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Transcript
ConcepTest 26.3b Wires II A wire of resistance R is 1) it decreases by a factor 4 stretched uniformly (keeping its 2) it decreases by a factor 2 volume constant) until it is twice 3) it stays the same its original length. What happens 4) it increases by a factor 2 to the resistance? 5) it increases by a factor 4 ConcepTest 26.3b Wires II A wire of resistance R is 1) it decreases by a factor 4 stretched uniformly (keeping its 2) it decreases by a factor 2 volume constant) until it is twice 3) it stays the same its original length. What happens 4) it increases by a factor 2 to the resistance? 5) it increases by a factor 4 Keeping the volume (= area x length) constant means that if the length is doubled, the area is halved. L Since R , this increases the resistance by four. A ConcepTest 26.4a Series Resistors I Assume that the voltage of the battery is 9 V and that the three resistors are identical. 1) 12 V 2) zero What is the potential difference across each 3) 3 V resistor? 4) 4 V 5) you need to know the actual value of R 9V ConcepTest 26.4a Series Resistors I 1) 12 V Assume that the voltage of the battery is 9 V and that the three resistors are identical. What is the potential difference across each resistor? 2) zero 3) 3 V 4) 4 V 5) you need to know the actual value of R Since the resistors are all equal, the voltage will drop evenly across the 3 resistors, with 1/3 of 9 V across each one. So we get a 3 V drop across each. 9V Follow-up: What would be the potential difference if R= 1 W, 2 W, 3 W ConcepTest 26.6b Two lightbulbs A and B are connected in series to a constant voltage source. When a wire is connected across B, bulb A will: Short Circuit II 1) glow brighter than before 2) glow just the same as before 3) glow dimmer than before 4) go out completely 5) explode ConcepTest 26.6b Two lightbulbs A and B are connected in series to a constant voltage source. When a wire is connected across B, bulb A will: Short Circuit II 1) glow brighter than before 2) glow just the same as before 3) glow dimmer than before 4) go out completely 5) explode Since bulb B is bypassed by the wire, the total resistance of the circuit decreases. This means that the current through bulb A increases. Follow-up: What happens to bulb B? ConcepTest 26.8a More Circuits I What happens to the voltage 1) increase across the resistor R1 when the 2) decrease switch is closed? The voltage will: 3) stay the same R1 S R3 V R2 ConcepTest 26.8a More Circuits I What happens to the voltage 1) increase across the resistor R1 when the 2) decrease switch is closed? The voltage will: 3) stay the same R1 With the switch closed, the addition of R2 to R3 decreases the equivalent S resistance, so the current from the battery increases. This will cause an R3 V increase in the voltage across R1 . Follow-up: What happens to the current through R3? R2