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Transcript
```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.
5
Distributing power at high voltage
Voltage supplied by
power company
Voltage supplied to home.
power plant
Advantage: Massive reduction in power loss in wires
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
6
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
7
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
8
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.
9
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
10
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.
11
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)
14
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
15
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
16
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.
17
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)
18
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
20
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
21
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!
22
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
25
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
26
Generator demo
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
27
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
28
Nuclear/fossil fuel power plant
boiler
turbine
I
cooling pond
• Fuel is used to boil water and make steam pressure
• Steam rotates the turbine
29
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?
30
```
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