Magnetism

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Magnetism

  1. 1. Part of Physics Achievement Standard 2.6 – Demonstrate understanding of electricity and magnetism.
  2. 2. V 1 2E F  Eq E p  Eqd Ek  mv d 2 q E EI V V  IR P  IV P t q t 1 1 1 Rt  R1  R2 ...   ... Rt R1 R2F  BIL(sin  ) F  Bqv V  BvL
  3. 3. • Where have you had experience with magnetism in life so far?• Permanent Magnets• Electromagnetism• Can a frog really levitate?
  4. 4. All permanent magnets have a north pole and a south pole. What will happen if we cut a bar magnet in half?Opposite poles attract, like poles repel, just like charges in electricity.The earth has a magnetic field, which is very important to protect the planet from dangerous radiation.The Auroa (northern lights) is actually a result of the charged particles being channelled to the poles and colliding with the particles in the atmosphere.
  5. 5. Magnetic field lines are drawn in a similar way to electric field lines. They show the direction that a small north pole would move.Field lines go from NORTH to SOUTH.The force is stronger where the field lines arecloser together. Field lines never cross or meet.
  6. 6. Magnetic field lines can be visualised in two mainways: Compass needles always line up with the magnetic field lines. Iron filings will become induced magnets and temporarily line up with field lines.
  7. 7. The Earth has a magnetic field, as though there is a large bar magnet inside the Earth.Note that the geographic and the magnetic poles are different.When we look at a compass, we read the needle pointing towards the North pole, so we assume that is the north end of the earth’s magnet.But is the north end of the compass attracted to the north pole of a magnet?
  8. 8. Some metals are easily magnetised (e.g. iron). This means that the dipoles inside the metal are free to move around. When they feel a magnetic field around them the dipoles align and this causes the object to become a temporary magnet.The method of induced magnetism is used to store most of the worlds information. Hard Disk Drives (HDDs) use temporary(ish) magnetism to store data. This is why you should never put your computer too close to a speaker.
  9. 9. Sometimes it is necessary for us to draw a current going into or out of the page. These are the symbols we use... Magnetic Field Lines Current Into Page Current Out Of PageIt turns out that there is a magnetic field that exists around the wire, depending on the direction of the current. The magnetic field circles the current.
  10. 10. So we can see that magnetism and electricity are linked in a very important way. It turns out that currents create circular magnetic fields around themselves.The magnetic field createdcan be found using theright hand rule:1. Your right thumb points in the direction of the current.2. Your fingers will wrap in the direction of the magnetic field lines.
  11. 11. The symbol for magnetic field strength is B, with units NA-1m-1 , more commonly called Tesla, T.A bit more about Tesla...
  12. 12. Solenoids are an example of an application of the magnetic field created by a current. There are three ways to increase the magnetic field produced by a solenoid:1. Increase the current.2. Increase the number of turns.3. Inserting an iron core.There is a right hand rule for solenoids which can be used to find the direction of the magnetic field:1. Curl your fingers in the direction of the current through the coil.2. Your thumb points in the direction of north.
  13. 13. Below: The right hand solenoid rule.Above: The magnetic fieldproduced by a solenoid.
  14. 14. Magnetic fields interact with each other when there is more than one in the same space.This interaction results in a force.Therefore, a wire carrying a current will experience a force when it is in a magnetic field.This is how we turn electrical energy into kinetic energy – an electric motor!It’s called the motor effect.
  15. 15. The direction of this force can be found using the Right Hand Slap rule:Your thumb should point inthe direction of the current.Your fingers should be inthe direction of the magneticfield.The direction of your “slap”will show how the force willact.
  16. 16. We are able to calculate the size of the force that the wire will feel by considering 3 factors that will have an effect on that force:The size of the force depends on:1. The strength of the magnetic field, B2. The size of the current in the wire, I3. The length of the wire that is inside the magnetic field, LThis gives the equation: F = BILNote that in some instances, the wire and the field are not at right angles. These cases will introduce a factor of angle, making the equation F = BILsin(θ), where θ is the angle between the wire and the magnetic field. Note that θ will always be between 0° and 90°.
  17. 17. The most important application of the motor effect is the DC motor. Approximately 85% of power generated is used to power DC Motors.The DC motorworks by takingadvantage of themotor effect tomake a loop ofwire turn.
  18. 18. Note that the split ring commutator is the main component that make the motor possible.There are a number of ways to make the motor force more powerful:1. Thicker wire to allow a bigger current2. Insert an iron core to make the magnetic field larger3. Put more than one coil on the loop.
  19. 19. The actual reason that the current feels a force when in a magnetic field is because a current is made up of moving charges. It is these charges that are feeling the force and making the wire move.This means that a free charge moving through a magnetic field will also experience a force. This force will depend on:1. The magnetic field strength, B.2. The velocity of the charged particle, v.3. The size of the charge, q.F = Bvq is the equation. Note that the charge must be moving in perpendicular direction.What kind of path would a moving charge in a magnetic field follow?
  20. 20. We know that a charge must follow a circular path , unless we change the direction of the magnetic field. This is a direct consequence of the right hand “slap” rule.(This is how old TV’s and Monitors work – they use a changing magnetic field to tell electrons exactly where to go.)The circular path can be calculated with precision: remember the equation for force in a circular situation: F = mv2/r.The forces are equivalent, so given the mass, we can find the radius, or vice versa.
  21. 21. A positron is the anti – matter equivalent of One type of radiation is an electron. A known to have a charge magnetic field is used of +3.2x10-19C, and a to keep the positron mass of 6.64x10-27 kg. contained. Calculate the size of B if r must Calculate the radius of it’s be at most 5cm, and path in a magnetic field the positron is moving of 1 Tesla if it is at 0.5c. travelling at 300ms-1
  22. 22. If (perchance) instead of sitting back and waiting for a current and magnetic field to interfere and cause a force on a wire, what if we gave the wire some force? What would this do?You shouldn’t be too surprised to hear that this would cause a current to exist in the wire.The free electrons in the wire feel a force on them, and all start moving in the direction of that force.Electron movement (flow) = current!We have just generated some electricity.
  23. 23. The movement of the wire in the magnetic field causes the current to flow in the wire – this induces a potential difference (voltage) in the wire.The induced voltage is proportional to the velocity the wire is moving, the length of the wire in the magnetic field, and the magnetic field strength: ������ = ������������������Note that it could be the field moving instead of the wire, or even the field oscillating in strength to generate the voltage. As long as the field is changing somehow relative to the wire.We could increase the size of the voltage/current generated by…
  24. 24. There is a right hand slap rule for the induced current:Your thumb is in the direction ofthe motion, your fingers in thedirection of the magnetic field,and your slap shows the direction of the current.This is the conventional current – electron flow would be the opposite way.This is how electricity is generated around the world. Note that energy is conserved in this situation.
  25. 25. The diagram shows ametal rod moving tothe right through amagnetic field.The induced currentin the rod flows asshown.If energy is to be conserved, then the mechanical workdone must equal the electrical energy generated.
  26. 26. Let the speed of the wire be v, and its effective lengthL.• Then the induced voltage is given by V = vBL.• The electrical power is VI = vBLI.• The force the magnetic field B makes on the currentI is given by F = BIL. This force opposes the motion.• If the rod is to move at constant speed, then theforce sustaining the motion must be the same size,BIL.• Then mechanical power = F × v = BIL × v = vBLIWe see that the mechanical power put in equals theelectrical power produced and so energy isconserved.
  27. 27. The alternating current generator is also known as analternator. In such a device, a coil rotates in amagnetic field and a current is produced.The current flows first in one direction, then theother, then the first again, and so on.Because it keeps reversing direction, the current isknown as alternating current (AC).This is the most common type of electricity, because itis much easier to use (in motors and almost all otherappliances) but it does have one major drawback.Can you guess what the disadvantage is?We learn a lot more about AC in year 13!

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