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What do magnets have to do with electricity?

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Power transmission

- Power loss to wires- Delivering Power - transformers- Creating electrical current - generators.

Power transmission - Power loss to wires

• Voltage dropped in wire = IRwire

• Power wasted in wire = I2Rwire

• Significant when:– I is large (supplying lots of appliances)– R is large (long wires)

120 V

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Long wires, some R

V1

Close heater switch I increases V1 drops Light bulb dims, wire gets warm

I

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)

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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

Voltage supplied bypower company

Voltage supplied to home. (some voltage drop in wires)

power plant

Power distribution questions

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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 b) less, same, less c) more, same, mored) less, less, less e) more, more, more.

Voltage supplied bypower company

Voltage supplied to home. (some voltage drop in wires)

power plant

Power distribution questions

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Voltage supplied bypower company

Voltage supplied to home.

power plant

Distributing power at high voltage

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)

US- 60 hertz (60 oscillations/s) 120 Vrms (av. voltage diff)Europe-50 Hz, 230 Vrms

look at wall outlet with Oscilloscope (measures voltage difference)

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Oscilloscope

A B

Voltage at Alarger than at B

Voltage at Blarger than at A

No voltage diffCurrent = 0 Amps

+170V

-170V

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1. In light bulbs and heaters? a) yes, b) no

2. In computers, cell phones, and electronics? a) yes, b) no

Does AC work the same as DC?

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power plant

5000V 500,000 V (on towers) substation

7200 Vrunning around town.

120 Vshort wiresinto houses

Transformers enable us to:• Change voltage easily• Transfer power between circuits so one house doesn’t effect next.

Transformers in the power distribution system

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• Convert AC voltage up and down• Made of two coils of wire (around a core)

Primary coil (in)

AC current in primary coil (e.g. from power company)

Two Steps:

A) Changing electric current in primary coil produce changing magnetic field

B) Changing magnetic field produces a current in the secondary coil

Secondary coil (out)

produces AC current in secondary coil (e.g. current in your house)

How do transformers work?

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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.

SN

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 Bar magnetCoil of wire

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DC powersupply

compass with I = 0

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

Producing magnets using electric currents

North pole

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DC powersupply

Conclusion:Current through coil of wire produces magnetic field(electromagnet).

Magnetic field B depends on---

as equation shorthandB = k I N = (constant)(current)(number of turns)

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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

Secondary coil (out)

Back to transformersConclusion 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?

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Q: What will happen if I move coil more slowly? a) brighter, b) dimmer, c) same

Producing voltages and currents using magnets.

Bulb lights up if we move coil in and out of magnet

North South

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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.

c)

a)b)

Useful Phet on induced voltage

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Producing electric currents using magnets.

Bulb lights up if we move coil in and out of magnet

North South

Q: What will happen if I use coil with 3 turns instead of 500? a) brighter, b) dimmer, c) same (discuss reasoning)

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

Secondary coil (out)

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!

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Assume all B is channeled from primary through secondary Rate of change of B is the same in both coils V = k B/t, per loop the voltage per loop is the same for primary and secondary:

Vout / Nsecondary = Vin / Nprimary

Vout

Vin

Which leads to the transformer rule:

Vout = Vin x Nsecondary/Nprimary or

Vout / Vin = Nsecondary/Nprimary

Transformer rule

Primary Secondary

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Vout

Vin

Transformer rule for current

Primary Secondary

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

Iout

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current in

B current out

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.

Transformer construction detail - The core.

What will happen tolight 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!

Changing voltages

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

Vout = Vin × Nsecondary / Nprimary

Changing voltages

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

Vout = Vin × Nsecondary / Nprimary

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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.

step up transformer – increases voltage – decreases currentstep down transformer – decreases voltage – increases current

Transformer summary

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)

Secondary coil (out)Primary coil (in)

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Electric power generation

Q: How did I generate power earlier in class?

A: By moving a coil relative to a magnetic field.

N S

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Power plant generators: - Use steam or water to spin magnets past coils (or vice-versa). - Like transformer, but changing B created by moving magnet

I, V out

iron corespinning turbine

magnets

NN

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S

S

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http://phet.colorado.edu/simulations/sims.php?sim=Generator

1. How does frequency of voltage oscillation dependon how fast magnet is spun?

a) twice magnet rotation frequency, b) same, c) halfd) 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

Generator demo

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E = mgh, power = energy/sec = mass/sec x gh

h~ 40% efficientPelectrical out = .4 (mass water/s x gh)

Hydroelectric turbine

[In a wind or wave driven generator, the wind/waves turn the turbine directly]S N

N S

N

S

S

N

How is turbine driven in a real power plant?

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Iboiler

turbine

cooling pond

Nuclear/fossil fuel power plant

• 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.

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?

N S