PHOTOELECTRIC, THERMIONIC EMISSION AND X-RAY

TOPIC: -                                                                PHOTOELECTRIC, THERMIONIC                 EMISSION AND X-RAY

INSTRUCTIONAL MATERIALS: - Chart showing x-ray machine    

REFERENCES BOOK: -

1.       FARINDE O. E e tal, ESSENTIAL PHYSICS FOR SSS, Tonad Publishing Limited.

2.       M. W. ANYAKOHA (2011), NEW SCHOOL PHYSICS FOR SENIOR SECONDARY SCHOOLS, Africana first publishers.  

3.       INTERNET           

PREVIOUS KNOWLEDGE: - Students have been familiar with gadgets uses electromagnetic principles.

 OBJECTIVES: - At the end of the lesson students should be able to: -

i.                     Define photo-electron.

ii.                   State the application of photoelectric emission.

iii.                  Define thermionic emission

iv.                 Differentiate between photoelectric and thermionic emission.

v.                   Characteristics of x-ray

CONTENT: -

PHOTOELECTRIC, THERMIONIC EMISSION AND X-RAY

When light falls on metal surfaces, electrons are emitted. This is the photo-electric effect. The emitted electrons are known as photo-electron.

The following observations were made in the study of the photoelectric effect:

i.                     Electrons are emitted at the instant the surface is illuminated even with light of very weak intensity.

ii.                   For each metal, there is a well define frequency called the threshold frequency which must be exceeded for electron emission to occur, no matter how strong the intensity of light may be.

iii.                  The maximum kinetic energy of the emitted electron increases with the frequency of the incident light, but it is independent of the intensity.

Application of photoelectric emission

1.       Photocell             2. Photometer                  3. Cathode-ray tube

Photocell can be used for following purposes: automatic door; sound production from film track; television camera; burglar alarms.

Thermionic emission is a process of emitting electrons from the surface of a metal when heated in a vacuum. Temperature is the only factor that affects the emission of electrons from the surface of a cathode.

Application of thermionic emission

1.       Diode valve        2. X-ray tube                      3. Cathode ray tube

Differences between photoelectric emission and thermionic emission

Photoelectric emission

Thermionic emission

Electron are released in the present of light

Electrons are released in the presence of heat in a vacuum

Electrons are released at a threshold frequency, depending on the nature of the metal surface

Electron are released at a particular temperature, depending on the nature of cathode (hot filament)

 Similarities

I.                    They are both surface phenomena.

II.                  Electrons and energy are released in both cases.

Energy of photon, E = hf where ‘h’ is the Planck’s constant (6.62x10-34 Js or 4.14x10-15 eVs) and ‘f ‘ is the frequency of light.

Einstein’s ideal can be expressed in an equation: hf = w + ½ mv2

The term hf represent the total energy content of a single photon of incident light. Part of this energy w, is used to get the electron free from the atom and away from the metal surface. The energy ‘w’ is known as the work function ( w = hfo where fo is the threshold frequency). The reminder of the energy  is used to give the liberated electron a kinetic energy ( ½ mv2 where v= velocity of photoelectron of mass m)

hf = w + ½ mv2

hf = hfo + E (E= kinetic energy of the photoelectron)

E = hf – hfo

E(ev) = h (f-fo)                   (1ev = 1.6x10-19 J)

THRESHOLD FREQUENCY (ho): is the maximum frequency an electron has to attain before it break through the barrier surface of the metal; having acquired the maximum kinetic energy. It is measured in Hertz (Hz)

WORK FUNCTION (wo): is the minimum amount of energy required to move a free electron from the surface of a metal.

STOPPING POTENTIAL(Vs) / STOPPING VOLTAGE: is the negative voltage of an electron emitting body which top all electrons reaching the metal.

evs = Emax = hf -wo

vs =  -  ;

Example

Light of frequency 6.0x1014Hz is incident on a metal surface. The maximum energy of the photoelectron is

2.4x10-19J. Calculate (a) the work function of the metal (b) the frequency of the radiation which will produce photoelectron with maximum energy of 3.4x10-19J (c) the stopping potential for these electrons.

Solution

F=6.0x1014Hz; E= 2.4x10-19J; h=6.62x10-34 Js

(a)    hf = w + E

w = hf –E = (6.62x10-34 Js x 6.0x1014Hz) - 2.4x10-19J = 1.572x10-19J

(b)   f =  =  = 7.51x1014Hz

(c)    vs =  =  =  = 2.1v

X-RAY

 X-rays are produced when thermally generated electrons from a hot filament are accelerated through a high voltage and focused onto a tungsten target of high melting point where the electrons are suddenly stopped.

The following energy conversions take place during the process of producing X-rays. The electrical energy is converted into the thermal energy of the thermionic electrons. This thermal energy is converted into the mechanical kinetic energy of the accelerated electrons. This is in turn converted into the electromagnetic energy of the x-rays and thermal energy.



TYPES OF X-RAYS

There are two types of x-ray:    i. Hard x-ray                         ii. Soft  x-ray

CHARACTERISTICS OF X-RAY

I.                    They are electromagnetic waves of very high frequency.

II.                  They have very short wavelength much shorter than that of light waves.

III.                They have a high penetrating power.

IV.                It travels in straight lines.

V.                  They are not deflected by electric or magnetic field.

VI.                They cause fluorescence in zinc substance e.g.  zinc sulphide.

VII.              They cause the liberation of electrons when they fall on certain substances (photoelectric effect).

VIII.            They ionize gases.

IX.                They are diffracted by crystals.

APPLICATION OF X-RAY

1.       They are used for examining the body to locate broken bones or hidden metallic object.

2.       Used in airports to detect metal contraband in a baggage.

3.       Used to detect cracks and flaws in metal castings and welding joints.

4.       Used in analytical tool for investigating crystal structure.

5.       Used in the treatment of tumours and some skin diseases.

6.       Used to detect alterations which have been made on works of art.

7.       Used in radio-therapy; radiography; and in agriculture to kill germs.

 

 

PRESENTATION

Step I: The teacher revises the previous topic.

 Step II: The teacher introduces the new topic.

Step III: The teacher explains photoelectric emission and its application.

Step IV: The teacher explains thermionic emission and its application

Step V: The teacher explains x-ray

Step VI: The teacher leads the students to state the characteristic of x-ray.

 EVALUATION

The teacher evaluates the lessons by asking these questions:

i.                     Define photo-electron.

ii.                   State the application of photoelectric emission.

iii.                  Define thermionic emission

iv.                 Differentiate between photoelectric and thermionic emission.

v.                   Characteristics of x-ray 

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