2/1/2011
Preflight 7-1
• Two resistors of very different value are
connected in parallel. Will the resistance of
the pair be closer to the value of the larger
resistor or the smaller one?
– the larger resistor
– the smaller resistor
• Car Headlights are connected
• Connect 4 equal resistors so their Req is R
As more identical resistors R are added to
the parallel circuit shown here, the total
resistance between points P and Q
1. increases
2. remains the same
3. decreases
Q
0%
0%
1
0%
2
3
Charge flows through a light bulb. Suppose a wire
is connected across the bulb as shown. When the
wire is connected,
1. all the charge continues to flow
through the bulb.
2. half the charge flows through the
wire; the other half continues
through the bulb.
3. all the charge flows through the
wire.
4. None of the above
0%
1
0%
0%
2
3
0%
4
1
2/1/2011
Power
• Power is the rate at which energy is used or at
which work is done
• P = IV
• Units: A ⋅ V = C ⋅ J = J = Watt = (W )
s C
s
Practice:
Resistors in Series
R1=1Ω
ε0
Calculate the power provided by the battery
if the battery emf is 22 volts. Calculate the
power dissipated by each resistor
R2=10Ω
R12
ε0
R1=1Ω
ε0
R2=10Ω
Practice:
Resistors in Parallel
ε
R2
R3
Determine the current through the battery.
Let ε = 60 Volts, R2 = 20 Ω and R3=30 Ω.
Simplify: R2 and R3 are in parallel
1/R23 = 1/R2 + 1/R3
ε
R23
V23 = V2 = V3
I23 = I2 + I3
2
2/1/2011
Practice:
Resistors in Parallel
ε
R2
R3
What is the power delivered by the battery and what is
the power dissipated by each resistor.
Let ε = 60 Volts, R2 = 20 Ω and R3=30 Ω.
Calculate IV for the battery.
ε
R23
R1
Try it!
ε
R2
Calculate current through each resistor.
R1 = 10 Ω, R2 = 20 Ω, R3 = 30 Ω,
R3
ε=44 V
Simplify: R2 and R3 are in parallel
R1
ε
R23
Simplify: R1 and R23 are in series
ε
R123
ε
R123
Power delivered by battery?
Try it! (cont.)
Calculate current through each resistor.
R1 = 10 Ω, R2 = 20 Ω, R3 = 30 Ω,
ε=44 V
Expand: R1 and R23 are in series
R1
ε
R23
Expand: R2 and R3 are in parallel
R1
ε
R2
R3
3
2/1/2011
If the 4 light bulbs in the figure are identical,
which circuit puts out more light?
1. I
2. They emit the same
amount of light
3. II
I
II
0%
1
0%
2
0%
3
4