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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)

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Presentation on theme: "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)"— Presentation transcript:

1 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)

2 continuous connection Current can only flow if there is a continuous connection from the negative terminal through the bulb to the positive terminal. This is only the case for Fig. (3). 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)

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

4 Ohm’s law is obeyed since the current still increases when V increases 1) Ohm’s law is obeyed since the current still increases when V increases Ohm’s law is not obeyed 2) Ohm’s law is not obeyed this has nothing to do with Ohm’s law 3) this has nothing to do with Ohm’s law V = I R linear Ohm’s law, V = I R, states that the relationship between voltage and current is linear. Thus for a conductor that obeys Ohm’s Law, the current must double when you double the voltage. ConcepTest 21.2Ohm’s Law You double the voltage across a certain conductor and you observe the current increases three times. What can you conclude? Follow-up: Where could this situation occur?

5 ConcepTest 21.3aWires 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

6 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 21.3aWires 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

7 ConcepTest 21.3bWires 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

8 doubledhalved Keeping the volume (= area x length) constant means that if the length is doubled, the area is halved. four Since, this increases the resistance by four. ConcepTest 21.3bWires 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

9 ConcepTest 21.4aSeries 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

10 equal evenly 3 V drop 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. ConcepTest 21.4aSeries 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 Follow-up: What would be the potential difference if R=  Follow-up: What would be the potential difference if R= 1 

11 ConcepTest 21.4bSeries 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

12 ConcepTest 21.4bSeries 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 The voltage drop across R 1 has to be twice as big as the drop across R 2.V 1 = 8 V The voltage drop across R 1 has to be twice as big as the drop across R 2. This means that V 1 = 8 V and V 2 = 4 V. Or else you could find the current I = V/R = (12 V)/(6  = 2 A, then use Ohm’s Law to get voltages. Follow-up: What happens if the voltage is doubled?

13 ConcepTest 21.5aParallel 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

14 voltagesame V 1 = I 1 R 1 I 1 = 2 A The voltage is the same (10 V) across each resistor because they are in parallel. Thus, we can use Ohm’s Law, V 1 = I 1 R 1 to find the current I 1 = 2 A. ConcepTest 21.5aParallel 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 Follow-up: What is the total current through the battery?

15 ConcepTest 21.5bParallel 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?

16 ConcepTest 21.5bParallel Resistors II 1) increases 2) remains the same 3) decreases 4) drops to zero resistance of the circuit drops resistance decreasescurrent must increase As we add parallel resistors, the overall resistance of the circuit drops. Since V = IR, and V is held constant by the battery, when resistance decreases, the current must increase. 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? Follow-up: What happens to the current through each resistor?

17 ConcepTest 21.6aShort 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

18 zeroALL The current divides based on the ratio of the resistances. If one of the resistances is zero, then ALL of the current will flow through that path. ConcepTest 21.6aShort 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 Follow-up: Doesn’t the wire have SOME resistance?

19 ConcepTest 21.6bShort 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

20 total resistance of the circuit decreasescurrent through bulb A increases Since bulb B is bypassed by the wire, the total resistance of the circuit decreases. This means that the current through bulb A increases. ConcepTest 21.6bShort 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 Follow-up: What happens to bulb B?

21 ConcepTest 21.7aCircuits 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)

22 ConcepTest 21.7aCircuits 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) parallel lowering the total resistance #1 willdraw a higher current P = I V In #1, the bulbs are in parallel, lowering the total resistance of the circuit. Thus, circuit #1 will draw a higher current, which leads to more light, because P = I V.

23 ConcepTest 21.7bCircuits 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)

24 ConcepTest 21.7bCircuits 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 We can use P = V 2 /R to compare the power: P A = (= 100 W P A = (V A ) 2 /R A = (10 V) 2 /1  = 100 W P B = (= 25 W P B = (V B ) 2 /R B = (5 V) 2 /1  = 25 W 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) Follow-up: What is the total current in the circuit?

25 ConcepTest 21.8aMore 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

26 ConcepTest 21.8aMore 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: decreases the equivalent resistancecurrent from the battery increases increase in the voltage across R 1 With the switch closed, the addition of R 2 to R 3 decreases the equivalent resistance, so the current from the battery increases. This will cause an increase in the voltage across R 1. V R1R1 R3R3 R2R2 S Follow-up: ? Follow-up: What happens to the current through R 3 ?

27 ConcepTest 21.8bMore 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?

28 V R1R1 R3R3 R4R4 R2R2 S A B C increase in the voltage across R 1 V AB constant V AB increasesV BC must decrease We just saw that closing the switch causes an increase in the voltage across R 1 (which is V AB ). The voltage of the battery is constant, so if V AB increases, then V BC must decrease! What happens to the voltage across the resistor R 4 when the switch is closed? increases 1) increases decreases 2) decreases stays the same 3) stays the same ConcepTest 21.8bMore Circuits II Follow-up: ? Follow-up: What happens to the current through R 4 ?

29 ConcepTest 21.9ircuits ConcepTest 21.9Even 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

30 I 1 = I 2 R 5 R 3 + R 4 branch containing R 5 has less resistance The same current must flow through left and right combinations of resistors. On the LEFT, the current splits equally, so I 1 = I 2. On the RIGHT, more current will go through R 5 than R 3 + R 4 since the branch containing R 5 has less resistance. ConcepTest 21.9ircuits ConcepTest 21.9Even More Circuits 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 Which resistor has the greatest current going through it? Assume that all the resistors are equal. V R1R1 R2R2 R3R3 R5R5 R4R4 Follow-up: ? Follow-up: Which one has the smallest voltage drop?

31 ConcepTest 21.10Dimmer 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)

32 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 21.10Dimmer 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?

33 ConcepTest 21.11aLightbulbs 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

34 P = V 2 / R, lower power ratinghigher resistance Since P = V 2 / R, the bulb with the lower power rating has to have the higher resistance. ConcepTest 21.11aLightbulbs 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 Follow-up: Which one carries the greater current?

35 ConcepTest 21.11bSpace 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

36 P = V 2 / R, smaller resistance larger power Using P = V 2 / R, the heater with the smaller resistance will have the larger power output. Thus, heater 2 will give off more heat. ConcepTest 21.11bSpace 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 Follow-up: Which one carries the greater current?

37 ConcepTest 21.12Junction Rule ConcepTest 21.12 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?

38 red One exiting branch has 2 A other branch (at P) must have 6 A The current entering the junction in red is 8 A, so the current leaving must also be 8 A. One exiting branch has 2 A, so the other branch (at P) must have 6 A. 5 A 8 A 2 A P junction 6 A S 1) 2 A 2) 3 A 3) 5 A 4) 6 A 5) 10 A What is the current in branch P? ConcepTest 21.12Junction Rule

39 ConcepTest 21.13Kirchhoff’s Rules ConcepTest 21.13 Kirchhoff’s 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

40 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 21.13Kirchhoff’s Rules ConcepTest 21.13 Kirchhoff’s Rules 24 V Follow-up: Follow-up: What happens if the bottom battery is replaced by a 24 V battery?

41 ConcepTest 21.14Wheatstone 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

42 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 21.14Wheatstone 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

43 ConcepTest 21.15Kirchhoff’s Rules ConcepTest 21.15 More Kirchhoff’s Rules 2 V 2  2 V 6 V 4 V 3  1  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 2 + 6 = 0 5) 2 – I 1 – 3I 3 – 6 = 0

44 ConcepTest 21.15Kirchhoff’s Rules ConcepTest 21.15 More Kirchhoff’s Rules 2 V 2  2 V 6 V 4 V 3  1  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 2 + 6 = 0 5) 2 – I 1 – 3I 3 – 6 = 0

45 ConcepTest 21.16aCapacitors I ConcepTest 21.16a 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?

46 series inverses1/2 C parallel directly 3/2 C The 2 equal capacitors in series add up as inverses, giving 1/2 C. These are parallel to the first one, which add up directly. Thus, the total equivalent capacitance is 3/2 C. ConcepTest 21.16aCapacitors I ConcepTest 21.16a 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?

47 ConcepTest 21.16bCapacitors II ConcepTest 21.16b 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 )?

48 ConcepTest 21.16bCapacitors II ConcepTest 21.16b 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 The voltage across C 1 is 10 V. The combined capacitors C 2 +C 3 are parallel to C 1. The voltage across C 2 +C 3 is also 10 V. Since C 2 and C 3 are in series, their voltages add. Thus the voltage across C 2 and C 3 each has to be 5 V, which is less than V 1. How does the voltage V 1 across the first capacitor (C 1 ) compare to the voltage V 2 across the second capacitor (C 2 )? Follow-up: ? Follow-up: What is the current in this circuit?

49 ConcepTest 21.16cCapacitors III ConcepTest 21.16c 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 )?

50 ConcepTest 21.16cCapacitors III ConcepTest 21.16c Capacitors III C 1 = 1.0  F C 3 = 1.0  F C 2 = 1.0  F 10 V Q = CVsame since V 1 > V 2 Q 1 > Q 2 We already know that the voltage across C 1 is 10 V and the voltage across both C 2 and C 3 is 5 V each. Since Q = CV and C is the same for all the capacitors, then since V 1 > V 2 therefore Q 1 > Q 2. 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 )?


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