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Electro-magnetism

Electro-magnetism. NCEA AS2.6 Text Chapters:. Magnetism. Permanent or ferro-magnetism Like poles repel, unlike poles attract Magnetic field strength B measured in Tesla T. Magnetic Fields. Magnetic fields can be represented by field lines Closer together=stronger field Point from N to S.

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Electro-magnetism

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  1. Electro-magnetism NCEA AS2.6 Text Chapters:

  2. Magnetism • Permanent or ferro-magnetism • Like poles repel, unlike poles attract • Magnetic field strength B measured in Tesla T

  3. Magnetic Fields • Magnetic fields can be represented by field lines • Closer together=stronger field • Point from N to S

  4. Magnetism • Electromagnetism. • Caused by moving electric charges. • The earth has a magnetic field around it because of iron atoms spinning around the core.

  5. Electromagnetism • Fields are formed around current carrying wires

  6. Right Hand Curl Rule • Thumb = points in direction of current flow • Fingers curl in direction of field lines

  7. Magnetic Force • Two parallel current carrying wires • F=kI1I2L/d • Current flowing same direction in both – wires are attracted • Current flowing opposite directions – wires repel each other

  8. Solenoids • Fields are formed in solenoids or coils.

  9. Right Hand Curl Rule • Fingers curl in direction of current flow around the coil • Thumb = points towards the north pole

  10. Electromagnets • Electromagnets have a range of uses: • Attached to cranes in scrap metal yards • Electric bells • Relay switches • More useful than permanent magnets as they are stronger and can be switched off

  11. +Q B F v Magnetic force • Force on moving charges - a charged particle moving in a magnetic field will experience a force • F=Bqv (sinθ) • B=mag field strength • q= charge • v=velocity • (This formula applies to a small charge moving at right angles to magnetic field lines – if not right angles multiply by sinθ)

  12. Right Hand Slap Rule • For a positive charge moving in a magnetic field: • Thumb = direction of movement • Fingers = direction of magnetic field • Palm = direction of force acting on charge Right hand rule gives direction for positive charge – negative must be reversed

  13. Force on Moving Charge • This principle can be used to deflect the electron beam in a TV set to make it scan across the screen

  14. Force on Moving Charge • It can also be used to determine the mass of unknown substances in a device called a “mass spectrometer”

  15. Force on Current-carrying Wire • A wire carrying a current will experience a force when placed into a magnetic field • F=BIL (sinq) • Applies to current flowing in a wire running at right angles to magnetic field lines. (if not 90°, then multiply by sinq) • Direction (for conventional current) given by right hand rule.

  16. Right Hand Slap Rule • For a wire carrying current in a magnetic field: • Thumb = direction of current • Fingers = direction of field lines • Palm = direction of force on wire

  17. Magnetic Force • This principle can be used in magneto-hydro propulsion units…

  18. Magnetic Force • And in loud-speakers...

  19. Electric Motors • If magnetic force acts on opposite sides of a current carrying coil which is mounted on an axis, a torque is produced which makes the coil spin

  20. Electric Motors • This idea is the basis for all devices that run by an electric motor.

  21. Induction • If a wire is moved through a magnetic field then a voltage can be induced across the ends the wire. • If the wire is connected to a circuit then current will flow. The direction of induced current is determined by a right hand rule.

  22. Right Hand Slap Rule • v=direction of wire movement • B=direction of magnetic field lines • F= force on a positive charge (ie direction of current flow)

  23. Induction • The size of this induced voltage is given by: • V=BvL • (B=mag field strength, v=velocity of movement, L=length of wire in field) • This is known as Faraday’s Law

  24. Induction • The direction of the induced current is such that it creates an opposing force on the motion that is causing it. • This is known as Lenz’s Law • Induced voltage/current can be made larger if: • The mag field is stronger • The wire is longer • The movement is faster • (Solenoid has an iron core)

  25. Induction • Induction is the principle behind the microphone

  26. Induction • And the dynamo..

  27. If a coil is spun around inside a magnetic field then current can be induced that can be used to operate devices that run on electricity This is called generation Both DC and AC can be generated depending on whether slip rings or split rings are used Generators

  28. Transformers • These consist of 2 coils wound close to each other. • Changing the current in one coil makes the field around it change. This changing field induces current in a nearby coil.

  29. Transformers • The ratio of the windings determines how much voltage/current is induced • The voltage can be calculated using: N=number of turns V=Voltage P=primary coil S=secondary coil

  30. Transformers • No transformer is 100% efficient, but assuming it was: • Power in = Power out

  31. Transformers • 3 Types: • Step up : Vs> Vp • Step down : Vs < Vp • Isolating (a safety device) : Vs = Vp

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