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Electromagnetic Induction

Electromagnetic Induction. Current Induction. Current Induction. Basic Generator. Current Induction. Current Generator. Transformers. Transformers. Transformers. Transformer is just wire coiled around metal. Magnetic field is generated

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Electromagnetic Induction

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  1. Electromagnetic Induction

  2. Current Induction

  3. Current Induction

  4. Basic Generator

  5. Current Induction

  6. Current Generator

  7. Transformers

  8. Transformers

  9. Transformers

  10. Transformer is just wire coiled around metal • Magnetic field is generated • Secondary Voltage is V2 = (N2/N1) V1 • Secondary Current is I2 = (N1/N2) I1 • But Power in = Power out • negligible power lost in transformer • Works only for AC, not DC

  11. An Example • My laptop computer requires about 20 Volts AC, which comes from an adapter (transformer) that is plugged into the wall socket. • What is the approximate ratio of the number of turns on this transformer, given 120 VAC from wall socket? Which coil has more turns, the primary or the secondary?

  12. Current Induction between Coils

  13. Electric Guitars

  14. Microphones

  15. Speakers

  16. Electrical Power DistributionHousehold AC PowerTransformers

  17. Electronic Components • Resisters: Electrical Friction, energy released as heat, but not stored. • I = V/R • Capacitors: Store Electrical Energy • P = V * I • Electric companies bill us for Energy: E = P * t measured in units of kilowatt-hours • D - cell battery stores about 27 watt-hours • Car Battery stores 120 watt-hours

  18. Electricity is a Medium for Transporting Energy Where does the energy come from? Is energy “lost” in the transmission wires? What’s the goal, in terms of energy transfer?

  19. Putting Electricity to Work • Recall the power consumed by an electrical device is given by the product of the current through it times the voltage drop across it P = VI • Many ways to get the same useful work done, i.e., same power output from electrical device Voltage Current or Voltage Current

  20. Segment of an Electrical Power Transmission Cable • Recall Power dissipated in a resistor is P = I2R • How can we minimize power dissipated in the cable? • Minimize R • Short cables with large cross sections • Use high conductivity materials (silver is good!) • Economic considerations limit the cross section and materials • Distribution requirements establish needed lengths • Minimize I • In order to do this, while keeping power delivered to the household appliances (P = VI) the same, must raise the voltage difference between the 2 transmission lines • Which as a bigger impact, halving I or R?

  21. Power Dissipated in an Electricity Distribution System 150 miles • Estimate resistance of power lines: say 0.001 Ohms per meter, times 200 km = 0.001 W/m  2105 m = 20 Ohms • With one light bulb on, we can figure out the current it draws using P = VI so I = P/V = 120 Watts/12 Volts = 10 Amps • Power in transmission line is P = I2R = 102 20 = 2,000 Watts!! • “Efficiency” is e = 120 Watts/2120 Watts = 0.6%!!! • What could we change in order to do better? 120 Watt Light bulb Power Plant 12 Volt Connection Box

  22. Answer: Must reduce either resistance or the current 1. Reduce resistance in the power lines • Already we’re using pretty hefty copper lines, not very cost-effective to do anything else (superconductors?). 2. Raising the voltage in the system reduces current! • Repeating the above calculation with 12,000 Volts delivered to the house draws only I = 120 Watts/12 kV = 0.01 Amps for one bulb, giving P = I2R = (0.01)220 = 2010-4 Watts, so P = 0.002 Watts of power dissipated in transmission line Efficiency in this case is e = 120 Watts/120.002 = 99.998%

  23. Need a Way to Convert! • We need a way to transform from a high voltage electrical distribution system to a low voltage electricity within a household..... • So, use a transformer! High voltage in Low voltage out...

  24. A way to provide high efficiency, safe low voltage: step-up to 500,000 V step-down, back to 5,000 V ~5,000 Volts step-down to 120 V High Voltage Transmission Lines Low Voltage to Consumers

  25. Examples

  26. Class Problem

  27. Class Problem

  28. Class Problem

  29. Class Problem

  30. Class Problem What happens to the reading on the Galvanometer when the switch in circuit 1 is a) first closedb) kept closedc) opened again?

  31. Class Problem When the switch is first closed, a current is established in coil 1 and creates a magnetic field which extends to coil 2. This build-up of field in coil 2 induces current which is registered in the Galvanometer. The current is brief, however, because once the field is stabilized and no further charge takes place, no current is induced and the Galvanometer reads zero current. When the switch is opened, the current ceases in coil 1 and the magnetic field in the coil and the part that extends to coil 2 collapses. This change induces a pulse of current in the opposite direction which is registered on the Galvanometer.

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