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# Electric Fields

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SACE Physics Section 2 Topic 1

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### Electric Fields

1. 1. Electric Fields Section 2 Topic 1
2. 2. Charges <ul><li>All matter is made up of atoms and molecules ; </li></ul><ul><ul><li>contain charged particles, </li></ul></ul><ul><ul><li>the proton and electron. </li></ul></ul><ul><li>The charges on each are equal ; </li></ul><ul><ul><li>but opposite in sign. </li></ul></ul>
3. 3. Charges <ul><li>The quantity of charge is measured in coulombs (C). </li></ul><ul><li>1C = charge carried by 6.25 x 10 18 electrons </li></ul><ul><li>The charge on one electron = -1.6 x 10 -19 C </li></ul><ul><li>The charge on one proton = +1.6 x 10 -19 C </li></ul>
4. 4. Charges <ul><li>Uncharged matter is electrically neutral ; </li></ul><ul><ul><li>same number of electrons and protons. </li></ul></ul><ul><li>By displacing electrons ; </li></ul><ul><ul><li>object can become charged. </li></ul></ul>
5. 5. Charges <ul><li>Like charges repel; unlike charges attract. </li></ul><ul><li>In most cases ; </li></ul><ul><ul><li>it is the electrons that move , </li></ul></ul><ul><ul><li>electrons are free to move . </li></ul></ul><ul><ul><li>positive can move in some cases . </li></ul></ul>
6. 6. Coulomb’s Law <ul><li>Charles Augustin de Coulomb ; </li></ul><ul><ul><li>in 1785 , </li></ul></ul><ul><li>I nvestigated the force acting ; </li></ul><ul><ul><li>between any two charged objects. </li></ul></ul><ul><li>He discovered that there was a relationship between ; </li></ul><ul><ul><li>force ( F ) , </li></ul></ul><ul><ul><li>two charges on the objects q 1 and q 2 . </li></ul></ul>
7. 7. Coulomb’s Law <ul><li>F  q 1 and F  q 2 </li></ul><ul><li>  D iscovered a relationship between ; </li></ul><ul><ul><li>force ( F ) , </li></ul></ul><ul><ul><li>distance ( r ) between the centres of the objects. </li></ul></ul>
8. 8. Coulomb’s Law <ul><li>The value of the constant is ; </li></ul><ul><ul><li>9.00 x 10 9 Nm 2 C -2 , in a vacuum. </li></ul></ul><ul><li>The constant is written as : </li></ul><ul><li> = permittivity of a medium ; </li></ul><ul><ul><li>or the ability to transmit electric force. </li></ul></ul><ul><li> o is the permittivity in a vacuum. </li></ul>
9. 9. Coulomb’s Law <ul><li>Force is a vector and so must have a direction. </li></ul><ul><li>The force is along a line joining the two centres. </li></ul><ul><li>If charges are the same, </li></ul><ul><ul><li>there is repulsion </li></ul></ul><ul><li>I f different, </li></ul><ul><ul><li>there is attraction. </li></ul></ul><ul><li>Coulomb’s Law states: </li></ul>
10. 10. Coulomb’s Law <ul><li>T he force acting between two charges q 1 and q 2 ; </li></ul><ul><ul><li>who are separated by a distance d , </li></ul></ul><ul><ul><li>is directly proportional to the product of the charges , </li></ul></ul><ul><ul><li>and inversely proportional to the square of the distance between them. </li></ul></ul><ul><li>The force is along the line joining the centres of the charges. </li></ul>
11. 11. Coulomb’s Law <ul><li>This is similar to Newton’s law of universal gravitation: </li></ul>
12. 12. Coulomb’s Law <ul><li>1. The interaction acts on both bodies. </li></ul><ul><li>2. Both forces act at a distance without the bodies touching. </li></ul><ul><li>3. Both directly proportional to the product of the properties causing the interaction. </li></ul><ul><li>4. Both inversely proportional to the distance between the bodies. </li></ul><ul><li>5. Forces are consistent with N III </li></ul>
13. 13. Coulomb’s Law <ul><li>They are dissimilar in that: </li></ul><ul><li>1. Gravitation is a force of attraction only while charges can attract and repel. </li></ul><ul><li>2. The force between charges depends on the medium while gravity does not. </li></ul>
14. 14. Principle of Superposition <ul><li>As force is a vector we cannot algebraically add forces if there is more than one point charge present. </li></ul><ul><li>The law that we use to determine to total force is called the law of superposition. </li></ul>
15. 15. Principle of Superposition <ul><li>When two or more point charges are present ; </li></ul><ul><ul><li>the total force is equal to , </li></ul></ul><ul><ul><li>the vector sum of the forces , </li></ul></ul><ul><ul><li>due to each of the other point charges. </li></ul></ul>
16. 16. Principle of Superposition <ul><li>To use this principle, follow the rules given below: </li></ul><ul><li>1. Draw a labelled diagram </li></ul><ul><li>Use coulombs law to determine the magnitude ; </li></ul><ul><ul><li>ignore the direction at this stage </li></ul></ul>
17. 17. Principle of Superposition <ul><li>3. Determine if the force is attractive or repulsive . </li></ul><ul><li>4. Repeat step 2 for any other combinations of charges . </li></ul><ul><li>5. Draw a vector diagram . </li></ul>
18. 18. Principle of Superposition <ul><li>Find the resultant ; </li></ul><ul><ul><li>using Pythagoras theorem , </li></ul></ul><ul><ul><li>trigonometry </li></ul></ul><ul><li>Determine the direction using trigonometry. </li></ul><ul><li>Coulomb's Law </li></ul><ul><li>Coulombs Law 2 </li></ul>
19. 19. Principle of Superposition Try Example 1
20. 20. Solution
21. 21. Solution <ul><li>q = +10  C = +10 x 10 -6 = 10 5 C </li></ul><ul><li>q 1 = +6  C = +6 x 10 -6 C </li></ul><ul><li>d 1 = 0.6 m </li></ul><ul><li>q 2 = +8  C = +8 x 10 -6 C </li></ul><ul><li>d 2 = 0.4 m </li></ul><ul><li>F total = F 1 + F 2 </li></ul>
22. 22. Solution <ul><li>Must use vector techniques as forces are in different directions. </li></ul><ul><li>F 1 = 1.50 N east (repulsion) </li></ul>
23. 23. Solution <ul><li>F 2 = 4.5 N south (attraction) </li></ul>
24. 24. Solution
25. 25. Solution <ul><li>Use Pythagoras’ Theorem; </li></ul><ul><li>F total = 4.74 N </li></ul>
26. 26. Solution <ul><li>From trigonometry, </li></ul><ul><li> = 71.6 o </li></ul>
27. 27. Solution <ul><li>The angle is 71.6 + 90 = 161.6 o T </li></ul><ul><li>F total = 4.74 N at 161.6 o T </li></ul>
28. 28. The Electric Field <ul><li>An electric field is a region in space where ; </li></ul><ul><ul><li>an object will experience a force due to , </li></ul></ul><ul><ul><li>its charge , </li></ul></ul><ul><ul><li>without the charges necessarily touching. </li></ul></ul>
29. 29. Lines of Electric Force <ul><li>A diagram representing the relative strength of a field at any point can be drawn. </li></ul><ul><li>The lines drawn give , </li></ul><ul><ul><li>direction of the force on a tiny positive charge . </li></ul></ul><ul><li>If the charge were allowed to move, </li></ul><ul><ul><li>the charge would move along the field line. </li></ul></ul>
30. 30. Lines of Electric Force <ul><li>  Rules for drawing electric field line diagrams. </li></ul><ul><li>1. Lines of electric force are always directed from positive to negative charges. </li></ul>
31. 31. Lines of Electric Force
32. 32. Lines of Electric Force <ul><li>Lines of electric force always start and end on a charged surface ; </li></ul><ul><ul><li>make an angle of 90 o to that surface. </li></ul></ul><ul><li>If the surface is curved ; </li></ul><ul><ul><li>construct a line at 90 o to the tangent at that point. </li></ul></ul>
33. 33. Lines of Electric Force
34. 34. Lines of Electric Force
35. 35. Lines of Electric Force <ul><li>3. Lines of electric force never cross. </li></ul><ul><li>There is no electric field inside a hollow conductor, </li></ul><ul><ul><li>hence no lines of electric force exist. </li></ul></ul><ul><li>This is used in electrical shielding such as ; </li></ul><ul><ul><li>Faraday’s Box , </li></ul></ul><ul><ul><li>Van De Graaf. </li></ul></ul><ul><li>Electric Field Charge Configuration </li></ul>
36. 36. Lines of Electric Force <ul><li>Lines of electric force are found to concentrate ; </li></ul><ul><ul><li>at regions of high curvature on a conductor. </li></ul></ul>
37. 37. Lines of Electric Force <ul><li>The field may be strong enough </li></ul><ul><ul><li>at the sharp point </li></ul></ul><ul><ul><li>to ionise the air. </li></ul></ul><ul><li>Charges may then move away </li></ul><ul><ul><li>from the conductor. </li></ul></ul><ul><li>This is called Corona Discharge. </li></ul><ul><li>Will be discussed in more detail later. </li></ul>
38. 38. Lines of Electric Force <ul><li>Make sure that the number of field lines per unit area represents the field strength ; </li></ul><ul><ul><li>when close together the field is strong , </li></ul></ul><ul><ul><li>when far apart the field is weak. </li></ul></ul><ul><li>  Where the field lines are parallel and equally spaced ; </li></ul><ul><ul><li>the field is said to be uniform. </li></ul></ul>
39. 39. Lines of Electric Force
40. 40. Lines of Electric Force <ul><li>The field becomes curved ; </li></ul><ul><ul><li>or non uniform. </li></ul></ul><ul><li>This is known as the end effect. </li></ul><ul><li>If the separation of the plates becomes too large ; </li></ul><ul><ul><li>the end effect encroaches on the region between the plates. </li></ul></ul>
41. 41. Lines of Electric Force <ul><li>This is the reason why the plates should be much longer ; </li></ul><ul><ul><li>than the separation between them. </li></ul></ul>
42. 42. Lines of Electric Force
43. 43. Lines of Electric Force <ul><li>U ncharged sphere placed between parallel plates </li></ul>Electric Field around 2 Point Charges
44. 44. Electric Field Strength <ul><li>The above diagrams only give an idea of the actual field strength. </li></ul><ul><li>In gravitational fields, field strength was determined by: </li></ul><ul><li>In an electric field ; </li></ul><ul><ul><li>the result is very similar. </li></ul></ul>
45. 45. Electric Field Strength <ul><li>The electric field strength, E , </li></ul><ul><ul><li>at a point in an electric field is given by the force, F , </li></ul></ul><ul><ul><li>acting on a unit positive charge placed at that point in the field. </li></ul></ul><ul><li>Units for E are N C -1 . </li></ul><ul><li>It is a vector with both magnitude and direction. </li></ul>
46. 46. Try Example 2
47. 47. Solution (Part a) <ul><li>q = + 2.50 x 10 -5 C </li></ul><ul><li>F = 1.25 N west </li></ul><ul><li>E = 5.00 x 10 4 N C -1 west </li></ul>
48. 48. Solution (Part b) <ul><li>M = 1.00 x 10 -2 kg </li></ul><ul><li>F = m a </li></ul><ul><li>a = 125 m s -2 West </li></ul>
49. 49. Derivation <ul><li>Consider two charges ; </li></ul><ul><ul><li>a fixed point charge q and a test charge q T , </li></ul></ul><ul><ul><li>separated by a vacuum by a distance r. </li></ul></ul><ul><li>Coulomb’s law gives the force each feels ; </li></ul><ul><ul><li>directions will be opposite (NIII). </li></ul></ul>
50. 50. Derivation <ul><li>E at the position of q T is given by : </li></ul>
51. 51. Derivation
52. 52. Derivation <ul><li>This expression does not include the test charge. </li></ul><ul><li>The strength of the electric field at a point ; </li></ul><ul><ul><li>is a feature of the field and , </li></ul></ul><ul><ul><li>is independent of any charge placed at that point . </li></ul></ul>
53. 53. Derivation <ul><li>This is analogous to gravitational field strength. </li></ul><ul><li>The field strength at the surface of the earth is 9.8 ms -2 ; </li></ul><ul><ul><li>regardless of whether a mass is present or not. </li></ul></ul>
54. 54. Try Example 3
55. 55. Solution (Part a) <ul><li>q = - 6.0 x 10 -6 C </li></ul><ul><li>r = 1.0 x 10 -2 m </li></ul><ul><li>E = 6 x 10 -7 N C -1 towards charge </li></ul><ul><li>E = 5.4 x 10 8 N C -1 toward the centre of the sphere </li></ul>
56. 56. Solution (Part b) <ul><li>r = 0.03 m </li></ul><ul><li>r = 0.03 m from the centre of the sphere or 0.02 m above its surface. </li></ul>
57. 57. Solution (Part c) <ul><li>At a point 0.03 m from the centre of the charge, </li></ul><ul><li>F = E q </li></ul><ul><li>F = 6 x 10 7 x 1.0 x 10 -6 </li></ul><ul><li>F = 60 N toward the centre of the sphere (attraction) </li></ul>
58. 58. Electric Field Strength Due to Several Charges <ul><li>As the electric field strength is also a vector ; </li></ul><ul><ul><li>the law of superposition also applies. </li></ul></ul>
59. 59. Electric Field Strength Due to Several Charges <ul><li>If more than one charge exists in an electric field, the total field at any one point is ; </li></ul><ul><ul><li>the vector sum of the electric field strengths due to each charge. </li></ul></ul><ul><li>E total = E 1 + E 2 + E 3 + …….+ E n </li></ul>
60. 60. Electric Field Strength Due to Several Charges Example
61. 61. Electric Field Strength Due to Several Charges <ul><li>There are three component fields at point P and so to find the resultant field ; </li></ul><ul><ul><li>a vector addition must be completed. </li></ul></ul><ul><li>Note the direction of the component fields is away from the positive charges. </li></ul>
62. 62. Electric Field Strength Due to Several Charges
63. 63. Electric Field Strength Due to Several Charges <ul><li>For two opposite charges ; </li></ul><ul><ul><li>the direction must also be taken into account. </li></ul></ul><ul><li>For negative charges ; </li></ul><ul><ul><li>the direction is towards the negative charge. </li></ul></ul>
64. 64. Electric Field Strength Due to Several Charges
65. 65. Try Example 4
66. 66. Solution Part (a) <ul><li>q 1 = + 5.0 x 10 -6 C r 1 = 0.05 m </li></ul><ul><li>q 2 = - 5.0 x 10 -6 C r 2 = 0.05 m </li></ul><ul><li>E 1 = 1.8 x 10 7 N C -1 toward q 2 </li></ul>
67. 67. Solution Part (a) <ul><li>E 2 = 1.8 x 10 7 N C -1 toward q 2 </li></ul><ul><li>As both are vectors in the same direction, their magnitudes can be added arithmetically. </li></ul><ul><li>E T = E 1 + E 2 </li></ul><ul><li>E T = 3.6 x 10 7 N C -1 toward the negative charge </li></ul>
68. 68. Solution Part (b) <ul><li>The three sides are 6, 8, 10. It must therefore be right angled as it is the Pythagorean ratio 3:4:5 </li></ul><ul><li>E T = E 1 +E 2 </li></ul>
69. 69. Solution Part (b) <ul><li>E 1 = 1.25 x 10 7 N C -1 away from q 1 </li></ul>
70. 70. Solution Part (b) <ul><li>E 2 = 7.03 x 10 6 N C -1 toward q 2 </li></ul>
71. 71. Solution Part (b) <ul><li>Use Pythagoras’ Theorem </li></ul>
72. 72. Solution Part (b) <ul><li>E T = 1.43 x 10 7 N C -1 </li></ul>
73. 73. Solution Part (b) <ul><li>From trigonometry </li></ul><ul><li> = 29.4 o </li></ul><ul><li>E T = 1.4 x 10 7 N C -1 29 o clockwise from E 1 </li></ul>
74. 74. Motion of Charges in a Field <ul><li>A charge that is free to move in a uniform electric field ; </li></ul><ul><ul><li>behaves in a similar way to a mass in a gravitational field. </li></ul></ul><ul><li>In a gravitational field, an object which moves towards the earth ; </li></ul><ul><ul><li>experiences a force that converts P.E. to K.E. </li></ul></ul>
75. 75. Motion of Charges in a Field <ul><li>When energy is converted from one form to another ; </li></ul><ul><ul><li>work is done. </li></ul></ul><ul><li>No work is done in the component that is parallel to the ground . </li></ul>
76. 76. Motion of Charges in a Field <ul><li>In an electric field, the same applies. </li></ul><ul><li>When a charge moves parallel to the conducting surface ; </li></ul><ul><ul><li>no work is done . </li></ul></ul><ul><li>The force only acts radially from the surface ; </li></ul><ul><ul><li>its velocity is unchanged. </li></ul></ul>
77. 77. Motion of Charges in a Field <ul><li>There cannot be a field inside a conductor no matter its shape. </li></ul><ul><li>Field lines show the direction ; </li></ul><ul><ul><li>a positive charge would move if placed in the field. </li></ul></ul><ul><li>As all points on the surface of the conductor are at the same potential ; </li></ul><ul><ul><li>the field lines would start and finish at exactly the same potential. </li></ul></ul>
78. 78. Motion of Charges in a Field <ul><li>No work would be done ; </li></ul><ul><ul><li>the charges would not experience a force , </li></ul></ul><ul><ul><li>compelling them to move. </li></ul></ul><ul><li>No charge actually resides within the conductor itself. </li></ul>
79. 79. Motion of Charges in a Field <ul><li>If the electric field is strong enough ; </li></ul><ul><ul><li>the air can be ionised. </li></ul></ul><ul><li>This is most likely to occur where charges accumulate ; </li></ul><ul><ul><li>about sharp points. </li></ul></ul><ul><li>When this occurs ; </li></ul><ul><ul><li>field lines are concentrated. </li></ul></ul>
80. 80. Motion of Charges in a Field <ul><li>When you see the ‘electric wind’ from a ‘windmill’ placed on top a Van de Graaf Generator ; </li></ul><ul><ul><li>it is because the air is being ionised , </li></ul></ul><ul><ul><li>charges are repelling. </li></ul></ul><ul><li>This is called Corona Discharge ; </li></ul><ul><ul><li>major application to modern electronic office equipment. </li></ul></ul>
81. 81. Motion of Charges in a Field <ul><li>  Another example of Corona Discharge is when a needle, held in place on a Van de Graaf Generator ; </li></ul><ul><ul><li>seen to deflect a candle flame. </li></ul></ul>
82. 82. Photocopiers & Laser Printers <ul><li>To produce a page from a photocopier or laser printer is essentially the same process. </li></ul><ul><li>There are five steps in the process. </li></ul><ul><li>Photocopier Stages </li></ul>
83. 83. Photocopiers & Laser Printers <ul><li>Step 1:Charging the Photoconductive Drum. </li></ul><ul><li>The drum has the special property of being an electrical insulator in the dark ; </li></ul><ul><li>an electrical conductor when exposed to light. </li></ul>
84. 84. Photocopiers & Laser Printers <ul><li>The material used to coat the earthed aluminium drum ; </li></ul><ul><ul><li>3 - 15 cm in diameter , </li></ul></ul><ul><ul><li>to achieve this effect is commonly selenium. </li></ul></ul>
85. 85. Photocopiers & Laser Printers <ul><li>Near the drum is a thin corona wire ; </li></ul><ul><ul><li>voltage of about 6000V between it and the drum , </li></ul></ul><ul><ul><li>extends for the length of the drum. </li></ul></ul><ul><li>The polarity can vary depending on the design. </li></ul>
86. 86. Photocopiers & Laser Printers
87. 87. Photocopiers & Laser Printers <ul><li>The electric field near the corona wire ; </li></ul><ul><ul><li>accelerates any ions in the atmosphere , </li></ul></ul><ul><ul><li>to high velocities. </li></ul></ul><ul><li>They in turn collide with neutral atoms in the air ; </li></ul><ul><ul><li>knocking out some electrons. </li></ul></ul>
88. 88. Photocopiers & Laser Printers <ul><li>These free electrons attach themselves to other neutral atoms. </li></ul><ul><li>From this process ; </li></ul><ul><ul><li>large amounts of positive and negative ions are formed , </li></ul></ul><ul><ul><li>as more and more collisions occur. </li></ul></ul>
89. 89. Photocopiers & Laser Printers <ul><li>These charged ions are attracted to either ; </li></ul><ul><ul><li>the corona wire , </li></ul></ul><ul><ul><li>or the drum. </li></ul></ul><ul><li>On reaching the drum ; </li></ul><ul><ul><li>they charge the photoconductive coating uniformly , </li></ul></ul><ul><ul><li>as the drum rotates. </li></ul></ul>
90. 90. Photocopiers & Laser Printers <ul><li>Step 2:Producing a Latent Image on the Drum </li></ul><ul><li>The image is then projected onto the drum. </li></ul><ul><li>The light areas become conductive and the charge is then deposited by step 1; </li></ul><ul><ul><li>move through the photoconductive coating , </li></ul></ul><ul><ul><li>to the aluminium drum underneath , </li></ul></ul><ul><ul><li>and then to ground. </li></ul></ul>
91. 91. Photocopiers & Laser Printers <ul><li>The dark parts of the image act as an insulator ; </li></ul><ul><ul><li>these areas of the drum remain uncharged. </li></ul></ul><ul><li>A latent image now exists in a charge pattern on the drum. </li></ul>
92. 92. Photocopiers & Laser Printers <ul><li>It is essentially in this step of how the image is projected onto the drum that ; </li></ul><ul><ul><li>a photocopier , </li></ul></ul><ul><ul><li>and laser printer differs. </li></ul></ul><ul><li>Photocopiers use mirrors as shown below. </li></ul>
93. 93. Photocopiers & Laser Printers
94. 94. Photocopiers & Laser Printers <ul><li>Laser printers use a laser to scan the photoconductive layer ; </li></ul><ul><ul><li>a line at a time. </li></ul></ul><ul><li>The laser switches on or off as required. </li></ul>
95. 95. Photocopiers & Laser Printers <ul><li>A 600 dpi (dots per inch) laser can switch on and off ; </li></ul><ul><ul><li>600 times per line and scan , </li></ul></ul><ul><ul><li>600 lines per inch. </li></ul></ul>
96. 96. Photocopiers & Laser Printers <ul><li>Step 3:Applying the Toner </li></ul><ul><li>A powder consisting of carbon and resins ; </li></ul><ul><ul><li>called toner . </li></ul></ul><ul><li>M ixed with another powder , </li></ul><ul><ul><li>called a carrier , </li></ul></ul><ul><ul><li>consisting of larger particles , </li></ul></ul><ul><ul><li>of iron coated in resin. </li></ul></ul>
97. 97. Photocopiers & Laser Printers <ul><li>As they mix ; </li></ul><ul><ul><li>they become oppositely charged . </li></ul></ul><ul><li>E lectrostatic attraction between the particles ; </li></ul><ul><ul><li>causes the toner particles to , </li></ul></ul><ul><ul><li>coat the carrier particles. </li></ul></ul>
98. 98. Photocopiers & Laser Printers
99. 99. Photocopiers & Laser Printers <ul><li>A magnetic roller, next to the drum ; </li></ul><ul><ul><li>attracts the iron particles in the carrier ; </li></ul></ul><ul><ul><li>which form hair like bristles on the roller. </li></ul></ul>
100. 100. Photocopiers & Laser Printers <ul><li>When they come in proximity to the relatively higher charged drum ; </li></ul><ul><ul><li>the toner particles are attracted to the drum , </li></ul></ul><ul><ul><li>and leave the magnetic roller. </li></ul></ul>
101. 101. Photocopiers & Laser Printers
102. 102. Photocopiers & Laser Printers <ul><li>Step 4:Transferring the Toner to the Paper </li></ul><ul><li>The paper is charged the same sign as that on the drum ; </li></ul><ul><ul><li>using another corona wire , </li></ul></ul><ul><ul><li>called the transfer corona . </li></ul></ul>
103. 103. Photocopiers & Laser Printers <ul><li>The paper then is brought into contact with the drum. </li></ul><ul><li>As the charge on the paper is much higher ; </li></ul><ul><ul><li>the toner transfers to the paper. </li></ul></ul>
104. 104. Photocopiers & Laser Printers <ul><li>So the paper does not cling to the drum, the extra charge on the paper is removed ; </li></ul><ul><ul><li>using another oppositely charged corona wire , </li></ul></ul><ul><ul><li>called the separation corona . </li></ul></ul>
105. 105. Photocopiers & Laser Printers
106. 106. Photocopiers & Laser Printers <ul><li>Step 5:Fixing the Image and Cleaning the Drum. </li></ul><ul><li>At this stage the toner is still a powder ; </li></ul><ul><ul><li>can easily be brushed away. </li></ul></ul>
107. 107. Photocopiers & Laser Printers <ul><li>The imaged is fixed by passing the paper between a pair of rollers ; </li></ul><ul><ul><li>heated to a high temperature. </li></ul></ul>
108. 108. Photocopiers & Laser Printers <ul><li>This melts the toner ; </li></ul><ul><ul><li>rollers press it , </li></ul></ul><ul><ul><li>into the fibres of the paper. </li></ul></ul><ul><li>This is why the paper is often warm ; </li></ul><ul><ul><li>when it emerges from the photocopier. </li></ul></ul>
109. 109. Photocopiers & Laser Printers <ul><li>The drum continues to rotate pas t a blade ; </li></ul><ul><ul><li>that removes any excess toner. </li></ul></ul><ul><li>The latent image is then removed by ; </li></ul><ul><ul><li>exposing the drum to a bright light , </li></ul></ul><ul><ul><li>ready for the next copy. </li></ul></ul><ul><li>The complete process is shown below. </li></ul>
110. 110. Photocopiers & Laser Printers