Power Transmission:
relating E(lectricity) & M(agnetism)
Phys 1020 Day 6
Power Transmission
Delivering safe / efficient power
Using A/C for transformers
Generating Power
Updates:
Keep up the good work
Eyes to the web re:
assignments & exams!
1
Recall the last question from Tuesday. We turn hairdryer
(heater) on and the light bulb dims because the parallel
path through the heater increases the total current in the
household wires, increasing the drop in those wires,
reducing the voltage drop across the light bulb, reducing its
brightness.
Now;
What will make bulb even dimmer?
I. Shorter wires
II. Longer wires
III. Adding another heater
IV. Thinner wires
V. Fatter wires
a. I and V
d. II, III, V.
b. I, II, and III
e. III only.
c. II, III, and IV
3
Power distribution and generation
Why use AC power?
1. Loss in wires
2. Virtues of high voltage
3. Transformers and how they work
Power system and how transformers work in it.
•Changing magnetic fields produce
voltages and currents
•Currents produce magnetic fields.
4
Why high voltage good
want thick wires, no longer than
necessary, still have some R over miles.
Deliver 100 W to house, P = I (DV)
V = 10 V, I has to be 10 A.
V= 100 V, I has to be 1 A
V= 1000 V, I has to be ? 0.1A
What is power loss in wires for each voltage if R wire = 1 ohm?
P = I DVdrop in wire (note: this is not Vcircuit)
P = I (IR)= I2R = (I2) 1 Ohm
10A -> P= 100 Watt = same as power to house
1A-> P = 1 W
0.1A -> P = .01 W = .0001 x power to house.
so tremendous advantage to transmitting power at high V!
power plant
5
Voltage supplied by
power company
power plant
Voltage supplied to home.
(some voltage drop in wires)
want thick wires, no longer than
necessary, still have some R over miles.
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?
Power to house = current x voltage supplied to home: P = IV.
If voltage supplied to home is 100 Volts,
I = Power/Voltage = 10,000 Watts/100 V = 100 Amps
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
6
Voltage supplied by
power company
power plant
Voltage supplied to home.
(some voltage drop in wires)
want thick wires, no longer than
necessary, still have some R over miles.
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.
What changes compared with home voltage of 100 Volts ?
Fill in the blanks:
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 b. less, same, less
d. less, less, less
e. more, more, more.
c. more, same, more
7
Voltage supplied by
power company
power plant
Voltage supplied to home.
(some voltage drop in wires)
want thick wires, no longer than
necessary, still have some R over miles.
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.
Giant advantage to having high V.
kills people!!
Disadvantage to high V
8
minimize both power lost in wires and customer loss
by electrocution.
power plant
transmit power long distance
at high V and low current
low V, high I
in house
How to change V up and down efficiently?
Use Alternating Current (AC) and transformers!!
Tesla and Westinghouse.
(big battle with Edison … wanted to use Direct Current (DC))
9
Electric power distribution system. Why physics
makes it the way it is.
Key features:
a. Very high voltage (50,000+ V) on the long transmission lines.
b. Voltage reduced in steps as get closer to home (120 V).
c. Uses alternating current (AC) and voltage. Change 60 times/s.
10
power distribution system, bunch of different voltages
500,000 V (on towers)
substation
power plant
5000V
Transformers- change V
120 V
short wires
into houses
7200 V
running around
town.
11
Remaining challenge.
How to change V up and down efficiently at various parts of
power grid?
Next: Physics of transformers.
i) Learn how electric currents produce magnetic fields.
ii) How changing magnetic fields produce voltages (and currents) in
wires.
iii) How can combine to change an oscillating voltage to higher or
lower voltage in a transformer. (use in house, cell phone charger…)
iv) Why only works with AC and so requires power system be AC.
measure voltage coming out of wall socket, what will we see?
12
What’s Alternating current?
look at wall outlet
with Oscilloscope
(measures voltage difference)
B
Oscilloscope
US- 60 hertz (60 oscillations/s)
120 V (average voltage diff)
Europe-50 Hz, 230 V
Voltage difference
A
No voltage diff
Current = 0 Amps
Voltage at A
larger than at B
0
time
Voltage at B
larger than at A
13
a. yes, b. no
Will AC work the same as DC?
light bulbs?
yes-- think like electron, flying around
heaters?
bumping into stuff. (resistor sim.)
AC also works for motors, but build different
than DC
computers, cell phones, and electronics?
--no, have to convert 120 V AC back to 3-5 V DC. (chargers)
~
O
14
Will AC work the same as DC?
See simulation
http://phet.colorado.edu/en/simulati
on/circuit-construction-kit-acvirtual-lab
~
O
15
Transformer - converts AC voltage up and down
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 flowing in your house)
Two Steps:
A) Magnetic Fields (changing ) produce a current (secondary coil)
B) Electric currents produce magnetic field (primary coil)
17
what is a magnet and a magnetic field?
• magnetism different force from electric.
No + and - charges.
North and South poles hooked together. ALWAYS.
• Opposite poles attract.
• Compass is a little bar magnet. Earth is big magnet.
N end of compass needle attract to S end of earth magnet.
“Magnetic field” force on a north pole of magnet at each
location.
N
S
18
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
19
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.
20
B. 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 use coil with 3 turns instead of
500? a. brighter, b. dimmer, c. same (discuss reasoning)
21
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.
22
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
23
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
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
24
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)
25
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
26
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)
27
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
28
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!
29
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
30
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
31