Lesson 3:
1)
[llel+llel]
Parallel Circuits
Connect the Circuit Picture with lines for wires using the Schematic Diagram Locate the junction indicated
with the large back dot on the Schematic Diagram with a large back dot the Circuit Picture.
(A)
AM M ETER
LOW
VOLTAGE
POWER SUPPLY
(B)
AM M ETER
(C)
AM M ETER
AC
ON
OFF
VOLTAGE
ADJUST
-
DC
+
0 -6 V 5 A MAX.
VOLTM ETER
12
Circuit Picture
Schematic Diagram
13
2)
When bulbs (or anything else) are connected side by side so that the current must divide at branch points,
we call it a parallel circuit.
3)
In the above circuit, if the voltage (potential) gain across the source is 1 Volt, what does our model predict
about the voltage drop across the light bulb will be? Explain.
All model predicts that the voltage (potential) drop across the light bulb will be 1 Volt. The Coulombs
have to fall this distance to return to the battery.
4)
What does our model predict the voltage drop across the other light bulb will be? Explain.
All model predicts that the voltage (potential) drop across the light bulb will be 1 Volt. The Coulombs
have to fall this distance to return to the battery. The other light bulb in parallel makes no difference
to the distance the Coulombs have to fall.
5)
If the voltage drop across the light bulbs is the same, what does our model predict the current through the
bulbs will be? Explain.
If the light bulbs are the same and the Coulombs give up the same amount of energy as they fall
through the light bulbs, then the current will stay the same.
6)
Assume both bulbs are identical. In the above circuit, if the electric current through Ammeter (C) is one
coulomb per second, then the current through Ammeter (B) will be _ one _ coulomb per second. What does
our model predict about the electric current through Ammeter (A)? Explain.
Because the light bulbs are identical to current through ammeters (B) and ammeter (C) will be the
same. If both light bulbs and one Coulomb per second then, every second two Coulombs must be
supplied through ammeter (A)
7)
If more bulbs are connected in parallel, what does our model predict about the brightness of each bulb?
Explain
Our model predicts the brightness of each light bulb will be the same. This is because the amount of
energy each Coulomb loses as it falls to the bulb stays the same.
8)
If more bulbs are connected in parallel, what does our model predict about the current in the main circuit
[Ammeter (A)]?
The current in the main circuit will increase as it supplies Coulombs each second to more and more
light bulbs.
9)
If more bulbs are connected in parallel, what does our model predict about the current in the branches
[Ammeter (B) and Ammeter (C) or any additional ammeter ]?
The current through each light bulbs stays the same because the Coulombs are falling the same
distance (losing the same amount of energy) through identical light bulbs.
10) As more bulbs are connected in parallel, what will happen to the number of paths for the current to flow?
The number of paths increase because each additional identical light bulb adds another path for the
Coulombs to flow.
11) In a parallel circuit, if one light bulb burns out, what does our model predict will happen to
A) the current in the main circuit [Ammeter (A)]?
The main branch current will decrease .
B) the current in the branch circuits [Ammeter (B) or (C)]?
The current in the branch circuits will stay the same. They are unaffected by the other branches.
C) the brightness of the remaining bulbs?
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The brightness of the remaining bulbs will say the same because the current stays the same.
12) Is there ever any parallel circuit where one branch gets all the current and the other gets none ?"
Hard question. -- The other branch could be an open switch, a broken wire, a really high resistance
in one of the branches or the Voltmeter!
More Practice Connecting Circuits from Schematic Diagrams
13)
a) Label all the points indicated on the schematic diagram on the circuit picture.
b) Assume power supply produces 6 amperes and 12 volts, write the expected readings beside the each
meter
(B)
(A)
AMMETER
LOW
VOLTAGE
POWER SUPPLY
3 Amps
A1
(D)
(D)
(C)
AMMETER
3 Amps
AC
ON
AT
(B)
6 Amps
6 Amps
-
VOLTAGE
ADJUST
OFF
DC
(C)
V2
VOLTMETER
+
0 -6 V 5 A MAX.
volts
12
(A)
12 volts
(E)
(E)
14) a) Label all the points indicated on the circuit picture on the schematic diagram.
b) Assume power supply produces 6 amperes and 12 volts, write the expected readings beside the each
meter
6 Amps
(B)
(B)
AMMETER
(E)
(C)
LOW
VOLTAGE
POWER SUPPLY
V2
OFF
VOLTAGE
ADJUST
-
DC
0 -6 V 5 A MAX.
(E)
AMMETER
3 Amps
AC
ON
(C)
(A)
12
volts
A1
3
Amps
AT
6
Amps
(A)
(D)
VOLTMETER
+
12 volts
(D)
15) a) On the following three sets of diagrams, show where the Ammeters wired in the Circuit Picture, are on the
Schematic Diagram THIS QUESTION IS DESIGNED TO INTRODUCE A VERY COMMON WIRING
ERROR BEFORE STUDENTS ACTUALLY WIRE IT
15
b) Assume power supply produces 6 amperes and 12 volts, write the expected readings beside the each
6 Amps
A1
3 Amps
AMMETER
6
Amps
AT
3 Amps
AMMETER
12
volts
LO W
V O LTA G E
PO W ERSU PPLY
V
VOLTMETER
AC
ON
DC
-
VOLTAGE
ADJ UST
OF F
12 volts
+
0 -6 V 5 A M AX.
meter
6 Amps
6 Amps
A1
AMMETER
6
Amps
AMMETER
LO W
V O LTA G E
PO W ERSU PPLY
V
12
volts
AT
6 Amps
VOLTMETER
AC
ON
OF F
VOLTAGE
ADJ UST
-
DC
+
12 volts
0 -6 V 5 A M AX.
16
16) a) Draw lines to represent conductors on the Circuit Picture in order to complete the circuits shown the
Schematic Diagram.
b) Assume power supply produces 6 amperes and 12 volts, write the expected readings beside the each
meter
6 Amps
VOLTMETER
2 Amps
12 volts
AT
A1
Amps
2
AMMETER
Amps
6
AMMETER
12 volts
LOW
VOLTAGE
POWER SUPPLY
AC
ON
VOL T AGE
ADJUS T
OF F
-
DC
V
+
0-6V 5A MAX.
17) a) Sometimes its hard to physically place a meter in the same relative position (where it should be) Draw
lines to represent conductors on the Circuit Picture in order to complete the circuits shown the Schematic
Diagram.
b) Assume power supply produces 6 amperes and 12 volts, write the expected readings beside the each
meter
6 Amps
Amps
6
AT
A2
12 volts
VOLTMETER
AMMETER
12 volts
2 Amps
AMMETER
2 Amps
LOW
VOLTAGE
POWER SUPPLY
V1
AC
ON
OF F
VOL T AGE
ADJUS T
-
DC
+
0-6V 5A MAX.
17
REALITY CHECK - Wiring Real Circuits
Wire the parallel circuits shown. Record the meter readings and
compare bulb brightness. Note all meters do not have to be connected at once. Connect the
meter in one particular position, take the reading, remove it and reconnect it in the next position.
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AT
A1
VT
VT
V1
example 10 V
Voltage
(Volts)
Schematic Diagram
A1
VT
A2
V1
1.34 A
A1
0.67 A
V2
A2
0.67 A
VT
AT
2.01 A
A1
0.67 A
A2
0.67 A
A3
0.67 A
V2
V1
AT
VT
A1
A2
V1
A3
V2
V1
V3
V2
V3
If one bulb is
turned out:
Normal
Brightness
AT
VT
Bulb
Brightness
0.67 A
0.67 A
example 10 V
AT
AT
A1
example 10 V
V1
Current
(Amperes)
Same
Brightness
compared to
the Normal
The other
bulb
Stays On
Same
Brightness
compared to
the Normal
The other
bulbs
Stay On
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Use the results from your table to answer the following;
1)
In any parallel circuit, what can you say about the voltage drop across any bulb is about equal as the voltage
gain across the source?
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2)
In a circuit containing
- ONE bulb, the current through the source is about = the current through the bulb.
- TWO identical bulbs, the current through the source is about 2X the current through
each bulb.
- THREE identical bulbs, the current through the source is about 3X the current through
each bulb.
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22
Circle
for increase, for decrease, for unchanged, and 0 for goes to zero
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3)
Look at all three circuits;
  
0
B) the current in the main circuit 
  0
C) the current in the branches   
0
D) the number of paths for the current 
  0
A) the brightness of each bulb
As more bulbs are connected in parallel;
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4)
In a parallel circuit, if one light bulb burns out,
   0
B) the brightness of the remaining bulbs   
A) the current in the main branch
0
5)
Voltmeters are connected in parallel to the rest of the circuit.
6)
A student has used the schematic diagram below to wire the circuit picture shown. The ammeter (A T) at the
source reads 6 Amps. The reading on ammeter (A1) should be 2 Amps. When the switch is closed,
everything works as it should but the reading on the ammeter (A1) is three times what it should be. What is
wrong?
Bold wire was on wrong
side of the Ammeter. The
correction is shown
A1 was really
wired here
A1
AT
AMMETER
A1
AMMETER
LO W
V O LTA G E
PO W ERSU PPLY
AC
ON
VOLTAGE
ADJ UST
OF F
-
DC
+
0-6V 5A M A X .
On the schematic diagram show where the ammeter (A1) is really wired. On the wiring diagram only one wire
needs to be moved to correctly wire ammeter (A1). Draw in where the wire should be.
7)
Another student has used the Schematic diagram below to wire the circuit picture shown. The ammeter (A T)
at the source reads 6 Amps. The reading on an ammeter placed at A2 should be 2 Amps. When the switch
is closed, the second light does not go on. The reading on the meter beside the second light is zero. What is
wrong?
AT
A2
AMMETER
VOLTMETER
LOW
VOLTAGE
POW ER SUPPLY
AC
ON
OF F
VOLTAGE
ADJ UST
-
DC
+
0-6V 5A M AX.
A voltmeter was mistakenly used at A2
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