Transcript document

DOOR
N
DOOR to
0046
Basement
Hallway
Stairs to
basement
MUEN 0046
Go down the outside steps and in through the
west entrance of Muenzinger. Take the stairs all
the way down to the basement (you’ll go past
the first floor landing). Once in the hallway, turn
left. There are signs to 0046 posted on the
hallway wall. Walk all the way down the hall until
you get to the double doors. 0046 is directly to
your right, through another set of doors. There
are more signs here to direct you.
Exam tonight in Muenzinger E-0046: 7:30 to 9:00
Exam: some essay, some calculations (like HW)
best preparation- go over homework and solutions,
also class notes (particularly questions in class).
Most importantly, be sure you make sense of the answers.
one 3 x 5 card. Important part of studying to make list of most important ideas
and formulas needed.
Group achievement reward-- if group average is higher
than 80% on exam( = .8 x 30 =24 pts), each member of
group gets +5 pts on exam.
Reading assignment thurs. review 7.1, 7.2 sound, new 10.1 amplifiers
Topics: Electrostatic forces … will things attract, repel, and why.
Circuits with wires, batteries or regular outlets, bulbs, heaters…
-- think like an electron, how fast will you be able to flow and where will you
lose your energy. Power, current, resistance, voltage drop.
Conductors, insulators, and semiconductors …
--how is conductivity influenced by how electrons are found in bands
--how can you change conductivity of semiconductor, why does it change.
Photocopiers, how they work…, role of photoconductors and forces between
electric charges.
Power Distribution …
--power loss in wires, why AC?, why HV and LV combo?
--transformers: how do they work, how would you design transformer for power
system to raise or lower voltages, currents creating magnetic fields and
CHANGING magnetic fields creating currents.
one 3 x 5 card. Important part of studying to make list of most important ideas
and formulas needed.
Reading assignment thurs. review 7.1, 7.2 sound, new 10.1 amplifiers
current through coil gives Magnetic field,
reverse current, reverse magnetic field.
current in
current out
What happens if oscillating current in primary but there is no core?
a. The light bulb will not light because there is no conduction path for
electrons to move from one coil to another.
b. The light bulb will not light because there is no changing magnetic
field present.
c. The light bulb will be dimmer than with a core.
d. The light bulb will be the same brightness as with core.
e. The light bulb will be brighter than with core.
ans. c. some field will miss secondary coil
Transformer construction detail. The core.
Magnetic field is always produced from current through primary coil.
Without core, magnetic field spreads out a lot. So field going through
secondary coil is weaker, doesn’t push as hard on electrons, produces
less current, less power transfer … power wasted.
Vsec = Vpri (Nsec/Npri)
current in
B
current out
What will happen to
light bulb?
iron core concentrates field (sucks it in), more through
second coil bigger current! (incredible graphics display…)
Does not carry current!
power distribution system
power plant
500,000 V (on towers)
substation
5000V
5000 V
running around
town.
Electric power generation
How did I generate power
in class?
120 V
short wires
into houses
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
magnets
N
I, V out
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
N
N
time
N
s
s
7
s
1
B
change in B/change in t (slope)
gives voltage-current out of coil
so current out of coil is biggest at a. 1, b.2, c. 3, d. 4, e. 7
d. 4. Magnetic field biggest at 1, 7, but changing slowly
V
8
2
3
4
5
6
N
N
time
N
s
s
7
s
1
B
change in B/change in t (slope)
gives voltage-current out of coil
so current out of coil is biggest at a. 1, b.2, c. 3, d. 4, e. 7
d. 4. Magnetic field biggest at 1, 7, but changing slowly
V
8
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
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
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
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
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.
Semi-conductor in the light.
little
gap
empty
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 1 and Band 2 have electrons with empty energy levels just above
them so in both bands we have electrons that can move.
Where does the power go?
Hair Dryer, Lights, and Stereo plugged into same outlet
Wires
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 in
to 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.
which would make the best core for a transformer if strength did not matter?
a.wood, b. copper, c. glass, d. iron wrapped in plastic insulator
d. iron- concentrates magnetic field. Does not conduct electrons so does
not matter if insulated. In fact cores are painted to insulate and avoid
rust. Also note, can not use permanent magnets as cores, because
cores have to be able to reverse back and forth!
If I took a transformer used to convert 100 V up to 1000 V and I hooked
the primary up to a 12 V car battery. If I then went to measure the
voltage across the secondary coil, what would I find?
a. 0 V, b. 12 V, c. 1200 V. d. 120 V, e. 1.2 V
a. 0 V. Battery would make a constant magnetic field through the
secondary. Need a changing magnetic field to give electrons push to get
voltage and current out of secondary.
What is ratio of turns on primary to secondary? a. 10 pri. to 1 sec., b. 1 to
10, c. 100 to 1, d. 1000 to 1, e. 1 to 1000
ans. 1 on primary for 10 sec.
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
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
exhausted!
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
e
three feet of
mud! takes lots
of energy to get
through. R (rottenness)
Can’t get past each other, ones slowed
down getting through much pile up others
behind. Almost all the energy is used up getting
through the mud. Sets limit on how many can get
through course each second.
#/s = vigor/rottenness (V/R). Nearly all the rottenness is the
mud. Going down wire is very small effect.
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
e
deep
mud!
energy used up getting
through course. Vigor (V)
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 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. Lose more energy
hitting bumps at high speed than with no bridge. Energy used going down road
not so small.
c. Ones that pick wrong path and go through mud have a little less energy to
get through it than they did without bridge path, so get through it slower.
#/s (I) through mud= (V - Vroad)/R, 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?
230 more dangerous than 120- 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
“ground”, always 0V
protection if wire touches case
electric appliance
+120 to –120 V
~0 V but goes up when
current flowing back