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Magnetism

Magnetism. Can you read through last lessons slide show?. Magnetism. ALL magnets have two poles. NORTH seeking pole. SOUTH seeking pole. Breaking a magnet produces two magnets!. N. S. N. S. N. S. N. S. Opposites attract!. Opposite poles attract and like poles repel.

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Magnetism

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  1. Magnetism Can you read through last lessons slide show?

  2. Magnetism

  3. ALL magnets have two poles NORTH seeking pole SOUTH seeking pole

  4. Breaking a magnet produces two magnets! N S N S N S N S

  5. Opposites attract!

  6. Opposite poles attract and like poles repel

  7. Magnetic materials

  8. Magnetic materials Iron (steel), Cobalt and Nickel

  9. Magnetic induction

  10. N S Magnetic induction When a magnetic material is close to a magnet, it becomes a magnet itself We say it has induced magnetism magnet S N

  11. Hard and Soft Magnetism

  12. N S Soft Magnetism Pure iron is a soft magnetic material It is easy to magnetise but loses its magnetism easily before after N S N S N Not a magnet Iron nail

  13. N S Hard Magnetism Steel is a hard magnetic material It is harder to magnetise, but keeps its magnetism (it is used to make magnets!) before after S N N S N N It’s a magnet! S Steel paper clip

  14. Magnetic field Magnets and electric currents produce magnetic fields around them. In a magnetic field, another magnet, a magnetic material or a moving charge will experience a magnetic force. www.physchem.co.za

  15. Magnetic field lines We can represent the magnetic field around a wire or magnet using field lines.

  16. Magnetic field lines The arrows show the direction a compass needle would point at that point in the field.

  17. Magnetic field lines The arrows show the direction a compass needle would point at that point in the field. Note that magnetic field is a vector quantity The closer the field lines are, the stronger the magnetic force felt

  18. S N Earth’s Magnetic Field Remember the North of a compass needle points to the geographic north pole (i.e. the geographic North pole is a magnetic south pole!)

  19. Moving charges (currents) Moving charges (electric currents) also produce a magnetic field Conventional current – electrons flow in the opposite direction http://www.sciencebuddies.org

  20. Magnetic field around a straight wire Stronger field closer to wire

  21. Magnetic field around a flat circular coil http://physicsed.buffalostate.edu

  22. Magnetic field around a solenoid

  23. The Motor Effect When a current is placed in a magnetic field it will experience a force. This is called the motor effect.

  24. The Motor Effect The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule. Thumb = Motion First finger = Field direction Centre finger = Conventional Current

  25. Sample question In this example, which way will the wire be pushed? (red is north on the magnets)

  26. Another sample question! An electron approaches a bar magnet as shown. What is the direction of the force on the electron? N S

  27. D.C.Motor Commutator ensures that every half rotaion the current direction reverses in the coil

  28. D.C.Motor

  29. Defining Magnetic Field B The size of the force on a wire in a field depends on the size of the field (B), the length of wire in the field (L) and the current in the wire (I)

  30. Defining Magnetic Field B In other words , F α BIL, or F = kBIL

  31. Defining Magnetic Field B F = kBIL We can make k = 1 by defining the Tesla as the magnetic field when the force on 1 m of wire carrying a current of 1 A is 1 N.

  32. Force on a current in a field Thus the force on a length L of wire carrying a current I in a magnetic field B is given by F = BILsinθ where θ is the angle between the current and the magnetic field.

  33. The force on a moving charge in a magnetic field Since a current experiences a force in a magnetic field, and a current is just made of moving charges, moving charges themselves must experience a force in a magnetic field. www.nearingzero.net

  34. The force on a moving charge in a magnetic field Consider a positive charge q moving with speed v. Magnetic field B out of the slide v q

  35. The force on a moving charge in a magnetic field In time Δt the charge will have moved a distance L = vΔt v q

  36. The force on a moving charge in a magnetic field The current is given by I = q/Δt v q

  37. The force on a moving charge in a magnetic field Given that F = BILsinθ F = B(q/Δt)vΔt =Bvqsinθ v q

  38. The force on a moving charge in a magnetic field The fact that this force is always at right angles to the velocity means that the charge will move in a circle (if the speed is constant) Note; If the force is perpendicular to the motion, no work is done. q v

  39. Homework – Due Monday Let’s do some questions! Page 344 onwards Questions 2, 6, 7, 9, 13, 15, 20.

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