Lecture 12


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Lecture 12

  1. 1. Magnetism
  2. 2. Magnetic Poles• Like poles repel,opposite poles attract– it’s a bit like the electricforce in this way• Every magnet has anorth and a south pole• Break a bar magnet inhalf and you get two barmagnets.
  3. 3. Magnetic Fields• Inside magnet field goessouth to north.• Outside field goes from northto south.• Denser lines = stronger field.• A smaller magnet will tend toalign its poles with the largermagnet.– See: Compass
  4. 4. Demo: Magnetic Fields• Sprinkle some iron filings over top of apermanent magnet.• Multiple magnets.
  5. 5. Magnetism is the result of movingcharges.• The electrons in an atomare in constant motion.• Electrons (and nuclei)may be ‘spinning’ whichis also motion of chargeand may produce amagnetic field.
  6. 6. Magnetic Domains• In most atoms, for every ‘spin’ inone direction there is a ‘spin’ in theopposite direction, canceling outthe magnetism.• Some atoms (iron, nickel, cobalt)have spins that add up in thesame direction.• The magnetic fields of manyatoms may become aligned,resulting in ‘domains’ with aligned,stronger magnetic fields.• When these domains tend to lineup you get a permanent magnet.
  7. 7. Example Problem: Breaking a magnet• When you break a permanent magnet in half,you get two permanent magnets. Explain, inphysical terms, how this could be so.
  8. 8. Magnetic Fields around ElectricCurrents• If a moving chargecreates a magnetic field,what would you expectfrom a current in a wire?– Current in a wiregenerates a magneticfield. It curls around thewire.– Curl the wire and you canget an electromagnet.
  9. 9. Demo• Compass and current in a wire.– I’m not sure how well this will work...
  10. 10. Example Question• What is the cause of a magnetic field about apermanent magnet, and about a currentcarrying wire?
  11. 11. Electromagnets• A coil of wire (often with aniron core).– Current flows through the wire.– Due to the wrapping, the smallmagnetic field from eachindividual wire adds up.– Lots of coils + high current =strong electromagnet.• May also besuperconducting– must be very cold– has near zero resistance tocurrent
  12. 12. Demo: Electromagnets• Iron filings again – what do you expect to see?
  13. 13. Magnetic Forces• If a moving charge creates a magneticfield it stands to reason a magneticfield affects a moving charge.• A moving charge is deflected when itmoves through a magnetic field.– if it is parallel it experiences no force.– This effect is often used for somethingcalled a ‘mass spectrometer’• This effect is very important fordeflecting high-energy particles awayfrom the earth’s surface.– This is why there is some concernabout magnetic field reversals.
  14. 14. Electric Motors• So if we have a coil ofwire in a magnetic fieldwe can force the loop tomake a partial turn.• If we cleverly arrange forthe direction of currentto reverse at the rightpoint, we get a loop thatwill turn continuously.
  15. 15. Demo: Electric Motors• Let’s talk about an electric motor in action:
  16. 16. Electromagnetic Induction• A moving charge creates amagnetic field.• A charged object mayexperience a force due to amagnetic field by movingthrough the field.• What if we attempt to movethe field around a stationarycharge?– Can we induce the charge tomove?• Indeed – this principle liesbehind many things such asmetal detectors andgenerators.
  17. 17. Faraday’s Law• The induced voltage in acoil is proportional to thenumber of loops,multiplied by the rate atwhich the magnetic fieldchanges within thoseloops.– Current induced will beproportional to theresistance of the coil.
  18. 18. Sample Problem: Guitar pickups• Electric guitars use steel strings. Under each string isa coil of wire containing a permanent magnet. The coilis connected to an amplifier and a speaker.• When you pluck a string, the string vibrates above themagnet and coil but does not make contact with it.• Explain how the vibrating string can cause anoscillating current to appear in the coil even thoughthe string isn’t connected to the coil.
  19. 19. Generators and AC• A generator works in muchthe same way as an electricmotor – but backwards.– By rotating a coil in a magneticfield you induce a current in thecoil (Faraday’s Law).• The voltage induced has achanging magnitude overtime, depending on howquickly the number ofmagnetic field lines throughthe coil is changing.
  20. 20. Demo: Electric Power Generation• Turning the handle spins a permanent magnetinside of a coil.
  21. 21. Power Transmission: why AC?• Transformers (not the robots).– Changing voltage is as easy as having different numbers ofcoils.• All back to faraday’s law – two linked sets of coils with differentnumbers of windings.– Why does this matter?• Remember P = I*V• By increasing the voltage we decrease the current.• Power dissipated by resistive heating: P = I2R– By increasing V we can dramatically decrease power loss in the lines.• Example problem: Why is the use of AC preferred forpowerlines?