11.2-11.3 Electric Power Distribution, 11.2-11.3 Electric Power Distribution, Generators and Motors Generators and Motors

New ideas for todayNew ideas for today

•Magnetic induction•Lenz’s law•Transformers and power transmission•Motors and Generators

Why such high voltage?Why such high voltage?

Transformers!Transformers!

Observations about Power Observations about Power DistributionDistribution Household power is AC (alternating current) Power comes in voltages like 120V & 240V Power is transmitted at “high voltage” Power transformers are everywhere

Power Consumption in wiresPower Consumption in wires Reminder:

power consumption = current × voltage drop voltage = resistance × current

power consumption = resistance × current2

So what? Wires waste power as heat Doubling current quadruples wasted power Better not transmit high current!

DCAC

EdisonTesla

Westinghouse

vs.vs.

AC = alternating currentAC = alternating current

Current switches direction 60 times per second (in N. America)

DC= “direct current” AC

Power TransmissionPower Transmission Power delivered to a city is:

power delivered = current × voltage drop Power wasted in transmission wires is:

power wasted = resistance × current2

For efficient power transmission: Use low-resistance wires

(thick, short copper) Use low current and high voltage drop

Can accomplish this with AC (alternating current) power transmission.

neutral hot

ground

7000 V7000 V

120 / 240 V120 / 240 V

160-800 kV160-800 kV

Voltage HierarchyVoltage Hierarchy

High voltage is dangerous High current is wasteful Use the following scheme:

low voltage circuits in neighborhoods (120/240 V) medium voltage circuits in cities (7000 V) high voltage circuits across the countryside (155,000-765,000 V)

Use transformers to change voltage

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Electromagnetic InductionElectromagnetic Induction Changing magnetic field electric field Electric field in conductor current Current magnetic field Induced magnetic field opposes the original

magnetic field change (Lenz’s law)

inductiveinductivechargingcharging

BB

Lenz’s LawLenz’s Law

That “emf” can be useful…That “emf” can be useful…

TransformerTransformer Alternating current in one circuit induces an alternating

current in a second circuit Transfers power between the two circuits Doesn’t transfer charge between the two circuits

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Current and VoltageCurrent and Voltage

Power arriving in the primary circuit must equal power leaving the secondary circuit

Power = current × voltage A transformer can change the voltage and

current while keeping the power unchanged!

Secondary voltage =Primary voltageSecondary turns

Primary turns

Step Down TransformerStep Down Transformer Fewer turns in secondary circuit so charge is

pushed a shorter distance Smaller voltage rise A larger current at low voltage flows in the

secondary circuit

Step Up TransformerStep Up Transformer More turns in secondary circuit so charge is pushed a

longer distance Larger voltage rise A smaller current at high voltage flows in

the secondary circuit

Transformers are oftenTransformers are often filled with nasty stufffilled with nasty stuff

You decide to use a transformer to increase the voltage from a battery, and hook it up in the circuit shown below. When you close the switch,

1.5

the voltage across the lightbulb is:(A) bigger than 1.5 V (B) smaller than 1.5 V (C) zero

Clicker questionClicker question

Electric Generators and MotorsElectric Generators and Motors

A generator provides electric power A generator requires a mechanical power A motor provides mechanical power A motor requires electric power

Alternator

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Electric GeneratorElectric GeneratorRotating magnet makes changing magnetic field induces AC current in the loop

Converts mechanical power into electrical power

Electric MotorElectric Motor Input AC power AC current makes changing magnetic field causes magnet to turn

Converts electrical power into mechanical power

A motor is a A motor is a generator run generator run backwards !backwards !