1/30/2013
Parallel Resistors Checkpoint
• 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?
checkpoint
Car Headlights • Are car headlights connected in series or parallel?
– the larger resistor
– the smaller resistor
• Car Headlights are connected
• Connect 4 equal resistors so their Req is R
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
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
Power
• Power is the rate at which energy is used or at which work is done
• P = IV
• Units: Practice:
Resistors in Series
R1=1
0
Calculate the power provided by the battery if the battery emf is 22 volts. Calculate the power dissipated by each resistor
0
R2=10
R12
R1=1
0
R2=10
1
1/30/2013
Practice: Resistors in Parallel

Practice: Resistors in Parallel
R2

R3
R2
R3
What is the power delivered by the battery and what is
the power dissipated by each resistor.
Let e = 60 Volts, R2 = 20  and R3=30 .
Determine the current through the battery.
Let e = 60 Volts, R2 = 20  and R3=30 .
Simplify: R2 and R3 are in parallel
Calculate IV for the battery.

1/R23 = 1/R2 + 1/R3

R23
R23
V23 = V2 = V3
I23 = I2 + I3
R1
Try it!
Calculate current through each resistor.

Try it! (cont.)
R2

R3
R1 = 10 , R2 = 20 R3 = 30  V

Calculate current through each resistor.

R123
R1 = 10 , R2 = 20 R3 = 30  V
Simplify: R2 and R3 are in parallel
Expand: R1 and R23 are in series
R1

R1

R23
Simplify: R1 and R23 are in series
R23
Expand: R2 and R3 are in parallel

R1
R123

R2
R3
Power delivered by battery?
Checkpoint
4 Resistor Combination
• Is it possible to connect 4 resistors of resistance R in such a way that their equivalent resistance is R?
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
2