Generators
How did I generate power in class?
moving coil through magnetic field.
so if moved coil or magnet could generate
electric power.
Power plants: use steam or water
to spin magnets past coils (or vice-versa)
S
I, V out
magnets
N
N
S
S
N
N
S
iron core
spinning turbine
hydroelectric turbine
E = mgh, power = mass/sec x gh
~ 40% efficient
Pelectrical out = .4 (mass water/s x gh)
h
steam plants same idea,
boil water to make steam pressure
to spin turbine.
boiler
turbine
I
cooling pond
2
3
4
5
6
7
N
N
time
N
s
s
8
s
1
B
(change in B) /(change in t) [slope of line]
gives voltage (and current) out of coil
so current out of coil is biggest at a. 1, b.2, c. 4, d. 7
c. 4. Note: magnetic field biggest at 1, 7, but changing slowly
V
Voltage = k (change in magnetic field/time to change) x number of turns
= k (DB/Dt) N
conduction of materials- energy levels and electron occupation.
3
2
1
How many energy levels are in band 1?
a. none, b. 1, c. between 1 and 10, d. an enormous number
Band 3
Band 2
Band 1
How many energy levels are in band 1?
a. none, b. 1, c. between 1 and 10, d. an enormous number
d. an enormous number. Each band is a whole bunch of very close levels. Each
Is filled with an electron.
How many empty and filled levels are there in band 3?
a. 1 filled, none empty, b. 1 of each, c. very many of each,
d. many filled, 1 empty
3
many empty
levels close
together
2
1
How many empty and filled levels are
there in band 3?
Answer is c: very many of each
True / False
1. the upper electrons in band 3 can easily
move because there are very close energy
levels they can move into.
2. the upper electrons in band 2 can easily
move because there are very close levels to
move into.
a. 1T 2T, b. 1F 2F, c. 1T 2F, d. 1F 2T
many filled levels close
together.
c. 1 is true but 2 is false.
Material with top band like 3 is
conductor
Material A. What is it?
Band 2
Band 1
empty
full
Fill in the blank:
This material is a ______________.
When hooked to a battery, electrons in Band 1 will ______________.
When hooked to a battery, electrons in Band 2 will ______________.
a. conductor, move, move
b. semi-conductor in the dark, not move, move.
c. semi-conductor in the light, move, move.
d. semi-conductor in the light, not move, move.
e. insulator, not move, not move.
conductor- empty levels
very close
empty
insulator- big jump to empties.
empty
full
ENERGY gap- no ALLOWED levels
full
full
electron like ball rolling on
almost flat ground
move easily
electron like in pit.
Can’t move
without big boost.
semiconductor-- half way in between. Little gap to empty levels, shallow
pit.
empty
full
Semi-conductor in the light.
little
gap
empty
full
E = energy of photon =
h x frequency (h)
=h x c/(wavelength) = hc/
Fill in the blank:
This material is a ______________.
When hooked to a battery, electrons in Band 1 will ______________.
When hooked to a battery, electrons in Band 2 will ______________.
a. conductor, move, move
b. semi-conductor in the dark, not move, move.
c. semi-conductor in the light, move, move.
d. semi-conductor in the light, not move, move.
e. insulator, not move, not move.
Small energy gap between band 1 and band 2. As shown, electrons are excited up
to Band 2 as would be the case for a semi-conductor in the light. Both Band 2 have
electrons with empty energy levels just above them so in both bands we have
electrons that can move.
Making a Copy
Part I: a charge image
Photoconductor
Corona wire
metal
Light
velocity
+++++
+++++
Document
Light
+++++
Charge image
+++++
11
Part II: Transferring Charge Image to Toner and Paper
Just like Part I, in reverse.
Roller and
brush
Release toner
toner
Light
+
+++++
Positive
toner particle
Charge image
+ ++
---
+++++
+
+
+++++
Black image
Heat
Charge Paper
--+ ++
+ ++
+++++
12
All Pretty Simple Charge-Physics (opposites attract), except
for selectively removing the charges
Document
Charge image
+++++
Rumor has it Carlson tried trained squirrels
with little fingers for long time. Gave up and used physics.
(yes kidding)
13
Photoconductorsway to get charges to leak away when shine light on surface.
1. before light.
------------------ -- -----------(side
view)
e’s want to go to bottom, but R too high, stuck
V
conducting plate
2. where light hits, R low, electrons flow
away.
--------- -- ------------
-
-
V
-
-
-
-
14
Semi-conductor physics
where light hits, R low, electrons flow away,
then add ink, sticks only where charges.
--------- -- ------------
-
-
V
-
-
-
-
-
Very special material- low R (“conducts”) only when
light hits. “Photoconductor”
To understand, have to understand what determines
resistance of a material.
• insulators (wood, ceramic, plastic)- very high resistance.
• conductors (metals)- very low resistance
• “semiconductors”- in the middle. Resistance depends on temp., light,
cleanliness.
15
Where does the power go?
Wires
Hair Dryer, Lights, and Stereo plugged into same outlet
Hair Dryer
Lightbulb 1
Stereo
10 Amps
0.5 Amps
Lightbulb 2
2 Amps
What is the current through the wires?
a. 10 Amps
b. 12.5 Amps
c. 7.5 Amps
d. more than 12.5 Amps.
Answer is b. 12.5 Amps. Currents flow together in wire. Wire carries total.
If the resistance of each light bulb is 100 Ohms, how much power is going into the
two bulbs combined?
a. 25 Watts,
b. 50 Watts
c. 100 Watts
d. 200 Watts
1. What is voltage drop across one bulb?
V_across bulb = I_bulb x R_bulb =0.5 amps x 100 ohms = 50 Volts
2. Power loss in each bulb = I_bulb x V_bulb = 0.5 amps x 50 Volts = 25 Watts
3. There are 2 bulbs. So total loss is 50 Watts. .. Answer is b.
120 V
Why does light dim when heater on, how much less current through light?
(need to think like an electron!)
Job for electron man!
(on rollerblades)
e
boot camp for electrons.
bunch of them going through
obstacle course.
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
exhausted!
Rules:
a) no electron deaths
b) no passing of electrons
c) electrons have energy
(high at start , low at end, determined by V)
e
e
e
e
e
e
e
e
e
feet of
mud! takes lots
of energy to get
through. R (rottenness)
V=IR
P=IV
d) Material has resistance
(lets electrons pass easily or not)
!! Careful: know which
elements / system these
apply to !!
What happens when bridge gets
fixed so have another route?
e
e
e
e’s piled up down both routes, so
still divide up and go down both,
just end up faster on bridge 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
lots of energy
at start.
e
e
e
e
(bulb)
deep
mud!
energy used up getting
through course. Vigor (V)
e
e
e
e
(heater)
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
exhausted!
What happens now compared with bridge out?
a. Can go easy path across bridge. Takes less energy, can go much faster.
b. Have to go down entry and exit road much faster. (wires fro outlet)
Lose more energy hitting bumps at high speed than with no bridge.
Energy used going down road not so small as when small current
c. Ones that go through mud have a little less energy to get through it than they did
without bridge path, so get through it slower.
#/s in bulb = I_bulb = (Vtotal - Vroad)/ Rbulb,
but going faster so Vroad lot bigger than before,
go through mud slower than if bridge out. I smaller. (Vroad = I Rroad)
why different voltages, why different plugs, why 3 wires?
Are different voltages more dangerous?
230V more dangerous than 120V
- easier to fibrillate heart (but wastes less power in wires).
Different plugs partly historical, but modern European
plugs harder to touch “live” plugs than US. Probably
designed that way because voltage more dangerous.
3rd wire- ground. Protection
electric appliance
+120 to –120 V
“ground”, always 0V
protection if wire touches case
~0 V but goes up when
current flowing back
End of Review
Back to Amplifiers
Unless You Have More Questions
Capacitors – two metal plates that store charge;
insulator in between plates.
No current or electron flow
across here.
+9V “Above Ground”
GROUND
(0 V)
E
in electric circuits hook on part to “ground”, infinite source or sink of electrons.
Does not change anything compared to if wire straight from negative terminal to
22
capacitor.
Hook up to battery and close switch,
what is the immediate current through the meter?
a. no current flows
b. electrons flow down for a little while then stop
c. current continues to flow until battery dead.
d. electron current oscillates back and forth
I?
+9V “Above Ground”
GROUND
(0 V)
E
in electric circuits hook on part to “ground”, infinite source or sink of electrons. Does not
change anything compared to if wire straight from negative terminal to capacitor.
23
b. Electrons flow down for a little while then
stop. Are attracted to + terminal of battery.
Stop flowing when shortage of electrons at
top (extra + charges) gives strong enough
force so no more electrons can flow.
Current
Capacitors – two metal plates that store charge; insulator in
between plates.
0
time after voltage
hooked up.
-  -  - -
+++++++
+++++++
A B
+9V “Above Ground”
GROUND
(0 V)
E
24
Capacitors – two metal plates that store charge; insulator in
What was current on the right
between plates.
side after hooked up battery?
a. no current flows.
b. same as on left, e’s flow down (away from
capacitor)
c. same as on left but opposite direction, e’s
flow up (towards capacitor)
d. e’s only go from battery to ground.
A B
I?
+9V “Above Ground”
GROUND
(0 V)
E
25
Capacitors – two metal plates that store charge; insulator in
between plates.
c. same as on left but opposite direction, electrons flow up towards capacitor. Extra
positive charge on left side of cap. attracts negatives. So end up with exactly equal
and opposite charge on each side of capacitor.
Flows until 9 V difference across Capacitor balances force from battery.
e’s flow out of ground. Equal number of e’s flow from battery into ground as flow
into + side of battery.
+9V
-  -  - -
-  -  - -
GROUND
(0 V)
-  -  - -
+9V “Above Ground”
-  -  - -
Capacitor
Demo
-- -- -- -- -- -- -+++++++
+++++++
A B
E
26
Capacitors – two metal plates that store charge;
insulator in between plates.
What is Voltage at B?
a. -9 V b. +9 V c. 0 V
Answer is c: 0 Volts. If not 0 Volts, would still have flow of electrons to or from
ground. Voltage difference across capacitor plates EQUAL to voltage difference
across Power Supply
+9V
-- -- -- -- -- -- -+++++++
+++++++
A B
0V
+9V “Above Ground”
Adjustable
Power Supply
GROUND
(0 V)
E
27
Capacitors – two metal plates that store charge;
insulator in between plates.
-  -  - -
-- -- -- -- -- -- -+++++++
+++++++
What if can control voltage instead of using
2) Attraction of electrons on Plate B to
battery? What happens if drop voltage to 4
positive charge on Plate A has decreased.
Volts?
Force of electrons repelling each other is
1) Plate A is at higher voltage than
greater than attraction to Plate A, so some
Supply. Electrons flow towards Plate A
electrons leave … flow back to Ground.
until voltage difference is 0. Excess
A B
positive charge on Plate A will decrease.
+4V
+9V
+0V
-  -  - -
4V“Above
“AboveGround”
Ground”
++9V
-  -  - -
E
-  -  - -
Adjustable
Power Supply
GROUND
28
Capacitors – two metal plates that store charge;
insulator in between plates.
So if we can vary voltage at Plate A, we can create an alternating current
through the speaker and drive the speaker.
A B
-  -  - -  -  - -
-  -  - -
Adjustable
Power Supply
------+++++++
-  -  - -
+9V to
+4V “Above Ground”
E
GROUND
29
Simulator!
http://phet.colorado.edu/en/simulation/circuit-construction-kit-ac
30
capacitor
thin metal plates with wires to each
hook up to battery, what is the
current through the meter?
a. no current flows
b. electrons flow down for a little while
then stop
c. current continues to flow until battery
dead.
d. electron current oscillates back and forth
I?
9V
+
-
in electric circuits hook
on part to “ground”,
infinite source or sink of
electrons. Does not
change anything compared
to if wire straight from
negative terminal to capacitor.
31
hook up to battery, what is the
current through the meter?
b. electrons flow down for a little while
then stop. Are attracted to + terminal of
battery. Sucked in until shortage of electrons
at top (extra + charges) gives strong enough
force so no more electrons can flow.
+
+
+
+
+
+
+
+
---
I
I??
9V
--- -
0
+
time after voltage
hooked up.
-
in electric circuits hook
on part to “ground”,
infinite source or sink of
electrons. Does not
change anything compared
to if wire straight from
negative terminal to capacitor.
What was current on the right
side after hooked up battery?
a. no current flows.
b. same as on left,e’s flow down
c. same as on left but opposite
direction, e’s flow up.
32
d. e’s only go from battery to ground.
---
I
+
+
+
+
+
+
+
+
-
- - -
I
9V!
-
9V
--- - +
0
time after voltage
hooked up.
----
What was current on the right side after
hooked up battery?
c. same as on left but opposite direction, e’s
flow up.
Extra positive charge on left side of cap.
attracts negatives.
So end up with exactly = and opp. charge on
each side
of capacitor. Flows until 9 V across Cap.
balance force
from battery. e’s flow out of ground. Equal
number of
e’s flow from battery into ground as flow into
+ side of batt.
Charges keep flowing
until Vbattery =Vcapacitor
Amount of charge piled up = CV
C is “capacitance”
1farad x 1 V = 1 coulomb
33
If suddenly reduced voltage to 6 V, capacitor would push
harder than batteries, charges would flow onto + side of C
and off of negative side. So changing V gives changing
current on each side of Cap., but constant voltage does not.
so capacitor saves the day, gets rid of all that DC
current, keeps the AC needed to drive speaker.
voltage
and current
0
0
time
time
in
V, I
34