Transcript step up transformer

Revisiting What We Know…
Reminders/Updates:
Day 7:
Review lecture
HW3 in/ HW4 out
Thurs: Exam 1, in class
Bring pencil, calculator, 3x5” formula card
Exam mechanics
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1 hr long
In class G125, 11am prompt!
1 3x5 handwritten formula card
Pencil
Calculator – (non-internet connectable device)
Do not cheat. It’s the surest way to piss me off
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 …
--what are the differences on a macroscopic scale?
-- how is conductivity influenced by how electrons are found in bands
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.
Review in reverse order today.
Power / Transformers
Energy Bands / Insulators, Conductors, Photoconductors.
Electric circuits
Transformer summary
Primary coil (in)
Secondary coil (out)
1) Oscillating current in primary creates oscillating B field
2) B field transfers power from primary to secondary. 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
4
Magnetic fields
• Magnetic field around a coil of wire – what does it look like?
• Q: Where is field strongest in this diagram?
C
B
A
E
D
Transformer construction detail. The core.
current in
current out
Q: What happens if current oscillates 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.
NOTE: Not everything curly is a transformer;
e.g. lightbulb filament is NOT.
Transformer questions
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
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
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
Power distribution system
power plant
500,000 V (on towers)
substation
5000V
5000 V
running around
town.
Why do we transmit power at
different voltages in different
parts of the system?
(2 main reasons)
120 V
short wires
into houses
Electric power generation
Q: In general, how do we generate electrical power
magnets
N
S
Power plants: use steam or water
to spin magnets past coils (or vice-versa)
N
I, V out
S
S
N
N
S
iron core
spinning turbine
9
Different types of power plants
• All power plants create electricity by moving a coil of wire
relative to a magnetic field (spinning one or the other)
• Use different energy sources to spin the turbine –
hydroelectric, nuclear, coal, gas, wind, waves. This is the
energy that is converted into electrical energy.
• Some sources more environmentally friendly than others
Generating electricity
2
3
4
5
6
N
8
N
time
N
s
s
7
s
1
time
B
Voltage = k (DB/Dt) N
Q: Voltage out of coil is biggest at a) 1, b) 2, c) 4, d) 7
V
time
Electrical conduction in different materials Energy levels and electron occupation.
Electron Energy
3
2
1
How many energy levels are in band 1?
a. none, b. 1, c. between 1 and 10, d. an enormous number
Electrical conduction in materials Energy levels and electron occupation.
Electron Energy
3
2
1
True / False
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
energy levels they can move into.
a) 1T 2T,
b) 1F 2F,
c) 1T 2F,
d) 1F 2T
What type of material 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.
Important circuit ideas
1. Current is conserved (electrons don’t disappear)
2. Change in V over circuit = V of battery, or energy source
3. V= I R (Ohm’s law)
- useful for whole circuit (R total, Vtotal, give I total)
- or individual component (e.g. Rbulb, Vbulb give I bulb),
…….Be careful about what R, and V are.
4. P = I V = I (IR) = I2R
= (V/R)V = V2/R
power dissipated across object R
5. Resistors in series:
Resistances add: Rtot = R1 + R2
Current through all resistors is the same
6. Resistors in parallel:
Voltage drop across parallel legs of circuit is same
15
Series Circuits
1
120V
2
200ohm
100ohm
What is the voltage drop across bulb 2?
a. 120V, b. 0V
c. 60V,
d. 80V,
e. 40V
Parallel circuits
I
120V
1
2
200ohm
100ohm
What is I?
a. 0.4A, b. 0.6A, c. 1.2 A,
d. 1.8A,
e. 120A
Where does the power go?
Hair Dryer, Lights, and Stereo plugged into same outlet
house wires
Hair Dryer
Lightbulb 1
Stereo
10 A
0.5 A
Lightbulb 2
2A
What is the current through the house wires?
a. 10 Amps
b. 12.5 Amps c. 7.5 Amps
d. more than 12.5 Amps.
If the resistance of each light bulb is 100 Ohms, how much power is going
into the two bulbs combined?
a. 25 Watts,
b. 50 Watts
c. 100 Watts
d. 200 Watts
Parallel circuits and power loss
120 V
Why does the light dim when the heater turns on?
Mains electricity in different countries
Different voltages, different plugs, 2 or 3 wires?
Some countries (e.g. UK) have 240V mains rather than the 120V used
in the US.
240V more dangerous than 120V
- easier to fibrillate heart (but wastes less power in wires).
Different plugs partly historical, but modern UK plugs harder to touch
“live” pin than US. Probably designed that way because higher voltage
more dangerous.
3rd wire- ground. Protection for case of appliance
electric appliance
Thin
Thick
Round
Plug
return
ground
\/\/\/\/\/
live
-Live: ±170V
-Return: ~0V
-Ground: Always 0V