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

3. Magnetism

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

5. Breaking a magnet produces two magnets! NS NS NNSS

6. Opposites attract!

7. Opposite poles attract and like poles repel

8. Magnetic materials

9. Iron (steel), Cobalt and Nickel

10. Magnetic induction

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

12. Hard and Soft Magnetism

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

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

15. 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

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

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

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

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

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

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

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

23. Magnetic field around a solenoid

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

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

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

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

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

29. D.C.Motor

30. 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)

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

32. 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.

33. 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.

34. 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

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

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

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

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

39. 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) v q Note; If the force is perpendicular to the motion, no work is done.

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