Why do lights dim when you turn on your hair dryer?
And how is this related to power distribution?
Day 4:
Questions?
Finish Photoconductors (Blm 12.1:424,433).
Start Power transmission Blm 11.2
Reminders/Updates:
EYES TO WEB
How are labs?
How was HW?1
More next week
Wrapping up physics of copying machine.
Making a Charge Image
Photoconductor
Corona wire
metal
velocity
+++++
+++++
Light
Document
+++++
Charge image
+++++
2
Transferring Charge Image to Toner and Paper
Roller and
brush
Release toner
toner
Light
+
+++++
Positive
toner particle
Charge image
Charge Paper
--- + +
+ ++
+++++
+++++
Black image
+ ++
+ ++
Heat
--+++++
Straightforward. All just good old physics of attraction
between opposite charges, plus photoconductor physics.
3
Semi-conductor physics
where light hits, R low, electrons flow away,
then add ink, sticks only where charges.
--------- -- ------------
-
- Very special material- low R (“conducts”) only when
light hits. “Photoconductor”
V
To understand, have to understand what determines
resistance of a material.
a. insulators (wood, ceramic, plastic)- very high resistance.
b. conductors (metals)- very low resistance
c. “semiconductors”- in the middle. Resistance depends
on temp., light, cleanliness.
4
Looking at atomic structure of solids… energy bands
many atoms
Energy
one atom
levels get shifted and shared
for all atoms and electrons.
5
In solid, billions of atoms, electrons, and levels!!
levels get shifted and shared
for all atoms and electrons.
so many individual levels,
just talk about bands of levels.
1 electron per level until run out of
electrons.
allowed levels but no electrons
up there
3
2
1
orange- levels mostly
full of electrons
actually
1023 electrons
and more
levels!!
6
Microscopic look at material
insulator- big jump to empties.
conductor- empty levels
very close
empty
gap- no levels
empty
full
full
electron like ball rolling on
almost flat ground
move easily
electron like ball in pit.
Can’t move
without big boost.
semiconductor-- half way in between.
empty
Little gap to empty levels
full
like shallow pit.
7
If apply voltage to make electron move, it must go into
a. higher energy level, b. lower energy level, c. does not matter, these
are the energy levels of electrons in atoms, no connection with motion.
3
2
1
moving a. higher- moving, more energy,
really only slightly higher level.
stationary
If push on electrons (apply voltage
to material) which ones will move?
a. all of them in bands 1,2,3
b. none of them
c. all of them in band 3
d. only the top one in band three
d. only the top one in band 3.
The others have no higher level
they can move into, all filled with
other electrons. After top one has
gotten pushed up room to move
next one below it. so maybe include
billion or so really close ones.
8
Worth “seeing” in practice
conductivity simulation
http://phet.colorado.edu
http://phet.colorado.edu/simulations/sims.php?sim=Conductivity
RULE: for electrons to move (with a voltage applied)
There must be an empty energy level immediately above
where they are
9
Which band structure goes with which material?
(be ready to give reasoning)
empty
full
1. Diamond
2. copper
3. germanium (poor conductor)
a. 1=w, 2=x, 3=y b. 1=z, 2=w, 3=y c. 1=z, 2=y, 3=x
d. 1=y, 2= w, 3=y. e. 1=w, 2=x, 3=y
Energy
25 eV
element w
0
x
y
z
10
Review of semiconductors.
little
gap
empty
full
Little, but not tiny gap between filled and empty energy levels.
Big enough to stop current (high R).
Small enough so humans can find ways to boost electrons up.
Concept behind all modern electronics!
Use semiconductors to control electric currents.
Which will decrease resistance of a semiconductor?
add heat, light,
shake,
add dirt
a.
T
T
T
T
b
T
T
F
F
c
T
T
F
T
d
F
T
F
F
e
F
T
F
T
11
Can also give electron kick of energy with light.
Photo-conductor
little
gap
empty
full
E = energy of photon =
h x frequency (h)
=h x c/(wavelength) = hc/
If green light has just enough energy to excite electron into empty
level and allow it to move (conduct electricity)
a. both blue and red light will also make it a conductor
b. blue light will, red light will not
c. neither blue nor red light will make it into a conductor
blue light has shorter wavelength so higher energy than green,
so it has more than enough energy. Red has longer wavelength
than green so has less energy, not enough to boost electron to
empty level where it can move/conduct. Photon energy hc/ needs
to be bigger than energy of gap or will have no effect.
12
End of physics of copiers. Will return to more about
semiconductors and use in electronics in couple of weeks.
Final questions???
13
On to power distribution system and the
physics that makes it the way it is.
Question to think about, and be ready to answer in a
couple of classes.
What is the relationship between the fact that lights
sometimes dim with you turn on a hair dryer and that
we have giant power transmission towers with cables
hanging from them all over the country?
14
Power distribution and generation
Why use AC power?
1. Energy loss in wires
2. Virtues of high voltage
3. Transformers and how they work
~1.5-2 days Power system and how transformers work in it.
.5 days- power generation- same physics
Big ideas:
• Changing magnetic fields produce
voltages and currents
• Currents produce magnetic fields.
15
Distributing Power Across the Country
• Long wires from power generating plants
to homes (100’s of miles)
• How much power will we lose in wires?
(wasted power)
• How can we minimize power lost in the
wires?
Explore what affects power loss.
16
Electron man!
e
1) Lose energy by bumping into stuff in atoms.
2) Lose energy everywhere but in some parts more than others.
3) Voltage change across something represents amount of energy
lost across it.
lots of energy
at start.
Lotse of volts
Wires: glide down pretty easily, just a few bumps.
Lose a little bit of energy.
e
e
-e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
Rtotal = Rlight + Rwires
Usually
small
e
e
e
e
e
e
e
+
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
Exhausted! energy used up getting
through course.
e
e
e
e
e
e
e
Light Bulb…
high resistance like
trudging through three
feet of mud!
takes lots of energy to
get through.
17
R (rottenness)
How will brightness of bulb1 in case i compare to brightness of bulb1 in case ii (no #2)?
a. brighter, b. dimmer, c. same
b) dimmer, 2 bulbs have more resistance
RTotal = R1+R2 so less current flows
through them. P=IV=I2R
1
case i
+
1.5 V
case ii
1
2
1.5 V
NOTE: Resistors (bulbs) in series (one after other) ADD,
Resistors in parallel (different paths) act independently
18
Wall =
120 V AC
Lights and power
Wiring inside home
“Power Source”
(We can vary
voltage output)
120 V
Light
125 Watts
Light
60 Watts
Wiring inside home
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 the
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
19
No matter what the path, electrons lose all voltage
(energy per electron) by the time they return to
outlet.
Vpath1 = Vpath2 = Vbattery (approx. for ideal R=0 wires)
Appox! assume wires don’t waste energy
lots of energy
at start. (V)
e e
e
ee
e
e
e
+
Thinking like an electron.
V1 = Vstart
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
V2 = Vstart
e
e
e
e
e
e
e
-
e
e
e
e
Path 1
e
Path 2
e
e
+
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
No energy left
20
Wall =
120 V AC
Lights and power
Wiring inside home
“Power Source”
(We can vary
voltage output)
Light
125 Watts
Light
60 Watts
What if hook 60 Watt and 125 Watt light bulbs in series, which
will be brighter? (Share reasoning before experiment. )
a. 125 Watt bulb will be brighter
b. 60 Watt bulb will be brighter
21
Thought process:
Same current must pass through both bulbs.
Can figure out current from total resistance:
Rtotal = (R 60W bulb + R 120 W bulb + Rwires)
+
60W
120W
lots of energy
at start.
e
e
60 Watt,
Higher resistance
e
-e
e
e
e
e
120 Watt bulb
Lower resistance
e
e
e
+
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
22
Home wiring
Wall =
120 V AC
Wiring inside home
“Power Source”
(We can vary
voltage output)
HEATER
120 V
Wiring inside home
60 Watts
\/\/\/\/\/
1500 Watts
Light
Turn on Heater (close switch), observe light bulb.
Light bulb dims. Why? Discuss with your group and come up
with possible reasons.
Share with class.
23
glide down easily, just a few bumps. Hardly any energy.
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
lots of energy
at start.
e
e
e
!?#%, bridge
out, stuck.
e
energy used up getting
through course. Vigor (V)
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
exhausted!
How much power and where is it going? Work through together.
Plug voltage = 120 Volts
Total resistance of wires = 0.1 ohms
Resistance of lightbulb = 240 ohms
Total resistance = Rwires + R bulbs = 240.1 ohms
Current = Plug Voltage / Total R = 120 Volts / 240.1 ohms = 0.5 amps
Voltage drop in wires = Current x resistance = 0.5 amps x 0.1 ohms = 0.05 Volts
Power into wires = Current x voltage drop in wires
= 0.5 amps x 0.05 volts = 0.025 Watts
Power into bulb = current x voltage drop in bulb
= 0.5 amps x 119.95 volts = 59.975 Watts
24
e’s piled up down both routes,
so still divide up and go down
both, just end up faster on
bridge route
What happens when bridge gets
fixed so have another route?
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
lots of energy
at start.
e
e
e
deep
mud!
(bulb)
energy used up getting
through course. Vigor (V)
e
e
e
pretty
easy
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
exhausted!
What changes compared with bridge out (heater off) ?
I. Current through wire is now larger
II. Voltage drop across wire is now larger
III. Voltage drop around whole circuit is now larger
IV. Current through bulb is smaller
V. Voltage drop across bulb is smaller
a. I and III
b. I and II
c. I, II, V
d. I, II, IV, V
e. I, II, III, IV, V
25
What will make bulb even dimmer?
I. Shorter wires
II. Longer wires
III. Adding another heater
IV. Thinner wires
V. Fatter wires
a. I and V
d. II, III, V.
b. I, II, and III
e. III only.
c. II, III, and IV
26