Understanding the Uses of the Cathode-Ray Oscilloscope

(CRO)

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

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

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

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

• Maltese Cross Tube

• Deflection Tube

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 ?

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

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

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

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