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ConcepTest 21.1Connect the Battery Which is the correct way to light the lightbulb with the battery? 4) all are correct 5) none are correct 1) 3) 2)

Ohms law is obeyed since the current still increases when V increases 1) Ohms law is obeyed since the current still increases when V increases Ohms law is not obeyed 2) Ohms law is not obeyed this has nothing to do with Ohms law 3) this has nothing to do with Ohms law ConcepTest 21.2Ohms Law You double the voltage across a certain conductor and you observe the current increases three times. What can you conclude?

ConcepTest 21.3Wires I Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? d A = 4 d B 1) d A = 4 d B d A = 2 d B 2) d A = 2 d B d A = d B 3) d A = d B 4) d A = 1/2 d B 5) d A = 1/4 d B

area is less arearadiussquared diameter of A must be two times less than B The resistance of wire A is greater because its area is less than wire B. Since area is related to radius (or diameter) squared, the diameter of A must be two times less than B. ConcepTest 21.3Wires I Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare? d A = 4 d B 1) d A = 4 d B d A = 2 d B 2) d A = 2 d B d A = d B 3) d A = d B 4) d A = 1/2 d B 5) d A = 1/4 d B

ConcepTest 21.4Wires II A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance? it decreasesby a factor 4 1) it decreases by a factor 4 it decreasesby a factor 2 2) it decreases by a factor 2 it stays the same 3) it stays the same 4) it increasesby a factor 2 4) it increases by a factor 2 5) it increasesby a factor 4 5) it increases by a factor 4

ConcepTest 21.5Series Resistors I 9 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? 1) 12 V 2) zero 3) 3 V 4) 4 V 5) you need to know the actual value of R

ConcepTest 21.6Series Resistors II 12 V R 1 = 4 R 2 = 2 In the circuit below, what is the voltage across ? In the circuit below, what is the voltage across R 1 ? 1) 12 V 2) zero 3) 6 V 4) 8 V 5) 4 V

ConcepTest 21.7Parallel Resistors I In the circuit below, what is the current through ? In the circuit below, what is the current through R 1 ? 10 V R 1 = 5 R 2 = 2 1) 10 A 2) zero 3) 5 A 4) 2 A 5) 7 A

ConcepTest 21.8Parallel Resistors II 1) increases 2) remains the same 3) decreases 4) drops to zero Points P and Q are connected to a battery of fixed voltage. As more resistors R are added to the parallel circuit, what happens to the total current in the circuit?

ConcepTest 21.9Short Circuit Current flows through a lightbulb. If a wire is now connected across the bulb, what happens? all the current continues to flow through the bulb 1) all the current continues to flow through the bulb half the current flows through the wire, the other half continues through the bulb 2) half the current flows through the wire, the other half continues through the bulb all the current flows through the wire 3) all the current flows through the wire none of the above 4) none of the above

ConcepTest 21.10Short Circuit II Two lightbulbs A and B are connected in series to a constant voltage source. When a wire is connected across B, bulb A will: glow brighter than before 1) glow brighter than before glow just the same as before 2) glow just the same as before glow dimmer than before 3) glow dimmer than before 4) go out completely 5) explode

ConcepTest 21.11Circuits I circuit 1 1) circuit 1 circuit 2 2) circuit 2 both the same 3) both the same it depends on R 4) it depends on R The lightbulbs in the circuit below are identical with the same resistance R. Which circuit produces more light? (brightness power)

ConcepTest 21.12Circuits II twice as much 1) twice as much the same 2) the same 1/2 as much 3) 1/2 as much 1/4 as much 4) 1/4 as much 4 times as much 5) 4 times as much 10 V A B C The three lightbulbs in the circuit all have the same resistance of 1 By how much is the brightness of bulb B greater or smaller than the brightness of bulb A? (brightness power)

ConcepTest 21.13More Circuits I increase 1) increase decrease 2) decrease stay the same 3) stay the same What happens to the voltage across the resistor R 1 when the switch is closed? The voltage will: V R1R1 R3R3 R2R2 S

ConcepTest 21.14More Circuits II increases 1) increases decreases 2) decreases stays the same 3) stays the same V R1R1 R3R3 R4R4 R2R2 S What happens to the voltage across the resistor R 4 when the switch is closed?

ConcepTest 21.15ircuits ConcepTest 21.15Even More Circuits Which resistor has the greatest current going through it? Assume that all the resistors are equal. V R1R1 R2R2 R3R3 R5R5 R4R4 1) R 1 and 2) both R 1 and R 2 equally and 3) R 3 and R 4 4) R 5 5) all the same

ConcepTest 21.16Dimmer When you rotate the knob of a light dimmer, what is being changed in the electric circuit? 1) the power 2) the current 3) the voltage 4) both (1) and (2) 5) both (2) and (3)

increases the resistancedecreases the current The voltage is provided at 120 V from the outside. The light dimmer increases the resistance and therefore decreases the current that flows through the lightbulb. ConcepTest 21.16Dimmer When you rotate the knob of a light dimmer, what is being changed in the electric circuit? 1) the power 2) the current 3) the voltage 4) both (1) and (2) 5) both (2) and (3) Follow-up: Why does the voltage not change?

ConcepTest 21.17Lightbulbs Two lightbulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance? 1) the 25 W bulb 2) the 100 W bulb 3) both have the same 4) this has nothing to do with resistance

ConcepTest 21.18Space Heaters I Two space heaters in your living room are operated at 120 V. Heater 1 has twice the resistance of heater 2. Which one will give off more heat? 1) heater 1 2) heater 2 3) both equally

ConcepTest 21.19Junction Rule ConcepTest Junction Rule 1) 2 A 2) 3 A 3) 5 A 4) 6 A 5) 10 A 5 A 8 A 2 A P What is the current in branch P?

ConcepTest 21.20Kirchhoffs Rules ConcepTest Kirchhoffs Rules The lightbulbs in the circuit are identical. When the switch is closed, what happens? 1) both bulbs go out 2) intensity of both bulbs increases 3) intensity of both bulbs decreases 4) A gets brighter and B gets dimmer 5) nothing changes

the point between the bulbs is at 12 VBut so is the point between the batteries When the switch is open, the point between the bulbs is at 12 V. But so is the point between the batteries. If there is no potential difference, then no current will flow once the switch is closed!! Thus, nothing changes. The lightbulbs in the circuit are identical. When the switch is closed, what happens? 1) both bulbs go out 2) intensity of both bulbs increases 3) intensity of both bulbs decreases 4) A gets brighter and B gets dimmer 5) nothing changes ConcepTest 21.20Kirchhoffs Rules ConcepTest Kirchhoffs Rules 24 V Follow-up: Follow-up: What happens if the bottom battery is replaced by a 24 V battery?

ConcepTest 21.21Wheatstone Bridge 1) I 2) I/2 3) I/3 4) I/4 5) zero An ammeter A is connected between points a and b in the circuit below, in which the four resistors are identical. The current through the ammeter is: I Vab

resistors are identical voltage drops are the same potentialsab same Since all resistors are identical, the voltage drops are the same across the upper branch and the lower branch. Thus, the potentials at points a and b are also the same. Therefore, no current flows. ConcepTest 21.21Wheatstone Bridge 1) I 2) I/2 3) I/3 4) I/4 5) zero An ammeter A is connected between points a and b in the circuit below, in which the four resistors are identical. The current through the ammeter is: I Vab

ConcepTest 21.22Kirchhoffs Rules ConcepTest More Kirchhoffs Rules 2 V 2 6 V 4 V I1I1 I3I3 I2I2 Which of the equations is valid for the circuit below? 1) 2 – I 1 – 2I 2 = 0 2) 2 – 2I 1 – 2I 2 – 4I 3 = 0 3) 2 – I 1 – 4 – 2I 2 = 0 4) I 3 – 4 – 2I = 0 5) 2 – I 1 – 3I 3 – 6 = 0

ConcepTest 21.22Kirchhoffs Rules ConcepTest More Kirchhoffs Rules 2 V 2 6 V 4 V I1I1 I3I3 I2I2 Eqn. 3 is valid for the left loop Eqn. 3 is valid for the left loop: The left battery gives +2V, then there is a drop through a 1 resistor with current I 1 flowing. Then we go through the middle battery (but from + to – !), which gives –4V. Finally, there is a drop through a 2 resistor with current I 2. Which of the equations is valid for the circuit below? 1) 2 – I 1 – 2I 2 = 0 2) 2 – 2I 1 – 2I 2 – 4I 3 = 0 3) 2 – I 1 – 4 – 2I 2 = 0 4) I 3 – 4 – 2I = 0 5) 2 – I 1 – 3I 3 – 6 = 0

ConcepTest 21.23Capacitors I ConcepTest Capacitors I o o C C C C eq 1) C eq = 3/2 C 2) C eq = 2/3 C 3) C eq = 3 C 4) C eq = 1/3 C 5) C eq = 1/2 C What is the equivalent capacitance,, of the combination below? What is the equivalent capacitance, C eq, of the combination below?

ConcepTest 21.24Capacitors II ConcepTest Capacitors II 1) V 1 = V 2 2) V 1 > V 2 3) V 1 < V 2 4) all voltages are zero C 1 = 1.0 F C 3 = 1.0 F C 2 = 1.0 F 10 V How does the voltage V 1 across the first capacitor (C 1 ) compare to the voltage V 2 across the second capacitor (C 2 )?

ConcepTest 21.25Capacitors III ConcepTest Capacitors III C 1 = 1.0 F C 3 = 1.0 F C 2 = 1.0 F 10 V 1) Q 1 = Q 2 2) Q 1 > Q 2 3) Q 1 < Q 2 4) all charges are zero How does the charge Q 1 on the first capacitor (C 1 ) compare to the charge Q 2 on the second capacitor (C 2 )?