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Resistor Potential Drop Within a material there is a + - voltage drop associated with a field. The material can be a weak conductor. • Charges may flow • Field and equipotentials exist The material will have a current I and voltage drop V. + - V Ed Ohm’s Law Many materials have a linear relationship between current and voltage. This relationship is called Ohm’s law. Resistance The ratio of the change in DP v d voltage to the current is the resistance. • Like resistance to water flow by stones A Resistance is measured in ohms (W). • 1 W = 1 V/A R V / I V IR I V / R Non-Ohmic Some materials have non-linear relations between current and voltage. • Non-ohmic materials • Fluorescent and incandescent lamps • Diodes and transistors Resistor Material can be fashioned to have a specific resistance. The symbol for a resistor is a zigzag line. • This is a resistor Resistors can be made from very long conductors. • Wound wire in a coil • Very precise Carbon composites make good resistors. Like other electrical symbols the orientation doesn’t matter. Resistivity The resistance of a material depends on the shape. • Length • Cross-sectional area The resistivity (r) is the material-dependent part of the resistance. Resistivity also depends on temperature. L Rr A r r0 (1 + 0 DT ) next

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