1 Physics of Circuits Created for BW Physics Middle School Science Teachers By Dick Heckathorn 23...
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Transcript of 1 Physics of Circuits Created for BW Physics Middle School Science Teachers By Dick Heckathorn 23...
1
Physics of Circuits
Created for BW PhysicsMiddle School Science Teachers
ByDick Heckathorn23 April 2K + 8
3
Table of Contents
4A. Finding Total or Equivalent Resistance of Resistors in Series12B. Finding Total or Equivalent Resistance of Resistors in Parallel19C. Circuit Analysis One Resistor 29D. Circuit Analysis Two Resistors Connected in Series38E. Circuit Analysis Two Resistors Connected in Parallel49 Final Summary with Some Problems64 Connecting a 3-Way Switch
6
R2 + R_______
2. Connect two of the resistors end to end (in series) and measure the resistance across each pair.
R1 + R2
_______R1 + R3
_______
7
3. A new resistor equal to the sum of the two resistors can replace the two resistors and has the same effect in a circuit as the two resistors.
We call the sum, an equivalent resistance or a total resistance.
8
4. Develop a rule from which you can predict the equivalent resistance of two resistors in series.
R1 + R2 R1 + R3
R2 + R3
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5. Connect all three resistors end to end and measure the resistance across all three.
R1 + R2 + R3
_____________
10
6. Does your rule for three resistors connected in series the same as for two resistors connected in series?
Yes ___ No ___
11
7. Your instructor hopes that you found the following to be true.
Remember this is for
ONLY resistors in series!
......321 RRRRT
13
1. Record the resistance of the three resistors that you measured before.
R1
_________
R2
_________
R3
_________
14
2. Connect two of the resistors as shown (in parallel) and measure the resistance across each end.
R1 : R2
_________
R1 : R3
_________
R2 : R3
_________
16
4. Connect all three resistors as shown, then measure the resistance across all three.
R1 : R2 : R3
____________
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5. Is the rule for finding the equivalent resistance of three resistors connected in parallel the same for finding the equivalent resistance of two resistors connected in parallel?
Yes ___ No ___
20
A
R
1. Measure the value of a resistor. R = ___ Set the ammeter to its greatest value. Then construct the following circuit.
22
VA
VRVB
Conclusion(s)
A
Compare VB with VA and VR and
I
R
VB, I, and R?
I?
Compare VB with VA and VR and VB, I, and R?
23
6. Divide the voltage (VB) by the resistance (R).
7. How does VB/R compare to the measured current?
Be sure to keep track of units.
____________________toCompares
__________R
VB
25
VRVB R
VB = _______ VR = _______
9. Connect and measure the voltage across the battery and then the resistor. Record the values.
27
10. Calculate:
__________R
V__________
R
V RR 2
2
1
1
How do they compare?
____________________tocomparesThey
28
11. What does your instructor say about this?
Your instructor says that VB and VR should be the same as the energy per charge given to the electrons by the battery VB and then removed by the resistor VR.
30
1. Measure the value of two resistors.
R1 = _________ R2 = _________
Then construct the following circuit.
32
3. Measure and record the voltage across the battery and both of the resistances.
VB = _______
VB
V1
V2
AI
R1
R2
VR! = _______ VR2 = _______
33
4. How does VB compare to VR1 and VR2?
5. Calculate VR1/R1 and VR2/R2
______ ______
What do these values compare to?
34
5. Calculate both VR1/R1 and VR2/R2.
__________R
V__________
R
V RR 2
2
1
1
What do these values compare to?
How do they compare?
35
6. Write a conclusion about the current and voltage in a circuit when two resistors are connected in series with the battery.
21
21
RRB
RRT
VVV
III
36
7. What does your instructor say about this?
Resistance in Series
1. Current (I) in the circuit is everywhere the same.
2. Potential difference (VB) supplied by the battery equals the sum of the potential difference (V1+V2) lost in the components connected in series.
39
1. Measure the value of both resistor.
R1 = _______ R2 = _______
Then construct the following circuit.
41
A2A1
R1 R2
I1 I2
3. If you have access to two meters, create the circuit below and measure the current through both resistors. Otherwise, hook up A1 measure the current I1 and then repeat for A2.
43
AR2A1
R1 R2
IR1 IR2
IT = _______ IR1 = _______ IR2 = _______
4. How is the does the current through each meter, IR1 and IR2 compare to the total current IT?
45
6. Measure and record the voltage across the battery VB, and the voltage across the two resistors VR1 and VR2
VB _______ VR1 _______ VR2 _______
VB
VR1 VR2R1 R2
46?relatedtheyareHow
?representvaluesthedoWhat
______R
V______
R
V______
R
V
R
R
R
R
T
B 2
2
1
1
7. How is the voltage of the battery VB related to the voltage through each resistor VR1 and VR2?
VB _______ VR1 _______ VR2 _______
Calculate the following:
47
11. Write a conclusion about the current and voltage in a circuit where 2 resistors are connected in parallel with the battery.
21
21
RRB
RRT
VVV
III
48
Summary
1. Current (I) from the battery equals the sum of the currents (I1 + I2) through the separate resistances.
2. Potential difference (VB) supplied by the battery equals the potential difference (V1 = V2) lost in the resistances connected in parallel.
50
Summary
What governs the amount of electric potential energy an electron will lose in each load?
The conservation of energy. The amount gained is equal to the sum of the total energy lost.
51
Summary
What governs the number of electrons that will take each path?
The conservation of charge. There is no gain or loss of electrons nor any accumulation at any point.
52
Series Circuit
321T I I I I
321T V V V V
RIV
R1 R2 R3
VTV1 V2 V3
332211 RIRIRIRI TT
321 RRRRT
onConservatiEnergy
onConservatieCharg
54
Resistance in Series
Three 30-Ω light bulbs and two 20-Ω heating elements
connected in series
The equivalent resistance is ….
130 ohms
55
Resistance in Series
two strings of Christmas tree lights connected in series, if the
1st string has eight 4-Ω bulbs & the 2nd has twelve 3-Ω bulbs
The equivalent resistance is ….
68 ohms
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Resistance in Series
a 20-Ω, a 18-Ω, and an unknown resistor are connected in series to give
an equivalent resistance of 64-Ω
26 ohms
Find the value of the unknown resistance in the following:
57
Parallel Circuit
onConservatieCharg 321T I I I I
R
VI
R1
R2
R3
T
T
R
V
1
1
R
V
2
2
R
V
3
3
R
V
TR
1
1
1
R 2
1
R
3
1
R
321T V V V V onConservatiEnergy
V1
V2
V3
VT
I1
I2
I3
IT
58
321
1111
RRRRT
Find the equivalent resistance when a 4-Ω and an 8-Ω bulb are
connected in parallel.
2.7 ohm
Resistance in Parallel
59
5.3 ohm
Resistance in Parallel
Find the equivalent resistance when a 16-Ω and an 8-Ω bulb are connected in parallel.
60
2.9 ohm
Resistance in Parallel
Find the equivalent resistance when a 20-Ω, a 10- Ω and a 5-Ω bulb are connected in parallel.
61
What resistance would have to be added in parallel with a 40- Ω hair dryer to reduce the equivalent resistance to 8- Ω?
10 ohm
Resistance in Parallel
62
30 Ω
Resistance in Parallel
Find the equivalent resistance of two, three, four, and five 60-Ω bulb are connected in parallel.
20 Ω 15 Ω 12 Ω
Is there a simple relationship for the equivalent resistance of ‘n’ resistors in parallel? R/n