4.1 understanding the uses of the cathode-ray oscilloscope (
TRANSCRIPT
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Understanding the Uses of the Cathode-Ray Oscilloscope
(CRO)
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Thermionic Emission• Metal surface has many free electrons• Bound to the surface not enough kinetic energy to break free• Example :
– Tungsten filament is heated (2500 K)
– Electrons are emitted
– Enough kinetic energy to break free
• This effect thermionic emission• A process involving the emission of electrons from a hot metal surface
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Cathode Rays• Electron gun an assembly of cathode and anode in a vacuum tube• To produce beam of fast-moving electrons cathode rays• Anode :
– Has hole in it
– Connected to (+ve) terminal high p.d supply
• Cathode :– Heated by tungsten filament
– Connected to (-ve) terminal of the supply
• Heated cathode emits electron accelerates towards anode• Anode focuses electrons fine beam cathode rays• High velocity strike fluorescent screen screen glows
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Factor Affecting the Rate of Thermionic Emission
• Surface area of metal– Thermionic emission rate increases as the surface area increases
– Larger surface area allows more electrons to be emitted
• Temperature of metal– Rate of thermionic emission increases with temperature of metal
– Higher temperature more electrons gain enough energy
• Types of metal– Different types of metals different rate of thermionic emission
– Different metals require different amounts of energy
• Nature of metal surface– Rate of thermionic emission increases when metal is coated with metal oxide
– Emit electrons at a lower temperature
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Properties of Cathode Rays
• Consist of fast-moving electrons straight line in vacuum• Possess momentum & kinetic energy due to the moving electrons
– Produce a bright spot when strike fluorescent screen (fluorescent effect)
• Negatively charged– Rays are deflected towards a positive plate
• Deflected by magnetic fields– Direction of deflection Fleming’s Left-hand Rule
• When strike heavy metal target X-Ray– Rest of the energy release as heat
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• Maltese Cross Tube
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• Deflection Tube
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Energy Conversion of Electrons • Electric current heats the filaments emits electrons
– Electrical energy heat energy
• Electric field across anode and cathode accelerates electrons towards fluorescent screen
– Electrical potential energy kinetic energy of electrons
• High speed electrons strike screen produce fluorescent (light)– Kinetic energy of electron light energy
• Potential difference across anode & cathode V• Electrical potential difference energy of electron eV
• Kinetic energy on an electron ½mev2
• Therefore :– Electrical potential energy = kinetic energy
• What is the final velocity of an electron ?
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The Cathode-Ray Oscilloscope (CRO)
• 3 main parts of the tube :– An electron gun– A deflection system– A fluorescent screen
• The main feature cathode-ray tube
• Air is removed avoid collision between electron beam & air molecules
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An Electron Gun• To produce a narrow beam of electrons• Heating filament :
– Heat the cathode to a high temperature
• Cathode :– Emit electrons from its surface
• Control grid :– Regulates the number of electrons reach the anode
– Control the brightness of the spot
• Focusing anode :– Focus electrons to a narrow beam
– Electrons arrive at the same spot
• Accelerating mode :– Accelerates the electron beam to a high velocity
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A Deflection System• Used to deflect the electron beam• Y-Plates :
– To move the electron beam vertically
• X-Plates :– To move the electron beam horizontally
• A sinusoidal wave form on the display• Due to the combined deflection by Y-Plates and X-Plates
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A Fluorescent Screen• Made up of 2 components• Inside surface coated with zinc sulphide
– Fluoresces when electron beam strikes it
– Kinetic energy of electron light energy
• Glass surrounding the fluorescent screen – Coated with graphite & connected to Earth
– Channel the electrons striking the screen to the Earth
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