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Resistance in Electrical Systems 4.3 Conductors ► Conduct electric current well ► Metals, some liquids and plasmas—because of their free electrons ► Superconductors—resistance disappears at very low temperatures Insulators ► Do not conduct electric current well ► Wood, plastic, glass, rubber, and minerals— because of tightly bound atoms or molecules Semiconductors ► Good conductors when small amounts of impurities are added ► Silicon, germanium, gallium, arsenic Electrical Resistance ► Caused by collision in the wire ► All electrical devices resist charge flow ► R = ΔV/I ( Electrical Resistance = change in potential difference [voltage] divided by current ► the Ohm(Ω) is the unit of electrical resistance ► 1 Ω = 1 V/A ( one ohm = one volt per ampere) Ohm’s Law ► resistance is constant, independent of potential difference and current ► ΔV = IR (change in voltage = current times resistance) ► is not obeyed by all materials and devices (but resistance always applies to all devices) Resistivity ► measure of the capacity of a material to resist electric charge flow ► unit of resistivity: Ω·m ρ(rho) is the symbol ► R = ρ L/A (Resistance = resistivity times length divided by area) Resistivity ► Depends on 3 things: length of the wire—longer wire, greater resistance the radius of the wire—smaller the wire, greater resistance material of which the wire is composed (p. 205 for resistivity chart) Series Circuits ► One path for current ► 4 important rules for series circuits: The current through each device and everywhere in the series circuit, is the same. The total resistance in a series circuit is the sum of the individual resistances. The voltage drop across each device is the product of the lamp’s resistance and the current. (ΔV = IR) The sum of the voltage drops across the devices = the total potential difference(voltage) of the power supply = the product of the total circuit resistance and the current. Parallel Circuits ► More than one path for current ► 4 important rules for parallel circuits: The total current through the circuit = the sum of the currents through all the devices. 1/Rtotal = 1/R1 + 1/R2 The current through each device is the ratio of the voltage drop across the device to the resistance of the lamp. (I = ΔV/R) The voltage drop across the lamps is the same, and equals the potential difference of the power supply. Resistors ► electrical devices that have a specific resistance ► made of long wires, carbon, or semiconductors ► value is labeled on the outside of a resistor ► used to reduce current in a circuit