Series and Parallel Circuits
Electric Power
How was power define before?
Time rate of doing work or using energy
Electric Power - Rate at converting
electrical energy into other forms of
energy.
P = W/t
or P = E/t
Unit: watt, w = J/s
60w light bulb converts 20 J of electric
energy to light and heat each second.
P=E
t
V=E
Q
E=QV
P=QV
t
P=IV
Example
What is the resistance of a 1200w electric
frying pan that draws a current of 11a?
9.9 Ω
Problem Set 1
1. An appliance draws 13 a when
connected to a 110v circuit.
a. What is the power of the appliance?
b. What is its resistance?
2. What is the power of a light bulb that has
a resistance of 190 in a 120 v circuit?
What does the power plant sell
you?
Problem Set 2
If the local power company charges
$0.080/kw * h, how much (to the nearest
cent) will it cost to use:
1. a 75 W CD player for 5.0 hours?
2. a 2.3 kw oven for 45 minutes?
3. a TV drawing 2.0 amps of current in a
120 v circuit for 3.5 hours?
Series Circuits
Resistors are connected in a single path.
Parallel Circuits
• Resistors are connected in separate
branches.
• There is more than one path for current.
Series Circuit Lab
•Make the following circuit. Remember an
ammeter goes into the circuit as shown in the
diagram.
A

Now add a second resistor in the circuit,
as shown in the diagram below.
A

Observe the brightness of the bulbs.

Now, add a third resistor into the circuit,
as shown in the diagram below.
A


Observe the brightness of the bulbs.
Remove one of the bulbs and record the
results.
Part B


Construct the following diagram with the
ammeter at the AT location.
Next, move the ammeter to the A1 and A2
sites and record the current.
Part C



Place the Multimeter in the voltage terminal and switch
the dial to Voltage setting.
Build the following circuit and place the voltmeter on
either side of the battery at the VT. Remember a
voltmeter is connected around the battery or resistor
since it measure a difference in potential energy
between two locations.
Now move the voltmeter to V1 and V2 locations.
Parallel Circuits Lab


Place the multimeter back in the ammeter
setting by switching to terminals and the
dial setting to A.
Construct the circuit depicted in the
diagram below.
A

Observe the brightness and the current
and record your observations.

Add a second resistor in parallel as
depicted in the diagram below.
A

Observe the brightness of the light bulbs
and the current and record your
observations.

Add a third bulb in parallel. Make
observations and record.
A


Unscrew a bulb and make observations.
Record
Answer conclusion questions.
Part B


Remove one of bulbs in
the circuit and keep the
ammeter in the same
position AT.
Now, move the
ammeter to the other
two sites to measure
the current in different
paths.
Part C



Switch the multimeter to the voltmeter
setting by moving the terminals and
turning the dial to the voltage setting.
Place the voltmeter in parallel with the
battery by reading the VT, terminal
voltage.
Move the voltmeter to the other two
Conclusions


Use ohm's law to calculate the resistance
in the different positions.
Answer the Conclusions.
Series Circuits
Current is the same in all parts of the circuit
It = I1 = I2 = ...
Series Circuit
Voltage drops across resistors adds up to
the total voltage provided by the battery.
Energy is conserved.
Vt = V1 + V2 + V3
Series Circuits
Total resistance is found by adding the
resistance of each device.
R t = R1 + R 2 + R 3
R1 = 15Ω
80v
R2 = 20Ω
R3 = 5Ω
RT _____
V1 _____
VT _____
V2 _____
IT _____
V3 ____
Video Problem Set 3
Problem Set 4
What would happen if your house
was wired in series circuits?
• What would happen as you added more
and more things to the circuit?
• What would happen if one appliance
malfunctioned?
Current in Parallel Circuits
4a
2a
2a
Current in Parallel Circuits
4
a
3
a
1
a
• Current in each branch adds up to the total
current of the circuit.
• Electric charge is conserved
Current in Parallel Circuits
• Current in each branch adds up to the total
current of the circuit.
• Electric charge is conserved.
• IT = I1 + I2 + I3 + …
Voltage in Parallel Circuits
6
v
6
v
6
v
Voltage in Parallel Circuits
• Voltage drops across resistors in each
branch equals total voltage provided by
the battery.
• VT = V1 = V2 = V3 = …
Resistance in Parallel Circuits
• Equivalent resistance is less than
resistance of each resistor alone.
1
1
1
1
=
+
+
+
…
Req R1
R2
R3
Examples
• What is the equivalent resistance of:
– Two 10Ω resistors in parallel?
• 5Ω
– Two 8Ω resistors in parallel?
• 4Ω
– Three 9Ω resistors in parallel?
• 3Ω
– A 60Ω, a 30Ω , and a 20Ω resistor connected
in parallel?
• 10Ω
Problem Set 1
Equivalent Resistance
• The two resistors in parallel can be
replaced with a single resistor
Example
Problem Set 2
Problem Set 3
1.In which type of circuit is there only one path
for the charges to travel?
2.In a series circuit, the voltage drops across the
resistors,
a.Equals the voltage of the battery
b.Adds up to the voltage of the battery
c.Is less than the voltage provided by the battery
3.When resistors are connected in ______,
voltage stays the same in all branches.
4.The term equivalent resistance is used for
resistors connected in _____.
5. Our homes are wired in ______.