1. Magnetic Fields

2. Definition : A magnetic field is a force field which surrounds either a magnet or a wire carrying an electric current and will act upon, without contact, another magnet or current carrying wire Like the other fields we have studied we represent magnetic fields diagrammatically using field lines or lines of magnetic flux

3. Magnetic Fields We name the ends of a magnet “the poles”. (North and & South). More correctly they should be referred to as the “North seeking pole” and “South seeking pole” The arrows on a magnetic field line represent the path which a “small, free north pole” would take Like poles repel each other Unlike poles attract each other

4. The Earth’s Magnetic Field The Earth has a magnetic field just like a giant magnet. The geographic North pole has a South magnetic pole associated with it (Since a north seeking pole of a magnet will point towards it)

5. The Earth’s Magnetic Field Watch terminology!!! At the geographic North pole there is a magnetic pole which we can refer to as “Magnetic North”. However it is a South pole! Watch the field lines! Geographic North Pole Geographic South Pole

6. Magnetic Fields around Wires Wires carrying an electric current produce a magnetic field. The “right-hand rule” can be used to establish the direction of this field. Note the current direction is the direction of “conventional current” positive to negative

7. The Motor Effect A current carrying wire, with its associated magnetic field will experience a “motor effect” if placed (at a non-zero angle) in a magnetic field. The force is perpendicular to both the current & the magnetic field

8. Factors Affecting the Motor Effect Experimentally, it can be shown that the size of the force due to the motor effect is related to the following : 1.The strength of the current 2.The strength of the magnetic field 3.The length of the wire 4.The angle between the field lines & current In terms of angles, the force is greatest when the current is perpendicular to the magnetic field and zero when parallel to the field

9. Fleming’s Left-Hand Rule The relationship between field, current and force can best be remembered using “Fleming's left hand rule” First finger (Field), seCond Finger (Current), Thumb (moTion) moTion

10. Magnetic Flux Density

11. Answers ForceMagnetic flux density CurrentLength 20 N0.3 T7 A9.52 m 3 N1 mT20 A150 m 500 N33 T200 mA75.8 m 67 N0.4 T2.5 A67 m 6 kN4 T60 A25 m Complete:

12. Question 1 Current East to West Field due North

13. Problems 2.4 x 10 -2 N West 4.5A East to West 0.2T Vertically down South 8.0 x 10 -3 N

14. Electric Motors The force that a current carrying conductor experiences in a magnetic field is the basic principle behind an electric motor. NS Consider the above rectangular coil which has n turns & can rotate about its vertical axis. The coil is arranged in a uniform magnetic field. The coil will experience a pair of forces where the direction is given by the left hand rule. Force (out of page) Force (into page) XY

15. Electric Motors

16. Looking from above : F F w F F  X Y X Y

17. Electric Motors: Practicalities In real motors, current must be delivered to the rotating coil (direct connections would twist!). In simple motors, sprung loaded carbon brushes push against a rotating commutator The second issue is that after half a rotation the force would change direction. We need to change the direction of the current so that as the coil rotates the force is always in the same direction. A split-ring commutator is used

18. Question 2

19. Part 7b- Charged Particles in Magnetic Fields

20. Moving Charges in Magnetic Fields Current carrying conductors experience a force in a magnetic field. In a similar way charged particles also experience a force in a magnetic field and are deflected This technology has been exploited in all sorts of Cathode Ray Tubes (CRTs), TVs, Monitors & Oscilloscopes

21. Moving Charges in Magnetic Fields The subtopics are a bit back to front... The reason why a current carrying conductor experiences a force is because the electrons moving along the wire experience a force and are moved to one side of the conductor, which exerts a force on it -- - - - - - - F F F F F F FF

22. Moving Charges in Magnetic Fields

23. Question 1

24. Path of a Charged Particle in a Field In the previous lesson we’ve seen that a moving charged particle is deflected in a magnetic field, in accordance with Fleming’s left hand rule Electron Gun Magnetic field coming out of the page Force Conventional Current Electron Gun Force Conventional Current Electron Gun Force The force always acts perpendicular to the velocity, causing the path to change... Circular motion is achieved! Conventional Current

25. Path of a Charged Particle in a Field As always with circular motion problems we are looking for the force to “equate” to the centripetal force From earlier, we know the force on a charged particle: F= BQv BQv = mv 2 / r  r = mv/BQ The path therefore becomes more curved (r reduced) if the flux density increases, the velocity is decreased, or if particles with a larger specific charge (Q/m) are used Instantaneous Velocity BQv

26. Question 2

27. Uses of Magnetic Fields Your task is to work in groups of 2-3, or individually, to investigate one application or use of electromagnets. You will have all lessons this week, plus prep, to prepare a presentation, which will be given on Tuesday 18 th March. If you don’t know where to start, there are some ideas in the book! Feel free to be adventurous though… And try to get some equations in there! -Thermionic emission - Superconducting magnets -Maglev - Synchrotrons -Mass spectrometers - Particle accelerators

28. Part 7c- Applications of Magnetic Fields

29. Application 1 : CRT Last lesson we mentioned the CRT (Cathode Ray Tube) Such devices can also be called: -Electron guns -Thermionic devices The cathode is a heated filament with a negative potential which emits electrons, a nearby positive anode attracts these electrons which pass through a hole in the anode to form a beam. This is called Thermionic emission. The potential difference between the anode and cathode controls the speed of the electrons.

30. Applications : Mass Spectrometers A very clever component known as a “velocity selector” is used to obtain a constant velocity. Firstly, we ionise the atoms. The +ve ions are acted upon by both an electric field & a magnetic field.

31. Applications : Mass Spectrometers

32. These machines can then be used to analyse the types of atoms, (and isotopes) present in a sample, using:

33. Applications : Cyclotrons Cyclotrons are a method of producing high energy beams used for nuclear physics & radiation therapy An alternating electric field is used to accelerate the particles while a magnetic field causes the particles to move in a circle, (forming a spiral) Compared to a linear accelerator, this arrangement allows a greater amount of acceleration in a more compact space

34. Applications : Cyclotrons Two hollow D shaped electrodes exist in a vacuum. A uniform magnetic field is applied perpendicular to the plane of the “D”s Charged particles are injected into a D, the magnetic field sets the particle on a circular path causing it to emerge from the other side of this D & to enter the next. As the particle crosses the gap between the Ds the supplied current changes direction (high frequency AC) & the particle is accelerated, (causing a larger radius each time)

35. Applications : Cyclotrons

36. Charged Particles in Magnetic Fields

37. Moving Charges in Magnetic Fields Current carrying conductors experience a force in a magnetic field. In a similar way charged particles also experience a force in a magnetic field and are deflected This technology has been exploited in all sorts of Cathode Ray Tubes (CRTs), TVs, Monitors & Oscilloscopes

38. Moving Charges in Magnetic Fields The subtopics are a bit back to front... The reason why a current carrying conductor experiences a force is because the electrons moving along the wire experience a force and are moved to one side of the conductor, which exerts a force on it -- - - - - - - F F F F F F FF

39. Moving Charges in Magnetic Fields

40. Question 1

41. Path of a Charged Particle in a Field In the previous lesson we’ve seen that a moving charged particle is deflected in a magnetic field, in accordance with Fleming’s left hand rule Electron Gun Magnetic field coming out of the page Force Conventional Current Electron Gun Force Conventional Current Electron Gun Force The force always acts perpendicular to the velocity, causing the path to change... Circular motion is achieved! Conventional Current

42. Path of a Charged Particle in a Field As always with circular motion problems we are looking for the force to “equate” to the centripetal force From earlier, we know the force on a charged particle: F= BQv BQv = mv 2 / r  r = mv/BQ The path therefore becomes more curved (r reduced) if the flux density increases, the velocity is decreased, or if particles with a larger specific charge (Q/m) are used Instantaneous Velocity BQv

43. Question 2

44. Applications of Magnetic Fields

45. Application 1 : CRT Last lesson we mentioned the CRT (Cathode Ray Tube) Such devices can also be called: -Electron guns -Thermionic devices The cathode is a heated filament with a negative potential which emits electrons, a nearby positive anode attracts these electrons which pass through a hole in the anode to form a beam. This is called Thermionic emission. The potential difference between the anode and cathode controls the speed of the electrons.

46. Applications : Mass Spectrometers A very clever component known as a “velocity selector” is used to obtain a constant velocity. Firstly, we ionise the atoms. The +ve ions are acted upon by both an electric field & a magnetic field.

47. Applications : Mass Spectrometers

48. These machines can then be used to analyse the types of atoms, (and isotopes) present in a sample, using:

49. Applications : Cyclotrons Cyclotrons are a method of producing high energy beams used for nuclear physics & radiation therapy An alternating electric field is used to accelerate the particles while a magnetic field causes the particles to move in a circle, (forming a spiral) Compared to a linear accelerator, this arrangement allows a greater amount of acceleration in a more compact space

50. Applications : Cyclotrons Two hollow D shaped electrodes exist in a vacuum. A uniform magnetic field is applied perpendicular to the plane of the “D”s Charged particles are injected into a D, the magnetic field sets the particle on a circular path causing it to emerge from the other side of this D & to enter the next. As the particle crosses the gap between the Ds the supplied current changes direction (high frequency AC) & the particle is accelerated, (causing a larger radius each time)

51. Applications : Cyclotrons