Transcript class2 1020A

Using Physics to Understand the World:
The Physics of Everyday Life
Physics 1020: Dr. Eleanor Hodby
Day 2:
Review of electric circuits
Begin copiers / photoconductors
Reminders:
Fill out group seating chart
Homework 1 due on Tuesday
Reading quiz Tuesday: 10.1, 10.2, 10.3
Attend HW problem solving sessions
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Problem solving strategy:
Analyzing explanations (in-class, homework etc).
People learn better if they frequently ask themselves
“Why did I do this step?”
“How does this fit into the solution as a whole?”
“Does this step or solution make sense, can I relate it to
something else I know or check in some other way that it
makes sense?”
Practice these questions during discussions in class, when
solving problems together, looking over solutions.
Research shows results in solving problems faster and
getting more correct.
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Review of circuits and moving charges
1. Ohms Law (V=IR) and Power (P = IV)
Car battery demo- paper clip across terminals.
What will happen
a) nothing,
b) drain battery slightly,
c) melt paper clip,
d) melt wires,
e) both c. and d.
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This is a task for Electron man!
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Circuits – Think like an electron
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Circuits – Think like an electron
Rules: a) no electron deaths
b) no passing of electrons
c) electrons have energy
(high at start , low at end)
d) Material has resistance
(lets electrons pass easily or not)
See: resistor heating simulation:
http://phet.colorado.edu or class website
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Reasoning a little more mathematically
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12 V
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Battery piles up electrons at negative side.
Pushes electrons to + terminal with 12 V of energy.
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Car battery demo II
Student volunteer grabs terminals with hands.
A. Nothing.
B. Will get zapped (flames etc.)
C. will get mild jolt,
D. other
How to figure out? 1st step in process?
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Review of circuits and moving charges
2. Bulbs in series and parallel
Given 2 bulbs and a battery, there are 2 different ways that you
can connect the circuit
1.5 V
1.5 V
+
+
Series
Parallel
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Bulbs in series
Compare the brightness of identical bulbs.
How does bulb 1 compare to bulb 2?
a. brighter,
b. dimmer,
1.5 V
c. same
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2
+
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Electron man picture of bulb problem
What does Current of 1 Amp mean?
(1 Coulomb of charge per second) =
6.25 x 1018 electrons per second passing by
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Remember rules:
- No Passing
- No electron man deaths or
births on route
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Series circuit question - current
case 1
+
All bulbs(R) and batteries (V) are identical.
How does current flow through circuit in case 1
compare to case 2?
a. Current in case 2 is 1/4 of current in case 1
b. Current in case 2 is 1/2 of current in case 1
c. Current in case 2 is same as current in case 1
d. Current in case 2 is 2 times current in case 1
e. Current in case 2 is 4 times current in case 1
case 2
+
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Series circuit question - current
case 1
+
case 2
+
When resistors are strung together in
SERIES then:
a) total resistance (R) is just the sum
of the individual resistances
b) Same current through each resistor
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Series circuit question - power
How will brightness of bulb 1 in left circuit compare to brightness of the
same bulb in the right circuit?
a. brighter,
b. dimmer,
+
1.5 V
c. same
1.5 V
+
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2
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Bulbs in parallel
Compare the brightness of identical bulbs.
How does bulb 1 compare to bulb 2?
a. brighter,
b. dimmer,
c. same
+
1.5 V
1
2
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Thinking like an electron – parallel circuits
No matter what the path, electrons lose all voltage (energy
per electron) by the time they return to outlet.
Vpath1 = Vpath2 = Vbattery
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Bulbs in parallel
+
1.5 V
For bulbs wired in parallel:
• Same voltage across each bulb
• V1 = V2
• (Assumes Rwires ~ 0)
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V1
2
V2
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Bulbs in parallel
B
A
+
1.5 V
1
+
1.5 V
1
2
How does brightness of bulb 1 in circuit A compare to the same bulb in B?
a. brighter,
b. dimmer,
c. same
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Bulbs in parallel
How does size of IB compare to I1 ?
(Bulbs 1 and 2 are identical)
a)
b)
c)
d)
1.5 V
1
IB is three times I1
IB is twice I1
IB is half I1
IB is the same as I1
+
IB
I1
2
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Bulbs in parallel
Q: What is different between the 60 Watt bulb and
the 125 Watt bulb?
a. the voltage difference (amount of energy lost
by each electron) across bulb
b. the number of electrons passing through the
filament each second
c. Choice a and the resistance of the filament
d. Choice b and the resistance of the filament
e. Choice a and Choice b and the resistance of
the filament
120V
60 W
125 W
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More bulb questions
What if we wire the 60 and 125 W bulbs in series, which
will be brighter?
120V
a) 60 W bulb is brighter
b) 125 W bulb is brighter
c) Same brightness
60 W 125 W
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Summary of some important ideas
1. Current is conserved (electrons don’t disappear)
2. Change in V over circuit = V of battery, or energy source
3. V= I R (Ohm’s law)
- useful for whole circuit (R total, Vtotal, give I total)
- or individual component (e.g. Rbulb, Vbulb give I bulb),
…….Be careful about what R, and V are.
4. P = I V = I (IR) = I2R
= (V/R)V = V2/R
power dissipated across object R
5. Resistors in series:
Resistances add: Rtot = R1 + R2
Current through all resistors is the same
6. Resistors in parallel:
Voltage drop across parallel legs of circuit is same
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Homework #1 due Tuesday at start of class:
Goal is to refresh what you learned in 1010 about electrostatic forces (like
charges repel) energy, and circuits.
Resources available to you:
1. Phys 1020 Lecture Notes (from course homepage)
2. Phys 1010 Lecture Notes from last semester
3. The book. Review chapters:
10.1 Static Electricity, 10.3 Flashlights,
4. Problem Solving Sessions (G2B87):
Monday 2-3.30pm
Friday 4-5.30pm
Late submission policy: No late homeworks will be
accepted. Two lowest homework scores will be
dropped. Save these for emergencies.
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