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Electric Power Distribution 1 Electric Power Distribution.

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Presentation on theme: "Electric Power Distribution 1 Electric Power Distribution."— Presentation transcript:

1 Electric Power Distribution 1 Electric Power Distribution

2 Electric Power Distribution 2 Introductory 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? 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? A. About 1% B. About 0.01% C. Exactly 0%

3 Electric Power Distribution 3 Observations about Electric Power Distribution Household electricity is alternating current (AC) Household electricity is alternating current (AC) Household voltages are typically 120V or 240V Household voltages are typically 120V or 240V Power is distributed at much higher voltages Power is distributed at much higher voltages Power transformers are common Power transformers are common Power substations are less common Power substations are less common

4 Electric Power Distribution 4 4 Questions about Electric Power Distribution Why isn’t power transmitted at low voltages? Why isn’t power transmitted at low voltages? Why isn’t power delivered at high voltages? Why isn’t power delivered at high voltages? What is “alternating current” and why use it? What is “alternating current” and why use it? How does a transformer transfer power? How does a transformer transfer power?

5 Electric Power Distribution 5 Question 1 Why isn’t power transmitted at low voltages? Why isn’t power transmitted at low voltages?

6 Electric Power Distribution 6 Electric Power and a Lightbulb An electric current passing through a lightbulb An electric current passing through a lightbulb experiences a voltage drop in the filament experiences a voltage drop in the filament and converts electric power into thermal power. and converts electric power into thermal power. The power provided to the lightbulb is The power provided to the lightbulb is the product of the current in its filament the product of the current in its filament times the voltage drop that current experiences, or times the voltage drop that current experiences, or power provided = current· voltage drop in filament. Doubling voltage or current doubles bulb power Doubling voltage or current doubles bulb power

7 Electric Power Distribution 7 Electric Power and a Wire An electric current passing through a wire An electric current passing through a wire wastes electric power as thermal power, wastes electric power as thermal power, power wasted = current· voltage drop in wire. Since the wire obeys Ohms law, Since the wire obeys Ohms law, voltage drop in wire = resistance· current, the power it wastes is the power it wastes is power wasted = resistance· current 2. Doubling current quadruples wasted power! Doubling current quadruples wasted power!

8 Electric Power Distribution 8 Large Currents are Wasteful The goal of a power distribution system is to The goal of a power distribution system is to transmit lots of electric power to a city, transmit lots of electric power to a city, power transmitted = current· voltage drop at city, while wasting only a little electric power in the wires, while wasting only a little electric power in the wires, power wasted = resistance· current 2. That goal can be met by That goal can be met by using a small current and a huge voltage drop, using a small current and a huge voltage drop, together with low-resistance wires. together with low-resistance wires.

9 Electric Power Distribution 9 Question 2 Why isn’t power delivered at high voltages? Why isn’t power delivered at high voltages?

10 Electric Power Distribution 10 High Voltages are Dangerous When large voltage drops are available, When large voltage drops are available, charges tend to jump toward the lower voltage charges tend to jump toward the lower voltage and currents tend to flow through unexpected paths. and currents tend to flow through unexpected paths. High-voltage electrical power in a home is High-voltage electrical power in a home is a spark hazard, a spark hazard, a fire hazard, a fire hazard, and a shock hazard. and a shock hazard.

11 Electric Power Distribution 11 The Voltage Hierarchy Large currents are too wasteful for transmission Large currents are too wasteful for transmission High voltages are too dangerous for delivery High voltages are too dangerous for delivery So electric power distribution uses a hierarchy: So electric power distribution uses a hierarchy: high voltage circuits in the countryside high voltage circuits in the countryside medium voltage circuits in cities medium voltage circuits in cities low voltage circuits in neighborhoods and homes low voltage circuits in neighborhoods and homes Transformers transfer power between circuits! Transformers transfer power between circuits!

12 Electric Power Distribution 12 Question 3 What is “alternating current” and why use it? What is “alternating current” and why use it?

13 Electric Power Distribution 13 Alternating Current (AC) In alternating current, In alternating current, the voltages of the power delivery wires alternate the voltages of the power delivery wires alternate and the resulting currents normally alternate, too. and the resulting currents normally alternate, too. Alternating voltage in the US Alternating voltage in the US completes 60 cycles per second, completes 60 cycles per second, reversing every 1/120 second. reversing every 1/120 second.

14 Electric Power Distribution 14 AC and Transformers AC has little effect on simple electric devices (e.g., lightbulbs, space heaters, toasters), AC 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). but it’s a nuisance for electronic devices (e.g., computers, televisions, sound systems). AC produces alternating magnetic fields AC produces alternating magnetic fields and transformers use alternating magnetic fields and transformers use alternating magnetic fields to move power between AC circuits! to move power between AC circuits!

15 Electric Power Distribution 15 Question 4 How does a transformer transfer power? How does a transformer transfer power?

16 Electric Power Distribution 16 Electromagnetism (Version 2) Magnetic fields are produced by Magnetic fields are produced by magnetic poles (but free poles don’t seem to exist), magnetic poles (but free poles don’t seem to exist), moving electric charges, moving electric charges, and changing electric fields [more later…]. and changing electric fields [more later…]. Electric fields are produced by Electric fields are produced by electric charges, electric charges, moving magnetic poles, moving magnetic poles, and changing magnetic fields. and changing magnetic fields.

17 Electric Power Distribution 17 Electromagnetic Induction Moving poles or changing magnetic fields Moving poles or changing magnetic fields produce electric fields, produce electric fields, which propel currents through conductors, which propel currents through conductors, and those currents, in turn, produce magnetic fields. and those currents, in turn, produce magnetic fields. Overall, changing magnetic effects induce currents and thereby produce magnetic fields. Overall, changing magnetic effects induce currents and thereby produce magnetic fields. Lenz’s law predicts the nature of those fields: Lenz’s law predicts the nature of those fields: “The effects of magnetic induction oppose the changes that produce them.” “The effects of magnetic induction oppose the changes that produce them.”

18 Electric Power Distribution 18 Transformer Alternating current in one circuit can induce an alternating current in a second circuit Alternating current in one circuit can induce an alternating current in a second circuit The transformer The transformer transfers power between the circuits transfers power between the circuits but it doesn’t transfer any charges between the circuits but it doesn’t transfer any charges between the circuits

19 Electric Power Distribution 19 Current and Voltage Power arriving in the primary circuit must equal power leaving in the secondary circuit Power arriving in the primary circuit must equal power leaving in the secondary circuit Since power is the product of voltage· current, Since power is the product of voltage· current, a transformer can obey energy conservation a transformer can obey energy conservation while exchanging voltage for current while exchanging voltage for current or current for voltage! or current for voltage!

20 Electric Power Distribution 20 Step-Down Transformer A step-down transformer A step-down transformer has relatively few turns in its secondary coil has relatively few turns in its secondary coil so charge is pushed a shorter distance so charge is pushed a shorter distance and experiences a smaller voltage rise and experiences a smaller voltage rise A larger current at smaller voltage flows in the secondary circuit A larger current at smaller voltage flows in the secondary circuit

21 Electric Power Distribution 21 Step-Up Transformer A step-up transformer A step-up transformer has relatively many turns in its secondary coil has relatively many turns in its secondary coil so charge is pushed a longer distance so charge is pushed a longer distance and experiences a larger voltage rise and experiences a larger voltage rise A smaller current at larger voltage flows in the secondary circuit A smaller current at larger voltage flows in the secondary circuit

22 Electric Power Distribution 22 Power Distribution System A step-up transformer increases voltage for efficient long-distance transmission A step-up transformer increases voltage for efficient long-distance transmission A step-down transformer decreases voltage for safe delivery to communities and homes A step-down transformer decreases voltage for safe delivery to communities and homes

23 Electric Power Distribution 23 Introductory 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? 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? A. About 1% B. About 0.01% C. Exactly 0%

24 Electric Power Distribution 24 Summary about Electric Power Distribution Electric power is transmitted at high voltages Electric power is transmitted at high voltages Electric power is delivered at low voltages Electric power is delivered at low voltages Transformers transfer power between circuits Transformers transfer power between circuits Transformers require AC power to operate Transformers require AC power to operate The power distribution system is AC The power distribution system is AC


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