ConcepTest PowerPoints Chapter 19 Physics: Principles with Applications, 6th edition Giancoli © 2005 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. ConcepTest 19.1a 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 9V ConcepTest 19.1a 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 19.1b Series Resistors II 1) 12 V In the circuit below, what is the 2) zero voltage across R1? 3) 6 V 4) 8 V 5) 4 V R1= 4 W R2= 2 W 12 V ConcepTest 19.1b Series Resistors II 1) 12 V In the circuit below, what is the 2) zero voltage across R1? 3) 6 V 4) 8 V 5) 4 V The voltage drop across R1 has to be twice as big as the drop across R2. This means that V1 = R1= 4 W R2= 2 W 8 V and V2 = 4 V. Or else you could find the current I = V/R = (12 V)/(6 W) = 2 A, then use 12 V Ohm’s Law to get voltages. Follow-up: What happens if the voltage is doubled? ConcepTest 19.2a Parallel Resistors I 1) 10 A In the circuit below, what is the 2) zero current through R1? 3) 5 A 4) 2 A 5) 7 A R2= 2 W R1= 5 W 10 V ConcepTest 19.2a Parallel Resistors I 1) 10 A In the circuit below, what is the 2) zero current through R1? 3) 5 A 4) 2 A 5) 7 A The voltage is the same (10 V) across each R2= 2 W resistor because they are in parallel. Thus, we can use Ohm’s Law, V1 = I1 R1 to find the R1= 5 W current I1 = 2 A. 10 V Follow-up: What is the total current through the battery? ConcepTest 19.2b Points P and Q are connected to a Parallel Resistors II 1) increases battery of fixed voltage. As more 2) remains the same resistors R are added to the parallel 3) decreases circuit, what happens to the total 4) drops to zero current in the circuit? ConcepTest 19.2b Parallel Resistors II Points P and Q are connected to a 1) increases battery of fixed voltage. As more 2) remains the same resistors R are added to the parallel 3) decreases circuit, what happens to the total 4) drops to zero current in the circuit? 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. Follow-up: What happens to the current through each resistor? ConcepTest 19.3a Current flows through a Short Circuit 1) all the current continues to flow through the bulb connected across the 2) half the current flows through the wire, the other half continues through the bulb bulb, what happens? 3) all the current flows through the wire lightbulb. If a wire is now 4) none of the above ConcepTest 19.3a Current flows through a Short Circuit 1) all the current continues to flow through the bulb connected across the 2) half the current flows through the wire, the other half continues through the bulb bulb, what happens? 3) all the current flows through the wire lightbulb. If a wire is now 4) none of the above 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. Follow-up: Doesn’t the wire have SOME resistance? ConcepTest 19.4a Circuits I The lightbulbs in the circuit below 1) circuit 1 are identical with the same 2) circuit 2 resistance R. Which circuit produces more light? (brightness power) 3) both the same 4) it depends on R ConcepTest 19.4a Circuits I The lightbulbs in the circuit below 1) circuit 1 are identical with the same 2) circuit 2 resistance R. Which circuit produces more light? (brightness power) 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. 3) both the same 4) it depends on R ConcepTest 19.4b The three lightbulbs in the circuit all have Circuits II 1) twice as much the same resistance of 1 W . By how 2) the same much is the brightness of bulb B greater 3) 1/2 as much or smaller than the brightness of bulb A? (brightness power) 4) 1/4 as much 5) 4 times as much A C B 10 V ConcepTest 19.4b The three light bulbs in the circuit all have Circuits II 1) twice as much the same resistance of 1 W . By how 2) the same much is the brightness of bulb B greater 3) 1/2 as much or smaller than the brightness of bulb A? (brightness power) 4) 1/4 as much 5) 4 times as much A We can use P = V2/R to compare the power: C B PA = (VA)2/RA = (10 V) 2/1 W = 100 W PB = (VB)2/RB = (5 V) 2/1 W = 25 W Follow-up: What is the total current in the circuit? 10 V ConcepTest 19.5a 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 19.5a 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 ConcepTest 19.5b More Circuits II 1) increases What happens to the voltage across the resistor R4 when the 2) decreases switch is closed? 3) stays the same R1 S R3 V R2 R4 ConcepTest 19.5b More Circuits II 1) increases What happens to the voltage across the resistor R4 when the 2) decreases switch is closed? 3) stays the same We just saw that closing the switch causes an increase in the voltage across R1 (which is VAB). The voltage of the battery is constant, so if VAB increases, then VBC must A R1 B S R3 V R2 decrease! Follow-up: What happens to the current through R4? C R4 ConcepTest 19.6 Even More Circuits 1) R1 Which resistor has the 2) both R1 and R2 equally greatest current going through it? Assume that all 3) R3 and R4 the resistors are equal. 4) R5 5) all the same V ConcepTest 19.6 Even More Circuits 1) R1 Which resistor has the 2) both R1 and R2 equally greatest current going through it? Assume that all 3) R3 and R4 the resistors are equal. 4) R5 5) all the same The same current must flow through left and right combinations of resistors. On the LEFT, the current splits equally, so I1 = I2. On the RIGHT, more current will go through R5 than R3 + R4 since the branch containing R5 has less resistance. V Follow-up: Which one has the smallest voltage drop? ConcepTest 19.7 Junction Rule 1) 2 A What is the current in branch P? 2) 3 A 3) 5 A 4) 6 A 5) 10 A 5A P 8A 2A ConcepTest 19.7 Junction Rule 1) 2 A 2) 3 A What is the current in branch P? 3) 5 A 4) 6 A 5) 10 A The current entering the junction S 5A 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. P 8A junction 2A 6A ConcepTest 19.8 Kirchhoff’s Rules The lightbulbs in the 1) both bulbs go out circuit are identical. When 2) intensity of both bulbs increases the switch is closed, what 3) intensity of both bulbs decreases happens? 4) A gets brighter and B gets dimmer 5) nothing changes ConcepTest 19.8 Kirchhoff’s Rules The lightbulbs in the 1) both bulbs go out circuit are identical. When 2) intensity of both bulbs increases the switch is closed, what 3) intensity of both bulbs decreases happens? 4) A gets brighter and B gets dimmer 5) nothing changes 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. Follow-up: What happens if the bottom battery is replaced by a 24 V battery? 24 V ConcepTest 19.9 Wheatstone Bridge An ammeter A is connected 1) I between points a and b in the 2) I/2 circuit below, in which the four 3) I/3 resistors are identical. The current 4) I/4 through the ammeter is: 5) zero a b V I ConcepTest 19.9 Wheatstone Bridge An ammeter A is connected 1) I between points a and b in the 2) I/2 circuit below, in which the four 3) I/3 resistors are identical. The current 4) I/4 through the ammeter is: 5) zero Since all resistors are identical, a the voltage drops are the same across the upper branch and the lower branch. Thus, the potentials at points a and b are b also the same. Therefore, no current flows. V I ConcepTest 19.10 More Kirchhoff’s Rules 1) 2 – I1 – 2I2 = 0 Which of the equations is valid 2) 2 – 2I1 – 2I2 – 4I3 = 0 for the circuit below? 3) 2 – I1 – 4 – 2I2 = 0 4) I3 – 4 – 2I2 + 6 = 0 5) 2 – I1 – 3I3 – 6 = 0 1W I2 2W 6V 22 VV 4V I1 1W I3 3W ConcepTest 19.10 More Kirchhoff’s Rules 1) 2 – I1 – 2I2 = 0 Which of the equations is valid 2) 2 – 2I1 – 2I2 – 4I3 = 0 for the circuit below? 3) 2 – I1 – 4 – 2I2 = 0 4) I3 – 4 – 2I2 + 6 = 0 5) 2 – I1 – 3I3 – 6 = 0 Eqn. 3 is valid for the left loop: The left battery gives +2V, then there is a drop through a 1W resistor with current I1 flowing. Then we go through the middle battery (but from + to – !), which gives –4V. Finally, there is a drop through a 2W resistor with current I2. 1W I2 2W 6V 22 VV 4V I1 1W I3 3W ConcepTest 19.11a Capacitors I 1) Ceq = 3/2 C What is the equivalent capacitance, 2) Ceq = 2/3 C Ceq , of the combination below? 3) Ceq = 3 C 4) Ceq = 1/3 C 5) Ceq = 1/2 C o Ceq o C C C ConcepTest 19.11a Capacitors I 1) Ceq = 3/2 C What is the equivalent capacitance, 2) Ceq = 2/3 C Ceq , of the combination below? 3) Ceq = 3 C 4) Ceq = 1/3 C 5) Ceq = 1/2 C The 2 equal capacitors in series add o up as inverses, giving 1/2 C. These are parallel to the first one, which Ceq add up directly. Thus, the total equivalent capacitance is 3/2 C. o C C C ConcepTest 19.11b Capacitors II How does the voltage V1 across 1) V1 = V2 the first capacitor (C1) compare 2) V1 > V2 to the voltage V2 across the 3) V1 < V2 second capacitor (C2)? 4) all voltages are zero C2 = 1.0 mF 10 V C1 = 1.0 mF C3 = 1.0 mF ConcepTest 19.11b Capacitors II How does the voltage V1 across 1) V1 = V2 the first capacitor (C1) compare 2) V1 > V2 to the voltage V2 across the 3) V1 < V2 second capacitor (C2)? 4) all voltages are zero The voltage across C1 is 10 V. The combined capacitors C2+C3 are parallel to C1. The voltage across C2+C3 is also 10 V. Since C2 and C3 are in series, their voltages add. Thus the voltage across C2 and C3 each has to be 5 V, which is less than V1. C2 = 1.0 mF 10 V C1 = 1.0 mF C3 = 1.0 mF Follow-up: What is the current in this circuit? ConcepTest 19.11c How does the charge Q1 on the first Capacitors III 1) Q1 = Q2 2) Q1 > Q2 capacitor (C1) compare to the charge Q2 on the second capacitor 3) Q1 < Q2 (C2)? 4) all charges are zero C2 = 1.0 mF 10 V C1 = 1.0 mF C3 = 1.0 mF ConcepTest 19.11c How does the charge Q1 on the first Capacitors III 1) Q1 = Q2 2) Q1 > Q2 capacitor (C1) compare to the charge Q2 on the second capacitor 3) Q1 < Q2 (C2)? 4) all charges are zero We already know that the C2 = 1.0 mF voltage across C1 is 10 V and the voltage across C2 and C3 each is 5 V. Since Q = CV and C is the same for all the capacitors, then since V1 > V2 therefore Q1 > Q2. 10 V C1 = 1.0 mF C3 = 1.0 mF