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Magnetism

Magnetism. Magnets. Most of you have had experience with a magnet before. A magnet is a piece of metal that has 2 poles – a North pole and a South pole. What happens when two poles are brought near each other?. History.

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Magnetism

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  1. Magnetism

  2. Magnets • Most of you have had experience with a magnet before. • A magnet is a piece of metal that has 2 poles – a North pole and a South pole. • What happens when two poles are brought near each other?

  3. History • Hans Christian Oersted discovered when a current carrying wire was near a compass, the needle moved. • This means… • And electric current has a magnetic field!!

  4. The Poles • A magnet consists of two poles – the North and the South. • What would happen if you took a 10 inch magnet and broke it in half? • You would still have a north pole and a south pole on each broken piece._ • There is no such thing as a monopole magnet!!!!!!

  5. Magnets vs Currents • Magnetic fields will behave a lot like electric charges except for one major feature. • Electric charges can be isolated but magnetic fields cannot. • For example, a north pole does not exist without a south pole. • But an electron can exist without a proton.

  6. Magnetic Fields • Around each magnet, there is a magnetic field. A magnetic field is a region in which a magnetic force can be detected. The direction of the magnetic field is always in relation to the North pole. The field lines go from the North pole to the South pole.

  7. The Earth’s Magnetic Field • The Earth has its own magnetic field. However, it’s a little bit tricky… The North pole is actually the South pole! Also, the true poles are slightly off from the geographic poles.

  8. Magnets and Charges • A magnetic field is produced by moving electric charges. • What moves in a bar magnet are the electrons of the atoms.

  9. Domains • In any given piece of material, the electrons are all spinning in random directions. In a magnet, the electrons are all spinning in the same direction. When the electrons are all spinning in the same direction, they induce a magnetic field. • A domain is a microscopic region of the magnet in which the magnetic fields are all aligned.

  10. Most objects are non-magnetic meaning that the electrons are all spinning in random directions. • Some materials are easily magnetized (iron). • Some materials are naturally magnetic (lodestones, magnetite)

  11. Wires • When a wire carries a current, it induces a magnetic field around it. • So everywhere in your house, there are magnetic fields!

  12. Particles in a Field • In a magnetic field, a charged object will feel a force. • This means that a charged object will move when it is placed in a magnetic field.

  13. Wires • We now know that a particle experiences a force when placed in a magnetic field. • What would happen to a wire that is placed in a magnetic field? • It also experiences a force!!!!

  14. Directions Anyone? • When the currents are in the same direction, the magnetic fields will reinforce one another. This means that the wires will attract. • When the currents are in opposite directions, the magnetic fields subtract from one another. This means that the wires will repel each other.

  15. Induction • We know that a current-carrying wire will experience a force when put into a magnetic field. • We can also induce a current in wire by moving it through a magnetic field. • This is called electromagnetic induction. • Remember that the wire has to be a closed circuit and it has to move. • No movement = no current

  16. Generators • We can use induction to convert mechanical energy into electric energy with the use of a generator. • In a diesel generator, a belt rotates in a circle and this moves a wire. The wire rotates in a magnetic field and electricity is made.

  17. Motors • Motors do the exact opposite of generators – they convert electrical energy into mechanical energy.

  18. Transformers • Transformers are used when we need to change one AC potential difference to another AC potential difference. An example would be the power lines that bring us our electricity.

  19. Transformers • When we vary the number of coils around the magnet, we can change the current and thus the potential difference. Transformers can either be step up transformers or step down transformers. • Step up transformers allow us to increase the voltage by increasing the number of coils. • Step down transformers allow us to decrease the voltage by decreasing the number of coils. • Realize that they are not 100% efficient and that some of the energy is lost.

  20. You can use a proportion to compare the voltages and number of coils. • ______________________________ • V’s are the voltages, N’s are the # of coils.

  21. Example • G: _________________________________________________ • U: _________________________________________________ • E: _________________________________________________ • S: _________________________________________________ • S: _________________________________________________

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