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From Ideas to Implementation

From Ideas to Implementation. Core Module 9.4. A word from the creator. This Powerpoint presentation was prepared by Greg Pitt of Hurlstone Agricultural High School.

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From Ideas to Implementation

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  1. From Ideas to Implementation Core Module 9.4

  2. A word from the creator This Powerpoint presentation was prepared by Greg Pitt of Hurlstone Agricultural High School. Please feel free to use this material as you see fit, but if you use substantial parts of this presentation, leave this slide in the presentation. Share resources with your fellow teachers and students.

  3. * * * and quantitatively (see next section of syllabus)

  4. Cathode Ray Tubes and Charged Particles A very simple cathode ray tube The development of various types of cathode ray tubes beginning in the mid-19th century allowed the manipulation, using electric and magnetic fields, of streams of charged particles. cathode

  5. Cathode Ray Tubes and Charged Particles • Observations of the behaviour of these charges led to an increased understanding of matter and the atom. Application of this understanding led to new technologies in the 20th century including the development of •  the oscilloscope •  television •  communications technologies •  the electron microscope •  photocopiers and fax machines

  6. cathode ray magnet velocity Cathode Ray Tubes and Charged Particles Charged particles entering a uniform magnetic field with a velocity at right angles to the field are deflected along a circular path. The path is circular because the force on the moving charge is of a constant magnitude (qvB) and it is always perpendicular to the direction of the velocity, making it a centripetal force. The maximum force is exerted on the charge when its velocity is perpendicular to the field.

  7. Cathode Ray Tubes and Charged Particles • Factors affecting the radius • the greater the velocity the greater the radius • the greater the particle mass the greater the radius • the greater the magnetic field strength the smaller the radius • the greater the charge of the particle the smaller the radius • Application • The mass spectrometer This image shows the circular path of a positively charged particle in a uniform magnetic field into the page Animation see: ChargeMotionBfield.avi

  8. Cathode Ray Tubes and Charged Particles A cathode ray tube is a highly evacuated glass tube containing a positive and a negative electrode. When a large DC potential difference is applied between the electrodes, the cathode releases electrons, forming a beam, which is attracted towards the positively charged electrode. This beam of electrons is called a cathode ray. A cathode ray is an electric current in a vacuum tube.

  9. Cathode Ray Tubes and Charged Particles The electrons themselves, making up the cathode ray, cannot be seen. This graphic shows how a cathode ray (electron beam) is made visible using a curved phosphor coated metal screen. Electrons travelling through the tube strike the phosphorescent screen, causing it to emit green light, thus making the path of the electrons visible.

  10. X X X X X X X X X X v X X X X X X X X X X F X X X X X – Cathode ray tubes and charged particles The magnet in this photo is just behind the cathode ray tube with one end pointing out of the page. What is the polarity of the magnet’s pole that is visible near the tube? It is a south pole. The cathode ray can be deflected from a straight-line path by a magnetic field, suggesting that the two were related in some way. The discovery of this effect in 1855 predates by some ten years the unification of electricity and magnetism by James Clerk Maxwell. B

  11. Cathode Ray Tubes and Charged Particles A moving charged particle, such as an electron, can be deflected by an electric field. An electric field can be produced by a potential difference applied across a pair of parallel charged conducting plates. The electron entering the field at right angles to the field is deflected along a parabolic trajectory in the field. For animation see: ElectronDeflectionPlates2.mov

  12. Debate Cathode Rays - Particles or Waves? • Cathode rays – charged particles or em waves - debate in late 1800s • Similar debate regarding light in the 1600s. Newton argued for particles. Huygens for waves. Young secured the wave model for light. • Lenard predicted that cathode rays would travel with the velocity of light but Thomson (1884) determined the velocity of cathode rays to be less than 1/100 of the speed of light • German and British rivalry between researchers concerning the nature of cathode rays. Germans  a wave British  particles. • Ultimately there was to be truth in both. • J.J. Thomson was awarded the Nobel Prize in Physics in 1906 for showing the electron to be a particle. • George Thomson (JJ’s son), was awarded the Nobel Prize in Physics in 1937 for showing that the electron is a wave!

  13. first Striation Patterns in Low Pressure Discharge Tubes These pictures show the same discharge tube with different high voltage sources. Left: Induction coil Right: Very high voltage transformer from a TV set

  14. first Maltese Cross Tube Electrons produced at the cathode are accelerated towards the cross, connected to the anode. The inertia of the electrons (due to their mass) carries them past the cross if they do not hit the metal cross itself. High energy electrons make the glass at the end of the tube glow. Identification of Properties

  15. first Maltese Cross Tube If an object is placed in the path of the cathode ray, a shadow of the object is cast on the glowing tube wall at the end. This showed that the cathode rays travelled in straight lines. Identification of Properties

  16. charged plates first Tubes with Electric Plates - Cathode Ray Control A potential difference applied across the charged electric plates causes the cathode ray to deflect to the right or the left, depending on the direction of the electric field produced. Identification of Properties

  17. velocity first Tubes with Electric Plates - Cathode Ray Control The deflection of electrons in the opposite direction to the electric field (away from the negative plate, towards the positive one) shows that electrons have a negative charge. Identification of Properties

  18. first Fluorescent materials and cathode rays These natural minerals fluoresce due to the absorption of UV radiation, which is subsequently emitted as visible light. Cathode rays cause certain materials, called phosphors, to behave in a similar manner. This is the basis of screens on CROs, TV and CRT based computer monitors. This demonstrates that cathode rays possess energy. Identification of Properties

  19. first Paddle Wheel Discharge Tube and Cathode Rays This cathode ray tube contains a small paddle wheel free to roll on its axle along glass tracks. Cathode rays cause hit the paddle wheel, causing it to turn and move along the track. This demonstrates that cathode rays have momentum. Identification of Properties

  20. first Cathode Rays (1) If an object is placed in the path of the cathode ray, a shadow of the object is cast on the glowing tube wall at the end. This showed that the cathode rays travelled in straight lines. (2) The cathode ray can push a small paddle wheel up an incline, against the force of gravity. This showed that the cathode ray carried energy and could do work. (3) The cathode ray can be deflected from a straight-line path by a magnetic field, suggesting that the two were related in some way. The discovery of this effect in 1855 predates by some ten years the unification of electricity and magnetism by James Clerk Maxwell. (4) Cathode rays cause phosphorescent materials to give off light. This also shows that the cathode ray carries energy and can do work. (5) Although there was some speculation that the cathode rays were negatively charged, it is not shown to be true by experiment until 1895, just two years before Thomson announced the discovery of the electron. (6) J.J. Thomson is the first individual to succeed in deflecting the cathode ray with an electrical field. He did so in 1897. The cathode rays bend toward the positive pole, confirming that cathode rays are negatively charged.

  21. Force on charged particle in magnetic field • Electric charges experience no force if they are stationary in a uniform magnetic field • Electric charges experience no force if they move with a velocity parallel to a uniform magnetic field • Electric charges experience a maximum force when they move with a velocity perpendicular to a magnetic field • The direction of the force is perpendicular to the velocity and the magnetic field direction Review

  22. Quiz - force on charged particle in magnetic field What is the direction of the force acting on the moving charged particles X and Y, both of which have the same magnitude charge? The force acting particle X is perpendicular out of the page The force acting particle Y is perpendicular out of the page Both the charge and the direction of the velocity are opposite to X What is the direction of the force acting on the moving charged particle Z? X The force acting particle Y is perpendicular out of the page Its magnitude is less than the force on X or Y. Y Z

  23. Force on Charged Particle in Magnetic Field The magnitude of the force (F) acting on a charged particle moving with velocity (v) in a magnetic field (B) is given by The force (F) is measured in newtons (N) The velocity (v) is measured in metres/second (ms–1) The magnetic field (B) is measured in teslas (T)

  24. Force on Charged Particle in Magnetic Field Q1. An electron (mass 9.1 x 10-31 kg) moves with a velocity of 3 x 107 m/s perpendicular to a magnetic field of 2 teslas. The charge on an electron is -1.6 x 10-19 coulombs. What would be the force on the electron? Q2. If a proton (mass 9.1 x 10-31 kg) entered the same field at the same speed of 3 x 107 m/s, compare its behaviour with that of the electron. Q3. A magnetic field and an electric field are arranged perpendicular to each other. A stream of charged particles moving perpendicular to both fields remains undeflected when the electric field has a strength of 5 x 103 NC-1 and the magnetic field is 2 x 10-2 T. What is the speed of the particles? Explain how this is independent of the mass. Q4. Compare (numerically) the mass to charge ratio of a beta particle and an alpha particle. State whether the mass or the charge of these particles has the greater effect on the radius of curvature of the particle in a magnetic field. Explain your answer. Solving Problems

  25. Electric Field Strength - Point Charges • Electric fields are represented arrows to indicate the direction of the field. • The closer the lines, the stronger the field represented. • These diagrams show the electric fields surrounding positive and negative charges. • The direction of an electric field is the direction of the force that it produces on a positive charge. Qualitative description

  26. Quiz - Electric fields and point charges Qualitatively describe the electric field surrounding a point positive charge. The electric field surrounding an isolated point positive charge is radial. The field lines point away from the positive charge. The strength of the field decreases with distance from the charge. A diagram can be used to augment the description. Qualitatively describe the electric field surrounding a point negative charge. Qualitative description

  27. – + – + Electric Field Strength - Parallel Plates The electric field between two parallel plates is uniform and has a direction from the positive to the negative plate. The field becomes less uniform as the distance between the plates increases. The field near the edges of the plates is non-uniform. The field direction is the direction of the force that would act on a positive charge placed in the field. Qualitative description

  28. Quiz - Electric field between parallel plates E Draw the electric field between the pair of square parallel plates seen edge on in the adjacent diagram. Under what conditions is the field between the plates uniform? The field is uniform providing the plates are parallel to each other and that the separation between the plates is small compared with the size (length of the sides) of the plate Solving Problems

  29. Charged Plates and Electric Field Production The magnitude of the electric field (E) between two parallel plates is • proportional to the potential difference (V) between the plates • inversely proportional to the separation (d) between them Potential difference is measured in volts (V) Distance is measured in metres (m) Electric field strength is thus measured in volts/metre* (Vm–1) *the alternative unit newton/coulomb is identical Quantitative description

  30. Quiz - Electric field strength The distance between the two square plates shown edge on in the adjacent diagram is 2 mm. The potential difference applied across the plates is 12 volts. What is the electric field strength? The electric field strength is 6000 volts/metre Solving Problems

  31. + + + + + – v – – – – – Thomson’s Experiment - Properties of Electrons • JJ Thomson used the vacuum tube above to determine the charge/mass ratio of the electron • He adjusted electric and magnetic fields so that the forces they produced on the electrons cancelled each other [Electric F = qE, Magnetic F = qvB] An electron moves to the right An electric field is applied Which way must a magnetic field be applied to counter the force produced by the electric field? E Charge to mass ratio of electron

  32. Thomson’s Experiment - Properties of Electrons Deduce the direction of the magnetic field required to make the electrons travel in a straight line in the tube. Answer: into the page Charge to mass ratio of electron

  33. Cathode Ray Tube - The Electron Gun The electron gun in a cathode ray tube produces electrons. • The cathode is heated by an electric current • Thermal vibrations give the electrons energy, releasing them from the metal surface

  34. Cathode Ray Tube - The Electron Gun An electric field accelerates and focuses the electron beam • What is the direction of the electric field in space in which the electrons are being accelerated? • The electric field is to the left (positive to negative)

  35. Cathode Ray Tube Operation A stronger electric field, produced by the high voltage of the accelerating anode, accelerate the electron beam further The kinetic energy of the fast moving electrons causes the screen phosphor to give off light. Movement of the electron beam in a vertical and horizontal direction is controlled by a combination of magnetic and electric fields.

  36. E force – force + Cathode Ray Tube - Electric Field Charged particles experience a force due to an electric field Electric field direction is by convention the direction of the force on a positive charge Electrons are therefore accelerated in the opposite direction to the electric field

  37. Cathode Ray Tube - Fluorescent Screen Electrons with high kinetic energies hit the phosphorescent screen, causing the atoms in the phosphor to produce light

  38. Cathode Ray Tube and the Oscilloscope Oscilloscopes have played a key role in scientific research. The oscilloscope is effectively a voltmeter capable of displaying variations in voltage (vertical axis) against time (horizontal axis). Any variable that can be measured electronically and converted to a voltage can be analysed with an oscilloscope. Many oscilloscopes now use computers to display the data rather than a dedicated instrument. Application

  39. Cathode Ray Tube and the Oscilloscope Oscilloscopes contain a cathode ray tube that is less complex than a television picture tube Application

  40. Cathode Ray Tube and the Electron Microscope Cathode ray tubes are the basis of the electron microscope. The electron microscope uses the wave properties of electrons to produce images of objects. The small wavelength permits a much greater resolution and hence magnification than a light microscope. Magnetic fields are used to focus and control the electron beam in the electron microscope. Image: Desktop electron microscope Application

  41. Cathode Ray Tube and the Electron Microscope Image: Electron microscope Application

  42. Cathode Ray Tube and the Television Set Application

  43. Magnetic Fields and the Television Set Application

  44. The Oscilloscope and Experimental Physics Image: cathode ray oscilloscope • Cathode ray tubes are the basis of many cathode ray oscilloscopes. • Computer based oscilloscopes are now common, as is LCD screen use. • The development of the oscilloscope as a key tool for measurement propelled research in many fields of science. Any variable that could be converted to a voltage could be displayed as a function of time on the oscilloscope screen. • Such variables include electronic voltages, sound levels, light intensities, biological signals such as heart and brain activity. Discussion of development

  45. The Photocopy Machine Discussion of application - see separate ppt on photocopier

  46. Lightning Conductors Lightning conductors are used on many buildings, towers and high voltage power lines. Lightning conductors Discussion of application

  47. Lightning Conductors See article “Lightning misses point” for an interesting example of how science can sometimes take things for granted - and get it wrong in the process! [Copied to notes page below] Discussion of application

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