Resistance and Resistivity Resistance depends on the geometry of the - PowerPoint PPT Presentation

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. r l  R A • Resistivity r depends on material properties like how often charges scatter.

Power in a circuit D E • Power:  D P t • In time D t, a charge D Q enters on the left and D Q exits on the right. V b V a

Energy dissipated by a resistor   P IV V IR    2 P IV I IR ( ) I R   2 V V      P IV V   R R

B 1 Four identical light bulbs are connected to a power B 2 supply as shown. Which light bulb consumes the B 3 most power? 1) B 1 B 4 2) B 2   3) B 3 4) B 4 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 S 1 is closed and the other two switches open. When the other two switches, S 2 and S 3 , are closed, the brightness of the first light bulb 1) is dimmer. 2) is unchanged. s 0 s 1 s 2 s 3 3) is brighter.  V B 1  B 2 B 3

Resistors in Series Figure 26.31a

Resistors in Parallel Figure 26.4a

Effective Resistance 1 1 1 • Parallel:     R R R eff 1 2 r l  R A     • Series: R R R eff 1 2

Effective Capacitance • Parallel:     C C C eff 1 2  0 A  C d 1 1 1 • Series:     C C C eff 1 2