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9.1 CRO

The meaning of thermionic emission

The emission of electrons from the surface of a heated metal or heated metal cathode.

The thermionic emission is a bit like electrons electrons being evaporated off from the hot wire.

The mechanism of thermionic emission

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A large number of electrons are free roam about inside a metal but an electron traveling outwards t the surface is held back by the attractive forces of the atomic nuclei near the surface.

However, when the metal is heated , some of electrons have gained enough kinetic energy (thermal energy) to escape from its surface.

Sources to produce the thermionic emission

Thermionic emission can be only be produced with certain metals, because it occurs at temperatures similar to their melting point.

A tungsten filament lamp was found to release electrons from it is filament at 2 300K.

It has been found that a metal filament coated with oxides of barium and strontium will release lots of thermal electrons at the much lower temperature of

1 300 K and will still emit some electrons at 1 000 K.

Factors that affect the rate of thermionic emission

(1) Surface area of the cathode

As the surface area of the cathode increases the rate of thermionic emission increases

(2) Temperature of the cathode

As the temperature of the cathode increases the rate of thermionic emission increases

(3) Types of metal

Different types of metal has different rate of

thermionic emission.

The good metals are tungsten, barium oxide and strontium oxide.

Cathode ray

Cathode ray is a narrow beam of a fast electrons moving in a vacuum.

Electron gun ( Cathode-ray tube)

Cathode ray can be produced by using an electron gun.

Such tubes, known as cathode-ray tubes ,have many applications including the television , cathode –ray oscilloscope (CRO) , Visual display unit (monitor) , radar screen , Maltese cross tube , Deflection Tube and X-Ray tube.

The design of a Cathode-ray tube and how it works?

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A cathode-ray tube consist of a vacuum tube , anode , cathode and a heating filament and screen.

The vacuum tube is an evacuated glass tube.

The anode has a hole in it to focusing the electrons.

The cathode is heated by a tungsten filament .

The heated cathode emits electrons and are accelerated at a high speed between anode and cathode because a high voltage is applied between the cathode and anode. The accelerated and fine beam electrons (cathode-ray) strikes the fluorescent screen causes the screen fluoresces with green light.

To investigate the properties of cathode rays

The properties of cathode rays is investigated by using Maltase cross tube and deflection tube.

Maltase cross tube

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Procedure

Observation

Explanation

Conclusion

6V heater supply is connected

A shadow of the cross is seen

The shadow is formed by the ray from the heated filament

Light rays travel in a straight line

6V heater supply and 3 kV power supply are connected

The green shadow of the cross is seen same size and at the same position as the shadow form by the light

The shadow is

formed by the cathode rays

Cathode rays travel in a straight line.

Cathode rays cause fluorescence.

Cathode rays carry kinetic energy and converts to light energy when they hit the screen.

A bar magnet is brought close to the cathode rays

The cathode ray shadow is moved and distorted

The catapult force is produced because and the cathode rays carry a charge

Cathode rays can be deflected by magnetic fields. The Fleming’s left-hand rule is used to determine the direction of motion.

Deflection tube

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Procedure

Observation

Explanation

Conclusion

6V heater supply and 3 kV power supply are connected

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No electric fields between two the metal plates

Light rays travel in a straight line

6V heater supply and 3 kV power supply are connected

and also 1000 V power supply is connected to the metal plates

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Electric field exists between two plates

Cathode ray is negatively charged

6V heater supply and 3 kV power supply are connected

and also 1000 V power supply is connected to the metal plates in reverse

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Electric field exists between two plates

Cathode ray is negatively charged

Properties of Cathode Rays

1. Travel in a straight lines in vacuum.

2. Possess kinetic energy and momentum

3. Produce fluorescent effect

4. Negatively charged

5. Deflected by an electric field towards a positive plate

6. Deflected by a magnetic field. The direction of deflection is determined by using Fleming’s Left-hand rule

7. Cause ionization of gas molecules

8. Can penetrate thin aluminium foil ,thin paper and thin graphite layer

9. Affect photographic plates

10. Produce heat and X-radiation in a X-ray tube

11. Charge of one electron ,e = 1.6 x 10-19 C

12. Mass of electron, me = 9 x 10-31 kg

Example 1

The diagram shows is applied to a cathode ray motion.

What is direction of the cathode ray is shifted?

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Solution

Types of motion of the cathode rays in a cathode rays tube

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Region

Types of motion

PQ : Cathode to anode

Uniform acceleration

QR: Anode to screen

Uniform velocity

Energy conversion of electrons in Cathode rays

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Region

Types of energy

P : Anode

Electrical potential energy

QR: Anode to cathode and screen

Kinetic energy

To determine the velocity of electrons

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From the principle of conservation of energy,

for each electron,

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v = velocity of the electrons

V = potential difference between anode and cathode

e = 1.6 x 10-19 C

me = 9 x 10-31 kg

Example 2

The potential difference between anode and cathode in an electron gun is 5 kV. Calculate the kinetic energy of the electrons?

(e = 1.6 x 10-19 C)

Solution

Example 3

In the vacuum tube of a television receiver , a cathode ray is produced and accelerated through a potential difference 7 kV. Determine the velocity of the cathode ray?

[ e = 1.6 x 10-19 C and me = 9 x 10-31 kg]

Solution

(a) Structure and the functions of the main parts of the Cathode Ray Oscilloscope ( CRO)

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Main part

Component

Function

Electron gun

Filament

Cathode

Control

Grid

Focusing anode

Accelerating anode

To heat up the cathode

Emits electrons through the thermionic emission process

Controls the number of electrons that will through it and hence control the brightness of the image on the screen

Focuses the electrons into a beam

To accelerate electrons to towards the screen

Deflection

system

Y-plates

X-plates

To deflect the electron beam vertically

To deflect the electron beam horizontally

Fluorescent

screen

Fluorescent

screen

Graphite coating

To convert the kinetic energy of the electron beam into the light energy

To channel the electrons striking the screen to the Earth

Handling CRO

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Knob / switch

Function / control

On/off

To on or off the CRO

Brilliance

To control the intensity of he bright spot

X-shift

To adjust the horizontal position of the bright spot

Y-shift

To adjust the vertical position of the bright spot

Y-gain

To amplified the small voltage across the Y-plates to deflect the electron beam. The control is calibrated in volt per cm

Time-base controls

Connected to the X-plates to control the frequency at which the beam sweeps horizontally across the screen. The control is calibrated in time per cm

X-input

To connect the source of potential difference to X-plates

Y-input

To connect the source of potential difference to Y-plates

AC/DC switch

Selected according to the type of input received

Uses of CRO

(1) Displaying waveforms

(2) As voltmeter (measuring potential difference)

(3) As a clock (Measuring short time intervals or frequency)

(1) Displaying waveforms

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(2) Measuring the potential difference power supply

The CRO is switched on.

The time-base circuit is switched off.

Adjust the spot to centre of the screen.

A dry cell is connected to the Y-input.

The vertical displacement of the spot is recorded = H cm

The Y-gain setting is recorded = Y volt/cm

The potential difference across the dry cell is calculated , V = YH volt

(3) Measuring a short time interval

The CRO is switched on.

The time-base circuit is switched on.

The Y-gain is adjusted so that the wave form displayed is easy to see.

A microphone is connected to the input-Y.

Two claps are made close to the

microphone.

The distance between two pulses on the screen is recorded = d

The time-base control setting is recorded = x ms / cm

The time lapse between the two claps is calculated , t = x d ms

Example 4

The diagram shows a waveform obtained from an a.c. power supply connected to Y-input of a CRO.

[ Y-gain setting = 20 V cm-1 and

Time-base control setting = 5 ms cm-1 ]

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Determine,

(a) the period of the signal

(b) the frequency of the signal

(c) the peak to peak voltage

(d) the peak voltage

Solution

Example 5

The figure shows a waveform obtained on the screen of CRO at an airport radar station. The point X and Y indicate the time transmission to an aero plane and time of receiving the reflected signals by the radar station .

[ Time-base control setting of the CRO = 50 ms cm-1 ]

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Determine

(a) The time travels of the radar from X to Y.

(b) The distance between the radar station and the aero plane.

[ Speed of light = = 3 x 108 ms-1 ]

Solution

One Response

  1. can i have the answer?

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