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Resistance and Resistivity • Resistance depends on the geometry of the resistor – its length and cross-sectional area – R is proportional to length – R is inversely proportional to area. rl R A • Resistivity r depends on material properties like how often charges scatter. Power in a circuit DE P Dt • Power: • In time Dt, a charge DQ enters on the left and DQ exits on the right. Vb Va Energy dissipated by a resistor P IV V IR P IV I ( IR ) I 2 R V2 V P IV V R R Four identical light bulbs are connected to a power supply as shown. Which light bulb consumes the most power? 1) B1 2) B2 3) B3 4) B4 B1 B2 B3 B4 V 5) They all consume the same amount of power. Three identical light bulbs are connected in parallel to a constant voltage power supply. Initially, switch S1 is closed and the other two switches open. When the other two switches, S2 and S3, are closed, the brightness of the first light bulb 1) is dimmer. s0 s1 s2 s3 2) is unchanged. 3) is brighter. V B1 B2 B3 Resistors in Series Figure 26.31a Resistors in Parallel Figure 26.4a Effective Resistance 1 1 1 • Parallel: Reff R1 R2 R • Series: Reff R1 R2 rl A Effective Capacitance • Parallel: Ceff C1 C2 C • Series: 1 1 1 Ceff C1 C2 0 A d