipate more heat because of the larger surface area. Common wattage ratings for carbon-based resistors are 1/8 watt, 1/4 watt, and 1/2 watt. Wire-wound and sand-filled resistors are used when a high wattage rating is required, such as 20 watts.

Furthermore, all real resistors also introduce some inductance and capacitance, which change the dynamic behavior of the resistor from the ideal equation.