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Day 5::Questions?Read Power transmission Blm 11.2and Transformers Blm 11.3

Reminders/Updates:

Labs Today! Build your own Electromagnet & Transformer

Homework #2 due this Friday

How can I efficiently deliver electrical power to your house?

What do magnets have to do with electricity?

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Announcements• Power Transmission (today and Thursday)

– Power loss to wires

– Delivering Power (role of transformers)

– Creating electrical current (role of generators)

• Big Picture– Electronics: all about manipulating charges,

transmitting power

Lab #2 (Electromagnets and Transformers) this week.Make sure to do pre-lab on separate sheet ahead of time.

Homework #2 on web page due on Friday at 5 pm.Problem Solving Sessions Thursday 2-4pm, Friday 1-2pm.

* Sessions in the same room as labs!Must be in a group for clickers by end of week!

Power distribution and generation

Why use AC power?1. Energy loss in wires2. Virtues of high voltage3. Transformers and how they work

Power system and how transformers work in it.

Power generation (importance of Niagara Falls)

Big ideas:

• Changing magnetic fields produce Voltages and currents

• Currents produce magnetic fields.

Distributing Power Across the Country

• Long wires from power generating plants to homes (100’s of miles)

• How much power will we lose in the wires?

Where does the “lost” power go?

• How can we minimize power lost in the wires?

Explore what affects power loss.

eElectron man!

lots of energyat start.

Lots of volts

Exhausted! energy used up gettingthrough course.

Light Bulb… high resistance like trudging through

three feet of mud! takes lots of energy

to get through.

Wires: glide down pretty easily, just a few bumps. Lose a little bit of energy.

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- e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e e e ee e e e ee e e e ee e e e ee e e

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Rtotal = Rlight + Rwires

Usuallysmall

1) Lose energy by bumping into stuff in atoms. 2) Lose energy everywhere but in some parts more than others.3) Voltage change across something represents amount of energy lost

across it. 1.5 V +

1 2

B) Brighter in case iiTwo bulbs have more resistance than one RTotal = R1+R2 so less current flows by V = I R.Then Power = P = IV = I2R

Compare the brightness of bulb #1 in both cases…A) Brighter in case i B) Brighter in case ii C) Same Brightness

1

case i

case ii

1.5 V

2

Light Bulb

120 Volts

“60 Watt” Bulb

Invented by Thomas Edison (incandescent type)

Tungsten filament with resistance R, gets hot and emits light

This means when hooked up to 120 Volts (typical outlet), you get

60 Watts of Power [energy/time].What is resistance of this filament?

A) 60 Ohms B) 120 Ohms C) 240 Ohms D) 1000 Ohms

Power = V I 60 = 120 I I = 0.5 AmpsV = I R 120 = 0.5 R R = 240 Ohms

Power = V IV = I R

240 Volts

“60 Watt” Bulb If you hooked up this “60 Watt” Bulb to a 240 Volt Battery (instead of

120 V), would you still get 60 Watts of power?

No. So the name is given for 120 Volts…

120 Volts

“125 Watt” BulbFor a light bulb labeled “125

Watts” is the resistance of the filament higher or lower than for

a “60 Watt” bulb?A) HigherB) LowerC) Same

Power = V I = I^2 R

“Power Source”(We can vary

voltage output)

Wiring inside home

Lights and Power

Light

Wall Outlet 120 Volts AC

60 WattsLight

125 Watts

What is different between a 60 Watt and a 125 Watt bulb? a. the Voltage difference (amount of energy lost by each electron)

across the bulbb. the number of electrons passing through the filament each secondc. Choice a and the resistance of the filamentd. Choice b and the resistance of the filamente. Choice a and Choice b and the resistance of the filament

120 V Wiring inside home

Thinking like an electron….

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lots of energyat start. (V)

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e e e e e e ee e e e ee e e e ee e e e ee e e

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No energy left

No matter what the path, electrons lose all Voltage (energy per electron) by the time they return to outlet.

Vpath1 = Vpath2 = Vbattery (approx. for ideal R=0 wires)

Path 1 Path 2

V1 = Vstart V2 = Vstart

Assume wires don’t waste energy

“Power Source”(We can vary

Voltage output)

Wiring inside home

Lights and Power

Light

Wall Outlet 120 Volts AC

60 WattsLight

125 Watts

What if we hook a 60 Watt and a 125 Watt light bulb in series, which will be brighter? (Share reasoning before experiment)

A) 125 Watt bulb will be brighterB) 60 Watt bulb will be brighterC) Same brightness for both 126

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60W 120W

lots of energyat start.

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60 Watt, Higher Resistance

125 Watt bulbLower Resistance

Thought process: Same current must pass through both bulbs. Can figure out current from total resistance:

Rtotal = (R 60 W bulb + R 120 W bulb + Rwires)current i = V / Rtotal

Power = i2 RSo which one is brighter?

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HEATER

“Power Source”(We can vary

voltage output)

Wiring inside home

Home Wiring

Light

Wall Outlet 120 Volts AC

Turn on Heater (close switch), observe light bulb.

120 V

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Wiring inside home

1500 Watts 60 Watts

Light bulb dims. Why? Discuss with your group and come up with possible reasons. Share with class.

e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e e e ee e e e ee e e e ee e e e ee e e

lots of energyat start.

exhausted!

energy used up gettingthrough course. Vigor (V)

e

e

e

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glide down easily, just a few bumps. Hardly any energy.e

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ee

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e !?#%, bridgeout, stuck.

How much power and where is it going? Work through together. Plug voltage = 120 Volts

Total resistance of wires = 0.1 OhmsResistance of lightbulb = 240 OhmsTotal resistance = Rwires + R bulbs = 240.1 Ohms

Current = Plug Voltage / Total R = 120 Volts / 240.1 Ohms = 0.5 Amps

Voltage drop in wires = Current x resistance = 0.5 Amps x 0.1 Ohms = 0.05 VoltsPower into wires = Current x Voltage drop in wires

= 0.5 Amps x 0.05 Volts = 0.025 Watts

Power into bulb = current x voltage drop in bulb = 0.5 Amps x 119.95 Volts = 59.975 Watts

e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e e e ee e e e ee e e e ee e e e ee e e

lots of energyat start.

exhausted!

energy used up gettingthrough course. Vigor (V)

deepmud!(bulb)

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What happens when bridge getsfixed so have another route?

e

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e’s piled up down both routes, so still divide up and go down both, just end up faster on bridge route

pretty easy

What changes compared with bridge out (heater off) ?A)Current through upper wire is now largerB)Current through the upper wire is the sameC) Current through the upper wire is smallerD)It depends on the kind of wire

e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e e e ee e e e ee e e e ee e e e ee e e

lots of energyat start.

exhausted!

energy used up gettingthrough course. Vigor (V)

deepmud!(bulb)

e

e

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What happens when bridge getsfixed so have another route?

e

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e’s piled up down both routes, so still divide up and go down both, just end

up faster on bridge route

pretty easy

What changes compared with bridge out (heater off) ?HINT: this is tricky – REMEMBER… the wires have small resistance!

I. Current through upper wire is now largerII. Voltage drop across upper wire is now largerIII. Voltage drop around whole circuit is now largerIV. Current through bulb is smallerV. Voltage drop across bulb is smaller

a. I and III b. I and II c. I, II, V d. I, II, IV, V e. I, II, III, IV, V

e e e ee e e e ee e e e ee e e e ee e e e ee e e

e e e e e e ee e e e ee e e e ee e e e ee e e

lots of energy

at start.

exhausted!

energy used up getting

through course. Vigor (V)

deep

mud!

(bulb)

e

e

e

e

e

e

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e

What happens when bridge gets

fixed so have another route?

a. Can go easy path across bridge. Moves faster through (higher current).

b. Have to go down entry and exit road much faster. Lose more energy in wires

hitting bumps at high speed than when no heater.

c. Ones that pick mud path have a little less energy to

get through it than they did without bridge path, so get through it slower.

Current through mud, I = (V - Vroad)/R, but going faster so Vroad bigger than before,

go through mud slower than if bridge out. I smaller. (Vroad = I Rroad)

e

e

e

e

e

e

e

e’s piled up down both routes,

so still divide up and go down

both, just end up faster on

bridge route

pretty

easy

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 b. I, II, and III c. II, III, and IV

d. II, III, V. e. III only.

Answer is C.

Anything that either increase resistance of wires (e.g. thinner or

longer) OR increases current through the wires (e.g. adding

another heater).

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Power distribution and generation

Why use AC (Alternating Current) power?1. Loss in wires2. Virtues of high voltage3. Transformers and how they work

Power system and how transformers work in it...

•Changing magnetic fields produce Voltages and currents

•Currents (moving charges) produce magnetic fields

PowerPlant

Why is High Voltage Good

want thick wires, no longer thannecessary, still have some small

Example: Deliver 100 Watts to house, P = IV

0.1A

What is power loss in wires for each Voltage if R wire = 1 Ohm?

P = I Vdrop in wire (note: this is not Vcircuit)P = I (IR)= I2R = (I2) 1 Ohm

10A P= 100 Watt = same as power to house1A P = 1 W

0.1A P = 0.01 W = 0.0001 x power to house.so tremendous advantage to transmitting power at high V!

V = 10 V, I has to be 10 A.V= 100 V, I has to be 1 AV= 1000 V, I has to be?