Pancaran Termion

(a)Pengertian

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

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

(3) Types of metalDifferent types of metal has different rate of thermionic emission.The good metals are tungsten, barium oxide and strontium oxide.

Cathode rayCathode 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?

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

1

LESSON 1-Understanding the uses of the Cathode Ray Oscilloscope (C.R.O)

Procedure Observation Explanation Conclusion6V 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

Procedure Observation Explanation Conclusion6V heater supply and 3 kV power supply are connected

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

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

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 momentum3. Produce fluorescent effect4. Negatively charged5. Deflected by an electric field towards a positive

plate

2

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

7. Cause ionization of gas molecules8. Can penetrate thin aluminium foil ,thin paper

and thin graphite layer9. Affect photographic plates10. Produce heat and X-radiation in a X-ray tube11. Charge of one electron ,e = 1.6 x 10 -19 C12. Mass of electron, me = 9 x 10 -31 kg

Example 1The diagram shows is applied to a cathode ray motion.What is direction of the cathode ray is shifted?

Solution

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

Region Types of motionPQ : Cathode to anode Uniform accelerationQR: Anode to screen Uniform velocity

Energy conversion of electrons in Cathode rays

Region Types of energyP : Anode Electrical potential

energyQR: Anode to cathode and screen

Kinetic energy

To determine the velocity of electrons

From the principle of conservation of energy,for each electron,

Electrical potential energy = Kinetic energy

eV = ½mv2

hence, v = 2eV m v = velocity of the electronsV = potential difference between anode and cathodee = charge of one electron, 1.6 x 10 -19 Cm= mass of electron 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?

3

(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 ]

Solution

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

Main part Component Function

Electron gun

Filament

Cathode

ControlGrid

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

Deflectionsystem

Y-plates

X-plates

To deflect the electron beam vertically

To deflect the electron beam horizontally

Fluorescentscreen

Fluorescentscreen

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

4

Knob / switch Function / controlOn/off To on or off the CROBrilliance To control the intensity of he

bright spotX-shift To adjust the horizontal

position of the bright spotY-shift To adjust the vertical position

of the bright spotY-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

(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 cmThe Y-gain setting is recorded = Y volt/cmThe 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 = dThe time-base control setting is recorded = x ms / cmThe time lapse between the two claps is calculated , t = xd ms

Example 4

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

5

[ Y-gain setting = 20 V cm-1 and Time-base control setting = 5 ms cm-1 ]

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 ]

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

1 The emission of electrons from the surface of a heated metal is called

A Vaporization

6

TUTORIAL 1

B CondensationC Cathode emissionD Thermionic emission

2 Thermionic emission is process involving

A the reflection of electrons from a heavy metal target

B the collision of electrons with the fluorescent screen

C the release of electrons from a hot metal surface

D the deflection of electron through an electric field

3 The rate of thermionic emission is not depends on

A The temperature of the metalB The surface area of the metalC The shape of the metalD The type of the metal

4 What is emitted by the hot metal filament in a cathode-ray tube?

A α-particles B ElectronsC Protons D X-rays

5 A narrow beam of a fast electrons moving in a vacuum is called

A X- ray B Alpha rayC Cathode ray D Gamma ray

6 When the electrons move through vacuum in an electron gun , the electron beam

A becomes narrow and is charged B becomes heavier and is charged C moves with a high velocity and is chargedD becomes narrow and moves with a high

velocity

7 Which of the following not involving the thermionic emission ?

A TransistorB TelevisionC Maltese cross tubeD Cathode Ray Oscilloscope

8 The diagram shows a cathode-ray tube. What is the types of motion of the cathode ray at sections JK and KL?

JK KLA Uniform

velocity Uniform acceleration

B Uniform acceleration

Uniform velocity

C Zero velocity Zero acceleration

D Zero acceleration

Zero velocity

9 Which of the following is not the property of cathode rays?

A Negatively chargedB Produce fluorescent effectC Cause ionization of gas moleculesD Moves with speed o light in vacuum

10 Which of the following is the property of cathode rays?

A Possess kinetic energy and momentum B An electromagnetic wavesC Has a high penetrating powerD Positively charged

11 In an electron gun ,the anode is connected to a positive potential to

A prevent the electrons hit the screenB attract the electrons to accelerateC control the number of electrons D heat the filament

12 The diagram shows a cathode-ray tube.

7

Which of the following is happened when the voltage of the high voltage supply is increased?

A The temperature of the filament increases B the rate of thermionic emission increasesC The velocity of the electrons increasesD The number of the electrons increases

13 The diagram shows to bar magnets are placed in between a Maltese Cross tube.In which direction will the shadow of the Maltese Cross is shifted?

14 The diagram shows a shadow of the Maltese Cross tube formed on the CRO screen.Where can the shadow be observed when the direction of the electron beam in an upward direction out of the paper?

15 The diagram shows an electron beam through a magnetic field.In which direction will the electron beam is deflected?

16 The potential difference between anode and cathode in an electron gun is 4 kV. Calculate the kinetic energy of the electrons?(e = 1.6 x 10 -19 C)

A 4.0 x 10-22 J B 4.0 x 10-20 JC 6.4 x 10-19 J D 6.4 x 10-16 JE 4.0 x 10-14 J

17 An electron is accelerated between anode and cathode through a potential difference 8 kV. What is the velocity of the electron?[ e =1.6 x 10 -19 C and me= 9 x 10 -31 ]

A 5.3 x 107 ms-1 B 4.3 x 106 ms-1

8

C 3.3 x 104 ms-1 D 2.3 x 103ms-1 E 1.3 x 102ms-1

18 Which components cannot be found in the electron gun of a CRO?

A Focusing anodeB Control grid C CathodeD Y-plates

19 What is the function of X-plates in CRO?

A To deflect the electron beam verticallyB To deflect the electron beam horizontallyC To convert a spot to a straight line D To convert the kinetic energy of the

electron beam into the light energy

20 Which of the components in a CRO controls the brightness of the image on the screen ?

A FilamentB Control gridC Focusing anode D Accelerating anode

21 What is the function the time-base switch in a CRO?

A Control the intensity of he bright spotB adjust the horizontal position of the bright

spotC connect the source of potential difference

to X-platesD Control the frequency at which the beam

sweeps horizontally across the screen.

22 Which of the following is not the uses of CRO?

A To measure the potential differenceB To measure the short time intervals C To measure the currentD To display the waveforms

23 The diagram shows the trace on the of a CRO when a power supply is connected to the Y-input of the CRO.

Which of the following is true?

Type of the power supply

Time-base control

A d.c. offB a.c. offC d.c. onD a.c. on

24 The diagram shows the trace on the of a CRO when an a.c. power supply is connected to the Y-input of the CRO.

Which of the following tracing is formed on the screen when the a.c. power supply is replaced by a dry cell?

25 The diagram shows a bright spot at the centre of the screen of a CRO.

9

What is the new position of the spot when P is adjusted so that its potential difference is higher than R and Q has a lower potential difference than S.

26 The diagram shows a bright spot formed on the screen of a CRO and a magnetic field is applied.

Where can the spot be observed now?

27 The diagram shows a bright spot at the centre of the screen of a CRO.

What is the new position of the spot when the switch is closed?

10

28 The diagram shows a waveform is displayed on the screen of a CRO when a moving d.c. generator supply is connected to Y-input of the CRO.

Which waveform will be displayed when the Y-shift is adjusted?

30 The diagram shows a waveform is displayed on the screen of a CRO when a moving an a.c. power supply is connected to Y-input of the CRO.

Which waveform will be displayed when the Y-gain is adjusted?

31 The diagram shows a waveform is displayed on the screen of a CRO when an a.c. power l is connected to Y-input of the CRO .The time-base is switched off

What is the peak voltage of the alternating current when the Y-gain of the CRO is adjusted at 20 V cm-1?

A 80.0 V B 56.6 V C 40.0 V D 28.3 VE 15.9 V

32 The diagram shows a waveform from a signal is displayed on the screen of a CRO.

What is the frequency of the signal when the time-base switch is adjusted at 20 ms cm-1?

A 12.5 Hz B 25 HzC 50 Hz D 100 HzE 200 Hz

33 The diagram shows a waveform is displayed on the screen of a CRO. The distance between two successive peaks is 3 cm.

What is the distance between two successive peaks when the frequency of the waveform is doubled?

A 0.75 cm B 1.50 cmC 3.00 cm D 4.50 cmE 6.00 cm

34 The diagram shows a waveform is displayed on the screen of a CRO when a moving an a.c.

11

power supply is connected to Y-input of the CRO. The frequency of the alternating current is 200 Hz.

What is the time taken from A to B?

A 5 x 10 -3 s B 7.5 x10 -3 sC 1 x 10 -2 s D 5 x 10-2 sE 7.5 x 10-2 s

35 The figure shows a waveform obtained on the screen of CRO at an airport radar station. The point M and N 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 = 5 ms cm-1 , the speed of light = 3 x 10 8 m s -1 ]

What is the distance between the radar station and the aero plane.

A 600 km B 1500 kmC 300 km D 100 kmE 50 km

36 The diagram shows a cathode-ray tube.

(a) What is the thermionic emission meant?

....................................................................

....................................................................

(b) What is the function of anode A?

....................................................................

.................................................................... (c) Explain why the tube must be a vacuum

tube?

....................................................................

....................................................................

....................................................................(d) What will happen to the electrons when

(i) the current flows through the filament is increased.

............................................................

............................................................(ii) the voltage of 5 KV power supply is

increases.

............................................................

............................................................

(iii) the switch is closed.

............................................................

12

............................................................

(e) What happen to the energy possesses by the electrons when they hit the screen of the cathode-ray tube?

....................................................................

.................................................................... (f) The potential between the filament and

anode A is 9 kV.[ e =1.6 x 10 -19 C and me= 9 x 10 -31 ]Calculate,(i) the kinetic energy of the electrons

(ii) the velocity of the electrons

37 The diagram shows the construction of a simple cathode-ray oscilloscope.

(a) State the name and the function of component P .

....................................................................

....................................................................

(b) The filament can be heated by d.c. or a.c. power supply. Explain why?

....................................................................

....................................................................

(c) Explain how is the control grid control the brightness the screen?

....................................................................

....................................................................(d) The time-base switch of the CRO is on.

Draw the waveform in the space below when the Y-input of the CRO is connected to (a) a dry cell

(ii) a transformer

(e) Give two advantages use the CRO to determine

the potential difference compare to the voltmeter.

....................................................................

....................................................................

....................................................................

....................................................................

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

13

(a) What is

(i) the frequency the current?

(i) the peak voltage of the current?

(b) Draw the waveform display on the screen of the CRO in figure below when

(i) the time-base switch is off.

(ii) the time-base switch is on and the a.c. power supply is replaced by a d.c. generator.

14