11.2-11.3 Electric Power Distribution, Generators and Motors

1. 11.2-11.3 Electric Power Distribution, Generators and Motors

2. New ideas for today • Magnetic induction • Lenz’s law • Transformers and power transmission • Motors and Generators

3. Why such high voltage?

4. Transformers!

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

6. Power 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!

7. AC DC vs. Edison Tesla Westinghouse

8. AC = alternating current Current switches direction 60 times per second (in N. America) DC= “direct current” AC

9. Power 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.

10. 160-800 kV 7000 V 120 / 240 V neutral hot ground

11. Voltage 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

12. Click me

13. Electromagnetic 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)

14. inductive charging B

15. Lenz’s Law

16. That “emf” can be useful…

17. Transformer • 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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19. Current 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 turns Secondary voltage = Primary voltage Primary turns

20. Step 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 thesecondary circuit

21. Step 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

22. Transformers are often filled with nasty stuff

23. Clicker question 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

24. Electric 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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26. Electric Generator Rotating magnet • makes changing magnetic field • induces AC current in the loop Converts mechanical power into electrical power

27. Electric Motor Input AC power • AC current makes changing magnetic field • causes magnet to turn Converts electrical power into mechanical power A motor is a generator run backwards !