2Introductory Question Electric power reaches this city via high voltage transmission lines. What fraction of the electric charges traveling on those transmission lines pass through this room?About 1%About 0.01%Exactly 0%
3Observations about Electric Power Distribution Household electricity is alternating current (AC)Household voltages are typically 120V or 240VPower is distributed at much higher voltagesPower transformers are commonPower substations are less common
44 Questions about Electric Power Distribution Why isn’t power transmitted at low voltages?Why isn’t power delivered at high voltages?What is “alternating current” and why use it?How does a transformer transfer power?
5Question 1Why isn’t power transmitted at low voltages?
6Electric Power and a Lightbulb An electric current passing through a lightbulbexperiences a voltage drop in the filamentand converts electric power into thermal power.The power provided to the lightbulb isthe product of the current in its filamenttimes the voltage drop that current experiences, orpower provided = current· voltage drop in filament.Doubling voltage or current doubles bulb power
7Electric Power and a Wire An electric current passing through a wirewastes electric power as thermal power,power wasted = current· voltage drop in wire.Since the wire obeys Ohms law,voltage drop in wire = resistance· current,the power it wastes ispower wasted = resistance· current2.Doubling current quadruples wasted power!
8Large Currents are Wasteful The goal of a power distribution system is totransmit lots of electric power to a city,power transmitted = current· voltage drop at city,while wasting only a little electric power in the wires,power wasted = resistance· current2.That goal can be met byusing a small current and a huge voltage drop,together with low-resistance wires.Demonstration: Distributing DC Power - Current Trouble.
9Question 2Why isn’t power delivered at high voltages?
10High Voltages are Dangerous When large voltage drops are available,charges tend to jump toward the lower voltageand currents tend to flow through unexpected paths.High-voltage electrical power in a home isa spark hazard,a fire hazard,and a shock hazard.
11The Voltage Hierarchy Large currents are too wasteful for transmission High voltages are too dangerous for deliverySo electric power distribution uses a hierarchy:high voltage circuits in the countrysidemedium voltage circuits in citieslow voltage circuits in neighborhoods and homesTransformers transfer power between circuits!
12Question 3What is “alternating current” and why use it?
13Alternating Current (AC) In alternating current,the voltages of the power delivery wires alternateand the resulting currents normally alternate, too.Alternating voltage in the UScompletes 60 cycles per second,reversing every 1/120 second.Demonstration: Alternating Current Really Alternates
14AC and TransformersAC has little effect on simple electric devices (e.g., lightbulbs, space heaters, toasters),but it’s a nuisance for electronic devices (e.g., computers, televisions, sound systems).AC produces alternating magnetic fieldsand transformers use alternating magnetic fieldsto move power between AC circuits!
15Question 4How does a transformer transfer power?
16Electromagnetism (Version 2) Magnetic fields are produced bymagnetic poles (but free poles don’t seem to exist),moving electric charges,and changing electric fields [more later…].Electric fields are produced byelectric charges,moving magnetic poles,and changing magnetic fields.
17Electromagnetic Induction Moving poles or changing magnetic fieldsproduce electric fields,which propel currents through conductors,and those currents, in turn, produce magnetic fields.Overall, changing magnetic effects induce currents and thereby produce magnetic fields.Lenz’s law predicts the nature of those fields:“The effects of magnetic induction oppose the changes that produce them.”Demonstration: Jumping RingsDemonstration: Eddy Current PendulumDemonstration: A Magnet Falling Through A Copper PipeDemonstration: A Magnet Sliding Through a Half-Copper, Half-Plexiglas TrackDemonstration: Superconductors and the Meissner Effect
18TransformerAlternating current in one circuit can induce an alternating current in a second circuitThe transformertransfers power between the circuitsbut it doesn’t transfer any charges between the circuitsDemonstration: Alternating Current and Transformers
19Current and VoltagePower arriving in the primary circuit must equal power leaving in the secondary circuitSince power is the product of voltage· current,a transformer can obey energy conservationwhile exchanging voltage for currentor current for voltage!
20Step-Down Transformer A step-down transformerhas relatively few turns in its secondary coilso charge is pushed a shorter distanceand experiences a smaller voltage riseA larger current at smaller voltage flows in the secondary circuitDemonstration: Step-Down Transformers
21Step-Up Transformer A step-up transformer has relatively many turns in its secondary coilso charge is pushed a longer distanceand experiences a larger voltage riseA smaller current at larger voltage flows in the secondary circuitDemonstration: Step-Up TransformersDemonstration: A Tesla Coil
22Power Distribution System A step-up transformer increases voltage for efficient long-distance transmissionA step-down transformer decreases voltage for safe delivery to communities and homesDemonstration: Distributing AC Power - Transformers Save the Day
23Introductory Question (revisited) Electric power reaches this city via high voltage transmission lines. What fraction of the electric charges traveling on those transmission lines pass through this room?About 1%About 0.01%Exactly 0%
24Summary about Electric Power Distribution Electric power is transmitted at high voltagesElectric power is delivered at low voltagesTransformers transfer power between circuitsTransformers require AC power to operateThe power distribution system is AC