Thursday, 20 July 2016
Announcements
Grade appeals due by the end of Monday (Office closes at
5pm)
➔ Pick up homeworks 1-3 from lab room
➔ Pick up exam corrections (Will be put in lab room as well.)
➔ Participation credit quiz
➔ Exam next Wednesday: ch 28-31
– see course home page for materials.
– Is there anything in particular that you want to review?
-> Email me by Monday evening.
➔
The outline
Today:
• Resistors in series and in parallel
• real batteries
• Power dissipater
• Power stored in capacitor
• Dielectrics
• RC circuits
Monday: grounding, circuit review, and midterm review
Intuition check – get your clickers
ready…
• Let all wires be ideal…
• Assume ideal battery unless stated otherwise.
For PH213, equate power dissipated with brightness…
The three bulbs are identical and the two batteries are
identical. Compare the brightnesses of the bulbs.
A.
B.
C.
D.
E.
A  B  C.
A  C  B.
A  B  C.
A  B  C.
A  B  C.
QuickCheck 31.2
The three bulbs are identical
and the two batteries are
identical. Compare the
brightnesses of the bulbs.
A.
B.
C.
D.
E.
A  B  C.
A  C  B.
A  B  C.
A  B  C.
A  B  C.
Slide 31-27
The three bulbs are identical and the two batteries are
identical. Compare the brightnesses of the bulbs.
A.
B.
C.
D.
E.
A  B  C.
A  C  B.
A  B  C.
A  B  C.
A  B  C.
QuickCheck 31.3
The three bulbs are identical
and the two batteries are
identical. Compare the
brightnesses of the bulbs.
A.
B.
C.
D.
E.
A  B  C.
A  C  B.
A  B  C.
A  B  C.
A  B  C.
Slide 31-29
A comparison
Images from : http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html
What does the pump create in the flow
of water?
Pumps create pressure difference.
You can pump water uphill.
What does the battery establish in the
electric circuit?
A battery create a potential difference
in a circuit.
A comparison
Images from : http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir2.html
Conservation of Water Flow
Conservation of Current
Resistor Combinations
Slide 23-22
Capacitor Combinations
Slide 23-30
Which resistor dissipates more
power?
A. The 9  resistor.
B. The 1  resistor.
C. They dissipate the same
power.
QuickCheck 31.7
Which resistor dissipates
more power?
A. The 9  resistor.
B. The 1  resistor.
C. They dissipate the
same power.
Slide 31-47
Example Problem
Slide 22-34
Circuit Diagrams
 The top figure shows a
literal picture of a
resistor and a capacitor
connected by wires to a
battery.
 The bottom figure is a
circuit diagram of the
same circuit.
 A circuit diagram is a
logical picture of what is
connected to what.
Circuit Elements
Slide 31-22
Terminology
• Ideal wire – R=0 ohms
• Ideal insulator – R=∞ ohms
• Ideal battery – emf (ε)=terminal voltage
The lightbulbs are identical. Initially both bulbs are
glowing. What happens when the switch is closed?
A.
B.
C.
D.
E.
Nothing.
A stays the same; B gets dimmer.
A gets brighter; B stays the same.
Both get dimmer.
A gets brighter; B goes out.
QuickCheck 31.15
The lightbulbs are identical. Initially both bulbs are
glowing. What happens when the switch is closed?
A. Nothing.
B. A stays the same;
B gets dimmer.
C. A gets brighter;
B stays the same.
D. Both get dimmer.
E. A gets brighter;
B goes out.
Short circuit.
Zero resistance
path.
Slide 31-89
Real Batteries
Real batteries have what is called an internal
resistance, which is symbolized by r.
Slide 31-67
Real battery
• You short circuit a real battery (internal resistance of 1
ohm) with a wire. Draw a circuit diagram, and calculate
the maximum possible current that can travel through
the wire.
A Short Circuit
Slide 31-71
Real Batteries
A single resistor connected to a real battery is in
series with the battery’s internal resistance, giving Req
= R + r.
Slide 31-68
Terminology
• Ideal wire – R=0 ohms
• Ideal insulator – R=∞ ohms
• Ideal battery – emf (ε)=terminal voltage
• Ideal ammeter
• Ideal Voltmeter
Kirchhoff’s Junction Law
Slide 23-15
Kirchhoff’s Loop Law
Slide 23-16
Tactics: Using Kirchhoff’s Loop Law
Slide 31-33
The current through the 3  resistor
is
A.
B.
C.
D.
E.
9 A.
6 A.
5 A.
3 A.
1 A.
QuickCheck 31.4
The current through the 3 
resistor is
A.
B.
C.
D.
E.
9 A.
6 A.
5 A.
3 A.
1 A.
Slide 31-35
•Solve the following circuit using
the tools that you have learned
about so far. Make sure that you
show all your working and that
your reasoning agrees with your
answers.
1.What is the current through
and the voltage across each
resistor?
2.If each of the resistors were
light bulbs, rank them in order of
brightness.
A circuit
Determine currents and voltages at each resistor.
If the resistors would be light bulbs, rank them by brightness
Slide 22-35