What was that that you said?
Day 7:
Questions?
Finish transformers and generators
Start Sound (hopefully), review: Blfmd:
9.2 Sound (p.311-313), 12.1Semiconductors,…(p431-437)
Reminders/Updates:
HW 4 will be due
Lab this week- transormers!
Eyes to web for drop dates,
Exam next week…
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A. Producing
electric currents using magnets.
Magnets have North and South Pole.
Like Poles repel, Opposite Poles attract
Magnetic field: force on a north pole
North
South
Bulb will lights up if move coil
in and out of magnet
What will happen if I move more slowly?
a. brighter, b. dimmer, c. same
dimmer, so how fast I move (how fast strength of magnetic field
inside coil changes) determines size of induced voltage and
current. MORE RAPID CHANGE, HIGHER VOLTAGE, MORE
CURRENT.
2
http://phet.colorado.edu/simulations/sims.php?sim=Faradays_Law
Move bar magnet up across front of coil.
Voltage will be biggest when
a. lined up with middle,
b. coming from far away,
c. half way in
demo-- when half way in. Most rapid change in B.
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A. Producing
electric currents using magnets.
Magnets have North and South Pole.
Like Poles repel, Opposite Poles attract
Magnetic field: force on a north pole
North
South
Bulb will lights up if move coil
in and out of magnet
4. What will happen if I use coil with 3 turns instead of
500? a. brighter, b. dimmer, c. same (discuss reasoning)
so dim cannot be seen- so induced voltage and current depends on how
many turns. Same thing as an equation is
Voltage = k (change in magnetic field/time to change) x number of turns
4
= k (DB/Dt) N
Conclusion: changing magnetic field through coil of wire
will give voltage drop across it, and if hooked to something
like light bulb, will give a current.
To House
Primary coil (in)
If we can produce a changing magnetic field through coil
to house, we can supply voltage and current to power house!
Next topic.
a. How to create a magnetic field with coil of wire?
b. How to change the field in time.
5
B. Producing magnets using electric currents
North pole
compass with I = 0
DC power
supply
6. What direction will compass point if turn on current to 5 amps?
a.
b.
c.
d.
e. could be b or d.
explain reasoning, then do experiment
e. depending on direction of current. Demo
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DC power
supply
North pole
What will happen if turn current to 1 amp?
a.
b.
c.
d.
e.
nothing, compass will point same as before.
compass will go back to original earth N oriented direction,
compass will reverse direction.
compass will point somewhere in between 5 A and N direction.
e. no idea
d. more current makes bigger magnetic fields, less current
makes smaller field, similar to earth’s field.
Repeat 5 amp experiment but with only one turn of wire instead of 50.
Which direction will compass point?
a. same as with 50 turns, b. almost entirely toward N pole. c. in between
b. B field from coil proportional to number of turns, so 1/50 the size. Much
less than earths field, so points almost towards earth N
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DC power
supply
Conclusion:
Current through coil of wire produces magnetic field
(electromagnet).
Magnetic field B depends on
number of turns N
amount of current I,
Direction of B depends on direction of current.
as equation shorthand
B = k I N = (constant)(current)(number of turns)
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Physics of transformer
1. Big current through lots of turns (Nprimary) gives big
magnetic field (B). If reverse current, reverse B.
current in
B
current out
2. Big B oscillating through many coils of wire Nsecondary
gives voltage-- makes current through bulb, etc.
Vout = Vin x Nsecondary/Nprimary
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In transformer, the voltage per loop is
the same for primary and secondary
Vout / Nsecondary= Vin / Nprimary
Which leads to
Vout = Vin x Nsecondary/Nprimary
Or
Vout / Vin = Nsecondary/Nprimary
Secondary (out)
Primary coil (in)
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How big a voltage can you get with a transformer?
Tesla tower demo
Vin to primary = 300 V.
Nprimary = 8 turns
N secondary = 8000 turns
Voltage at secondary (top ring) will be
a. 300 V, b. 2400 volts, c. 24,000 V, d. 300,000 V
think about what your prediction will imply about what will
happen when we turn it on.
d) Nsecondary/Nprimary = 8000/8 = 1000.
So voltage will increase by factor of 1000 300,000 V
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Transformer construction detail. The core.
B field from coil spreads out a lot, like in simulation for bar magnet.
Means less goes through second coil. Less current, wastes power.
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…)
Core does not carry current!
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Primary coil (in)
Secondary coil (out)
transformer basics1) Iron core concentrates field, couples primary to
secondary better, no wasted power.
2) If perfect coupling (real transformers pretty close)
Vsec =Vprimary x (# turns secondary/#turns primary)
I sec = I primary x (# turns primary/#turns secondary)
Know this from P=IV (power isn’t changing)
So step up voltage- more turns on secondary
step down voltage- fewer turns on secondary
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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
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turbine
boiler
I
cooling pond
All about turning the magnet / turbine… can do this with:
-Coal (produce steam)
-Nuclear (produce steam)
-Wind (directly rotate)
-Ocean waves (directly rotate)
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Today and next class:
• Sound waves and speakers – Short review of 1010 …
See Phys1010 Lecture notes. (Assumption by me)
• Audio Amplifiers – electronics that makes this work.
Looks complicated … new circuit part (capacitors, transistors,
resistors.)
Our goal is for you to be able to understand how components work
and reason through where electrons are flowing and why.
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