Transcript secondary coil

Power transmission
Voltage difference
What do magnets have to do with electricity?
time
- Power loss to wires
- Delivering Power - transformers
- Creating electrical current generators.
1
Power transmission - Power loss to wires
I
120 V
\/\/\/\/\/
• Voltage dropped in wire = IRwire
• Power wasted in wire = I2Rwire
• Significant when:
– I is large (supplying lots of appliances)
– R is large (long wires)
Long wires, some R
Close heater switch
 I increases
 V1 drops
 Light bulb dims, wire gets warm
V1
How can we efficiently supply a town with
power from a power station 30 miles away?
• P = I ×V
• Transmit power at high V and low I
 Voltage drop smaller (IRwire)
 Power wasted much smaller (I2Rwire)
Power distribution questions
Voltage supplied by
power company
Voltage supplied to home.
(some voltage drop in wires)
power plant
Q: Power plant decides to deliver power of 10,000 W to power a house:
How much current needed if voltage at home is 100 V?
A: Power to house = current x voltage supplied to home: P = IV.
I = Power/Voltage = 10,000 W/100 V = 100 A
Q: At this current, what is power loss in wires if Rwire = 1 ohm?
a) 100 W, b) 10 W, c) 1000 W, d) 10,000 W, e) 100,000 W
4
Power distribution questions
Voltage supplied by
power company
Voltage supplied to home.
(some voltage drop in wires)
power plant
Power plant still delivers 10,000 W to power a house, but now adjusts voltage
supplied so the voltage at home is 10,000 Volts.
Q: What changes compared with home voltage of 100 Volts ?
Current through wire needed to supply power will be ---------.
Voltage drop across segments of wire will be ----------.
Power going into heating the wires will be ----------.
a) same, same, same
d) less, less, less
b) less, same, less
c) more, same, more
e) more, more, more.
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Distributing power at high voltage
Voltage supplied by
power company
Voltage supplied to home.
power plant
Advantage: Massive reduction in power loss in wires
Disadvantage: Kills people
Solution:
• Transmit at high V over long distances
• Reduce to low V near houses
• Change V up and down efficiently with AC and transformers
• See Blm for history of power transmission
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Alternating current (AC)
look at wall outlet
with Oscilloscope
(measures voltage difference)
Oscilloscope
Voltage difference
A B
No voltage diff
Current = 0 Amps
Voltage at A
larger than at B
+170V
0
-170V
time
US- 60 hertz (60 oscillations/s)
120 Vrms (av. voltage diff)
Europe-50 Hz, 230 Vrms
Voltage at B
larger than at A
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Does AC work the same as DC?
1. In light bulbs and heaters? a) yes, b) no
2. In computers, cell phones, and electronics? a) yes, b) no
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Transformers in the power distribution system
5000V
500,000 V (on towers)
substation
power plant
Transformers enable us to:
• Change voltage easily
• Transfer power between circuits
so one house doesn’t effect next.
120 V
short wires
into houses
7200 V
running around
town.
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How do transformers work?
• Convert AC voltage up and down
• Made of two coils of wire (around a core)
Secondary coil (out)
Primary coil (in)
AC current in primary coil (e.g. from power company)
produces AC current in secondary coil (e.g. current in your house)
Two Steps:
A) Changing electric current in primary coil produce changing magnetic field
B) Changing magnetic field produces a current in the secondary coil
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What is a magnet and a magnetic field?
• Natural phenomenon closely related to electricity
• North and south poles, opposite poles attract
• Important difference:
Magnetism has no monopoles (like + and - charges.)
North and South poles are hooked together. ALWAYS.
• A magnetic field describes the force on
a north pole of a magnet at each location
in space
Compass is a little bar magnet.
Earth is a big bar magnet.
N end of compass needle
attracted to S end of earth
magnet.
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How can we make a magnet?
1. Magnetic material (e.g. Iron)
- Electrons behave like tiny bar magnets
- Usually paired in opposite orientations – cancel out
- Iron retains some unpaired electrons – billions of atomic
magnets combine to make a big magnet
2. Electric currents produce
magnetic fields
- Magnetic field around a
coil
of wire is much like that
around a bar magnet
- Electromagnet
Coil of wire
Bar magnet
Producing magnets using electric currents
North pole
compass with I = 0
DC power
supply
Q: 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
13
DC power
supply
Conclusion:
Current through coil of wire produces magnetic field
(electromagnet).
Magnetic field B depends on
as equation shorthand
B = k I N = (constant)(current)(number of turns)
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Back to transformers
Conclusion so far:
- Steady current in primary coil will produce a steady B field
- Direction of B field depends on direction of current
-Changing (AC) current will produce a changing B field – step A
Next:
-What effect does the changing B field have on the secondary coil?
Secondary coil (out)
Primary coil (in)
Two Steps:
A) Changing electric current in primary coil produce changing magnetic field
B) Changing magnetic field produces a current in the secondary coil
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Producing voltages and currents
using magnets.
North
South
Bulb lights up if we move
coil in and out of magnet
Q: What will happen if I move coil more slowly?
a) brighter, b) dimmer, c) same
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Useful Phet on induced voltage
c)
b)
a)
Move bar magnet up across front of coil.
Voltage will be biggest when
a) just starting , b) half way across c) lined up with middle of coil.
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Producing electric currents
using magnets.
North
South
Bulb lights up if we move
coil in and out of magnet
Q: What will happen if I use coil with 3 turns instead of 500?
a) brighter, b) dimmer, c) same (discuss reasoning)
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Back to transformers
Conclusion:
- Changing the magnetic field through secondary coil will give a voltage
drop across it.
- If secondary coil is part of a complete circuit, current will flow – step B
-Transformer physics complete!
Secondary coil (out)
Primary coil (in)
Two Steps:
A) Changing electric current in primary coil produce changing magnetic field
B) Changing magnetic field produces a current in the secondary coil
Transformer rule
Assume all B is channeled from primary through secondary
 Rate of change of B is the same in both coils
 V = k DB/Dt, per loop
 the voltage per loop is the same for primary and secondary:
Vout / Nsecondary = Vin / Nprimary
Which leads to the transformer rule:
Vout = Vin x Nsecondary/Nprimary or
Vout / Vin = Nsecondary/Nprimary
Vin
Vout
Primary
Secondary
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Transformer rule for current
Vout = Vin × Nsecondary / Nprimary
In an ideal transformer, no power (P = IV) is wasted:
 Iin Vin = Iout Vout
 Iout = Iin ×Vin / Vout = Iin × Nprimary / Nsecondary
Increase voltage  Decrease current and vice versa
Iin
Vin
Vout
Iout
Primary
Secondary
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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 B goes through second coil. Less current, wastes power.
current in
B
current out
What will
happen to
light bulb?
iron core concentrates B (sucks it in),  more changing B through
second coil,  bigger induced voltage  bigger current out!
Core does not carry current!
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Changing voltages
Vout = Vin × Nsecondary / Nprimary
Step up transformer: Nsecondary > Nprimary
Q: If Vin 5000V AC, Nprimary = 50 and Nsecondary = 5000, what is Vout ?
a) 50V b) 500V c) 5000V d) 50,000 V e) 500,000 V
Changing voltages
Vout = Vin × Nsecondary / Nprimary
Step down transformer: Nsecondary < Nprimary
Q: If Vin 120V AC, Nprimary = 500 and Nsecondary = 50, what is Vout ?
a) 12,000V b) 1200V c) 120V d) 12V e) 1.2 V
Q: What would happen to a 40W lightbulb if wired to the secondary?
a)
Filament burns out
b)
Same brightness as if wired to mains
c)
Just lights up a bit
d)
No light at all
Transformer summary
Primary coil (in)
Secondary coil (out)
1) Oscillating current in primary creates oscillating B field
2) Iron core concentrates B field, improving coupling between primary
and secondary  no wasted power.
3) Oscillating B through secondary coil creates voltage which drives a
current through bulb etc.
Transformer rule assumes perfect coupling (real transformers pretty close)
Vsec = Vprimary x (Nsec/Nprimary)
Also Isec = Iprimary x (Nprimary/Nsec)
(since P=IV is constant)
step up transformer – increases voltage – decreases current
step down transformer – decreases voltage – increases current
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Electric power generation
N
Q: How did I generate power earlier in class?
S
A: By moving a coil relative to a magnetic field.
Power plant generators:
- Use steam or water to spin magnets
past coils (or vice-versa).
- Like transformer, but changing B
created by moving magnet
S
magnets
N
I, V out
N
S
S
N
N
S
iron core
spinning turbine
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Generator demo
http://phet.colorado.edu/simulations/sims.php?sim=Generator
1. How does frequency of voltage oscillation depend
on how fast magnet is spun?
a) twice magnet rotation frequency, b) same, c) half
d) unrelated, e) 4 times rotation frequency.
2. How does size of voltage depend on how fast spun?
a) unrelated, b) faster gives more V, c) faster gives less V
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How is turbine driven in a real power plant?
Hydroelectric turbine
E = mgh, power = energy/sec
= mass/sec x gh
~ 40% efficient
Pelectrical out = .4 (mass water/s x gh)
h
S
N
S N
[In a wind or wave driven
generator, the wind/waves turn
the turbine directly]
N S
N
S
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Nuclear/fossil fuel power plant
boiler
turbine
I
cooling pond
• Fuel is used to boil water and make steam pressure
• Steam rotates the turbine
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How is energy conserved in a power plant?
The induced current in the coil produces a magnetic field that acts
back on the rotating magnet to oppose its motion. (Lenz’s Law)
Thus mechanical energy is taken from the rotor and converted to
electrical energy.
N
S
Generator demo
- open switch, no current to light bulb.
- closed switch, current flows through light bulb
In which case is it hardest to turn the generator?
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